An exoplanet or extrasolar planet is a planet that does not orbit Earth's Sun and instead orbits a different star, stellar remnant, or brown dwarf. Over 1800 exoplanets have been discovered (1821 planets in 1135 planetary systems including 467 multiple planetary systems as of 29 August 2014). There are also free floating planets, not orbiting any star, which tend to be considered separately, especially if they are free floating gas giants, in which case they are often counted, like WISE 0855–0714, as low-mass brown dwarfs.
The Kepler mission space telescope has also detected a few thousand candidate planets, of which about 11% may be false positives. There is at least one planet on average per star. Around 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone,[c] with the nearest expected to be within 12 light-years distance from Earth. Assuming 200 billion stars in the Milky Way,[d] that would be 11 billion potentially habitable Earths, rising to 40 billion if red dwarf stars are included. The free-floating planets in the Milky Way possibly number in the trillions.
On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity". Prior to these results, most confirmed planets were gas giants comparable in size to Jupiter or larger as they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.
The nearest known exoplanet, if confirmed, would be Alpha Centauri Bb, but there is some doubt about its existence. Almost all of the planets detected so far are within the Milky Way; however, there have been a small number of possible detections of extragalactic planets. As of March 2014[update], the least massive planet known is PSR B1257+12 A, which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is DENIS-P J082303.1-491201 b, about 29 times the mass of Jupiter, although according to most definitions of a planet, it is too massive to be a planet and may be a brown dwarf instead. There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit. Some are so far out that it is difficult to tell if they are gravitationally bound to the star. (See also: List of exoplanet extremes.)
For centuries philosophers and scientists supposed that extrasolar planets existed, but there was no way of detecting them or of knowing their frequency or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers. The first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods such as the transit method and the radial-velocity method.
The discovery of extrasolar planets has intensified interest in the search for extraterrestrial life, particularly for those that orbit in the host star's habitable zone where it is possible for liquid water (and therefore life) to exist on the surface. The search for extrasolar planets prompts the study of planetary habitability, which considers a wide range of factors in determining the suitability of an extrasolar planet for hosting life.
- 1 Definition
- 2 History of detection
- 3 Detection methods
- 4 Verification and falsification methods
- 5 Characterization methods
- 6 Nomenclature
- 7 Formation and evolution
- 8 Planet-hosting stars
- 9 Orbital parameters
- 10 Rotation and axial tilt
- 11 Physical parameters
- 12 Atmosphere
- 13 Climate and weather
- 14 Surface
- 15 Water
- 16 General features
- 17 Habitability
- 18 Planetary systems
- 19 Cultural impact
- 20 See also
- 21 Notes
- 22 Further reading
- 23 References
- 24 External links
The official definition of "planet" used by the International Astronomical Union (IAU) only covers the Solar System and thus does not apply to exoplanets. As of April 2011, the only definitional statement issued by the IAU that pertains to exoplanets is a working definition issued in 2001 and modified in 2003. That definition contains the following criteria:
- Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our solar system.
- Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located.
- Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).
However, the IAU's working definition is not universally accepted. One alternate suggestion is that planets should be distinguished from brown dwarfs on the basis of formation. It is widely believed that giant planets form through core accretion, and that process may sometimes produce planets with masses above the deuterium fusion threshold; massive planets of that sort may have already been observed. Brown dwarfs form like stars from the direct collapse of clouds of gas and this formation mechanism also produces objects that are below the 13MJup limit and can be as low as 1MJup. Objects in this mass range that orbit their stars with wide separations of hundreds or thousands of AU and have large star/object mass ratios likely formed as brown dwarfs; their atmospheres would likely have a composition more similar to their host star than accretion-formed planets which would contain increased abundances of heavier elements. Most directly imaged planets as of April 2014 are massive and have wide orbits so probably represent the low-mass end of brown dwarf formation.
Also, the 13 Jupiter-mass cutoff does not have precise physical significance. Deuterium fusion can occur in some objects with mass below that cutoff. The amount of deuterium fused depends to some extent on the composition of the object. The Extrasolar Planets Encyclopaedia includes objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 MJup in the observed mass spectrum reinforces the choice to forget this mass limit,". The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity." The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses. Another criterion for separating planets and brown dwarfs, rather than deuterium burning, formation process or location, is whether the core pressure is dominated by coulomb pressure or electron degeneracy pressure.
History of detection
|“||This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.||”|
—Giordano Bruno (1584)
In the sixteenth century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
In the eighteenth century the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centers of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."
Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 Capt. W. S. Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system. In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars. However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable. During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star. Astronomers now generally regard all the early reports of detection as erroneous.
In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations. The claim briefly received intense attention, but Lyne and his team soon retracted it.
The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of University of Victoria and University of British Columbia. Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990 additional observations were published that supported the existence of the planet orbiting Gamma Cephei, but subsequent work in 1992 again raised serious doubts. Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.
On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12. This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Followup observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits.
On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, namely the nearby G-type star 51 Pegasi. This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet passed in front of it.
Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters are a minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets. Other multiple planetary systems were found subsequently.
As of 29 August 2014, a total of 1821 confirmed exoplanets are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late 1980s. That count includes 1135 planetary systems, of which 467 are multiple planetary systems. Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.
17 October 2012 brought news of an unverified planet, Alpha Centauri Bb, orbiting Alpha Centauri B, which is one of three stars in a triple star system nearest to Earth's Sun. Alpha Centauri Bb is an Earth-size planet, but not in the habitable zone within which liquid water can exist.
As of March 2014, NASA's Kepler mission had identified more than 2,900 planetary candidates, several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.
Planets are extremely faint compared to their parent stars. At visible wavelengths, they usually have less than a millionth of their parent star's brightness. It is difficult to detect such a faint light source, and furthermore the parent star causes a glare that tends to wash it out. It is necessary to block the light from the parent star in order to reduce the glare, while leaving the light from the planet detectable; doing so is a major technical challenge.
All exoplanets that have been directly imaged are both large (more massive than Jupiter) and widely separated from their parent star. Most of them are also very hot, so that they emit intense infrared radiation; the images have then been made at infrared where the planet is brighter than it is at visible wavelengths. During the gas-accretion phase of giant planet formation the star-planet contrast may be even better in H alpha than it is in infrared - an H alpha survey is currently underway.
Specially designed direct-imaging instruments such as Gemini Planet Imager, VLT-SPHERE, and SCExAO will image dozens of gas giants, however the vast majority of known extrasolar planets have only been detected through indirect methods. The following are the indirect methods that have proven useful:
- If a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors. This method suffers from a substantial rate of false positives and confirmation from another method is usually considered necessary. The transit method reveals the radius of a planet, and it has the benefit that it sometimes allows a planet's atmosphere to be investigated through spectroscopy. Because the transit method requires that part of the planet's orbit intersect a line-of-sight between the host star and Earth, the probability that an exoplanet in a randomly oriented orbit will be observed to transit the star is somewhat small.
- As a planet orbits a star, the star also moves in its own small orbit around the system's center of mass. Variations in the star's radial velocity—that is, the speed with which it moves towards or away from Earth—can be detected from displacements in the star's spectral lines due to the Doppler effect. Extremely small radial-velocity variations can be observed, of 1 m/s or even somewhat less. This method has the advantage of being applicable to stars with a wide range of characteristics. One of its disadvantages is that it cannot determine a planet's true mass, but can only set a lower limit on that mass. However, if the radial velocity of the planet itself can be distinguished from the radial velocity of the star, then the true mass can be determined.
- Transit timing variation (TTV)
- When multiple planets are present, each one slightly perturbs the others' orbits. Small variations in the times of transit for one planet can thus indicate the presence of another planet, which itself may or may not transit. For example, variations in the transits of the planet Kepler-19b suggest the existence of a second planet in the system, the non-transiting Kepler-19c. If multiple transiting planets exist in one system, then this method can be used to confirm their existence. In another form of the method, timing the eclipses in an eclipsing binary star can reveal an outer planet that orbits both stars; as of August 2013, a few planets have been found in that way with numerous planets confirmed with this method.
- When a planet orbits multiple stars or if the planet has moons, its transit time can significantly vary per transit. Although no new planets or moons have been discovered with this method, it is used to successfully confirm many transiting circumbinary planets.
- Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Planets orbiting the lensing star can cause detectable anomalies in the magnification as it varies over time. Unlike most other methods which have detection bias towards planets with small (or for resolved imaging, large) orbits, microlensing method is most sensitive to detecting planets around 1–10 AU away from Sun-like stars.
- Astrometry consists of precisely measuring a star's position in the sky and observing the changes in that position over time. The motion of a star due to the gravitational influence of a planet may be observable. Because the motion is so small, however, this method has not yet been very productive. It has produced only a few disputed detections, though it has been successfully used to investigate the properties of planets found in other ways.
- A pulsar (the small, ultradense remnant of a star that has exploded as a supernova) emits radio waves extremely regularly as it rotates. If planets orbit the pulsar, they will cause slight anomalies in the timing of its observed radio pulses. The first confirmed discovery of an extrasolar planet was made using this method. But as of 2011, it has not been very productive; five planets have been detected in this way, around three different pulsars.
- Like pulsars, there are some other types of stars which exhibit periodic activity. Deviations from the periodicity can sometimes be caused by a planet orbiting it. As of 2013, a few planets have been discovered with this method.
- When a planet orbits very close to the star, it catches a considerable amount of starlight. As the planet orbits around the star, the amount of light changes due to planets having phases from Earth's viewpoint or planet glowing more from one side than the other due to temperature differences.
- Relativistic beaming measures the observed flux from the star due to its motion. The brightness of the star changes as the planet moves closer or further away from its host star.
- Massive planets close to their host stars can slightly deform the shape of the star. This causes the brightness of the star to slightly deviate depending how it is rotated relative to Earth.
- With polarimetry method, a polarized light reflected off the planet is separated from unpolarized light emitted from the star. No new planets have been discovered with this method although a few already discovered planets have been detected with this method.
- Disks of space dust surround many stars, believed to originate from collisions among asteroids and comets. The dust can be detected because it absorbs starlight and re-emits it as infrared radiation. Features in the disks may suggest the presence of planets, though this is not considered a definitive detection method.
Most confirmed extrasolar planets have been found using ground-based telescopes. However, many of the methods can work more effectively with space-based telescopes that avoid atmospheric haze and turbulence. COROT and Kepler were space missions dedicated to searching for extrasolar planets. Hubble Space Telescope and MOST have also found or confirmed a few planets. The Gaia mission, launched in December 2013, will use astrometry to determine the true masses of 1000 nearby exoplanets. CHEOPS and TESS, to be launched in 2017, and PLATO in 2024 will use the transit method.
Primary and secondary detection
|Transit||Primary eclipse. Planet passes in front of star.||Secondary eclipse. Star passes in front of planet.|
|Radial velocity||Radial velocity of star||Radial velocity of planet. This has been done for Tau Boötis b.|
|Astrometry||Astrometry of star. Position of star moves more for large planets with large orbits.||Astrometry of planet. Color-differential astrometry. Position of planet moves quicker for planets with small orbits. Theoretical method - has been proposed for use for the SPICA (spacecraft).|
Verification and falsification methods
Most exoplanets have catalog names which are explained in the following sections, but in 2014 the IAU launched a process for giving proper names to exoplanets. The process involves public nomination and voting for the new names, and the IAU plans to announce the new names in August 2015. The decision to give the planets new names followed the private company Uwingu's exoplanet naming contest, which the IAU harshly criticized. Previously a few planets had received unofficial names: notably Osiris (HD 209458 b), Bellerophon (51 Pegasi b), and Methuselah (PSR B1620-26 b).
The convention for naming exoplanets is an extension of the one used by the Washington Multiplicity Catalog (WMC) for multiple-star systems, and adopted by the International Astronomical Union. The brightest member of a star system receives the letter "A". Distinct components not contained within "A" are labeled "B", "C", etc. Subcomponents are designated by one or more suffixes with the primary label, starting with lowercase letters for the 2nd hierarchical level and then numbers for the 3rd. For example, if there is a triple star system in which two stars orbit each other closely with a third star in a more distant orbit, the two closely orbiting stars would be named Aa and Ab, whereas the distant star would named B. For historical reasons, this standard is not always followed: for example Alpha Centauri A, B and C are not labelled Alpha Centauri Aa, Ab and B.
Extrasolar planet standard
Following an extension of the above standard, an exoplanet's name is normally formed by taking the name of its parent star and adding a lowercase letter. The first planet discovered in a system is given the designation "b" and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size.
For instance, in the 55 Cancri system the first planet – 55 Cancri b – was discovered in 1996; two additional farther planets were simultaneously discovered in 2002 with the nearest to the star being named 55 Cancri c and the other 55 Cancri d; a fourth planet was claimed (its existence was later disputed) in 2004 and named 55 Cancri e despite lying closer to the star than 55 Cancri b; and the most recently discovered planet, in 2007, was named 55 Cancri f despite lying between 55 Cancri c and 55 Cancri d. As of April 2012 the highest letter in use is "j", for the unconfirmed planet HD 10180 j, and with "h" being the highest letter for a confirmed planet, belonging to the same host star).
If a planet orbits one member of a binary star system, then an uppercase letter for the star will be followed by a lowercase letter for the planet. Examples are 16 Cygni Bb and HD 178911 Bb. Planets orbiting the primary or "A" star should have 'Ab' after the name of the system, as in HD 41004 Ab. However, the "A" is sometimes omitted; for example the first planet discovered around the primary star of the Tau Boötis binary system is usually called simply Tau Boötis b.
If the parent star is a single star, then it may still be regarded as having an "A" designation, though the "A" is not normally written. The first exoplanet found to be orbiting such a star could then be regarded as a secondary subcomponent that should be given the suffix "Ab". For example, 51 Peg Aa is the host star in the system 51 Peg; and the first exoplanet is then 51 Peg Ab. Because most exoplanets are in single-star systems, the implicit "A" designation was simply dropped, leaving the exoplanet name with the lower-case letter only: 51 Peg b.
A few exoplanets have been given names that do not conform to the above standard. For example, the planets that orbit the pulsar PSR 1257 are often referred to with capital rather than lowercase letters. Also, the underlying name of the star system itself can follow several different systems. In fact, some stars (such as Kepler-11) have only received their names due to their inclusion in planet-search programs, previously only being referred to by their celestial coordinates.
Circumbinary planets and 2010 proposal
Hessman et al. state that the implicit system for exoplanet names utterly failed with the discovery of circumbinary planets. They note that the discoverers of the two planets around HW Virginis tried to circumvent the naming problem by calling them "HW Vir 3" and "HW Vir 4", i.e. the latter is the 4th object – stellar or planetary – discovered in the system. They also note that the discoverers of the two planets around NN Serpentis were confronted with multiple suggestions from various official sources and finally chose to use the designations "NN Ser c" and "NN Ser d".
The proposal of Hessman et al. starts with the following two rules:
- Rule 1. The formal name of an exoplanet is obtained by appending the appropriate suffixes to the formal name of the host star or stellar system. The upper hierarchy is defined by upper-case letters, followed by lower-case letters, followed by numbers, etc. The naming order within a hierarchical level is for the order of discovery only. (This rule corresponds to the present provisional WMC naming convention.)
- Rule 2. Whenever the leading capital letter designation is missing, this is interpreted as being an informal form with an implicit "A" unless otherwise explicitly stated. (This rule corresponds to the present exoplanet community usage for planets around single stars.)
They note that under these two proposed rules all of the present names for 99% of the planets around single stars are preserved as informal forms of the IAU sanctioned provisional standard. They would rename Tau Boötis b formally as Tau Boötis Ab, retaining the prior form as an informal usage (using Rule 2, above).
To deal with the difficulties relating to circumbinary planets, the proposal contains two further rules:
- Rule 3. As an alternative to the nomenclature standard in Rule 1, a hierarchical relationship can be expressed by concatenating the names of the higher order system and placing them in parentheses, after which the suffix for a lower order system is added.
- Rule 4. When in doubt (i.e. if a different name has not been clearly set in the literature), the hierarchy expressed by the nomenclature should correspond to dynamically distinct (sub)systems in order of their dynamical relevance. The choice of hierarchical levels should be made to emphasize dynamical relationships, if known.
They submit that the new form using parentheses is the best for known circumbinary planets and has the desirable effect of giving these planets identical sublevel hierarchical labels and stellar component names that conform to the usage for binary stars. They say that it requires the complete renaming of only two exoplanetary systems: The planets around HW Virginis would be renamed HW Vir (AB) b & (AB) c, whereas those around NN Serpentis would be renamed NN Ser (AB) b & (AB) c. In addition the previously known single circumbinary planets around PSR B1620-26 and DP Leonis) can almost retain their names (PSR B1620-26 b and DP Leonis b) as unofficial informal forms of the "(AB)b" designation where the "(AB)" is left out.
The discoverers of the circumbinary planet around Kepler-16 followed the naming scheme proposed by Hessman et al. when naming the body Kepler-16 (AB)-b, or simply Kepler-16b when there is no ambiguity.
Other naming systems
Another nomenclature, often seen in science fiction, uses Roman numerals in the order of planets' positions from the star. (This was inspired by an old system for naming moons of the outer planets, such as "Jupiter IV" for Callisto.) But such a system is impractical for scientific use, because new planets may be found closer to the star, changing all numerals.
Formation and evolution
- Initial Conditions of Planet Formation: Lifetimes of Primordial Disks, Eric E. Mamajek, 26 Jun 2009
- On the formation time scale and core masses of gas giant planets, W.K.M. Rice, Philip J. Armitage, 7 Oct 2003
- A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites, Yin, Qingzhu; Jacobsen, S. B.; Yamashita, K.; Blichert-Toft, J.; Télouk, P.; Albarède, F. Nature, Volume 418, Issue 6901, pp. 949–952 (2002)
- Origin and Loss of nebula-captured hydrogen envelopes from "sub"- to "super-Earths" in the habitable zone of Sun-like stars, H. Lammer, A. Stökl, N.V. Erkaev, E.A. Dorfi, P. Odert, M. Güdel, Yu.N. Kulikov, K.G. Kislyakova, M. Leitzinger, 13 Jan 2014
- Thermally-Driven Atmospheric Escape, R.E. Johnson, 7 Feb 2010
- Atmospheric mass loss by stellar wind from planets around main sequence M stars, Jesus Zendejas, Antigona Segura, Alejandro Raga, 31 May 2010
- Co-evolution of atmospheres, life, and climate, John Lee Grenfell et al., 20 May 2010
- Phase Separation in Giant Planets: Inhomogeneous Evolution of Saturn, Jonathan J. Fortney, William B. Hubbard, 1 May 2003
- Magnetodynamo Lifetimes for Rocky, Earth-Mass Exoplanets with Contrasting Mantle Convection Regimes, Joost van Summeren, Eric Gaidos, Clinton P. Conrad, 9 Apr 2013
- The effect of evaporation on the evolution of close-in giant planets, I. Baraffe, F. Selsis, G. Chabrier, T. S. Barman, F. Allard, P.H. Hauschildt, H. Lammer, 5 Apr 2004
- Observational Evidence for Tidal Destruction of Exoplanets, Brian Jackson, Rory Barnes, Richard Greenberg, 7 Apr 2009
- A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects, Fred C. Adams, Gregory Laughlin, 18 Jan 1997
Proportion of stars with planets
Planet-search programs have discovered planets orbiting a substantial fraction of the stars they have looked at. However, the overall proportion of stars with planets is uncertain because not all planets can yet be detected. The radial-velocity method and the transit method (which between them are responsible for the vast majority of detections) are most sensitive to large planets in small orbits. Thus many known exoplanets are "hot Jupiters": planets of Jovian mass or larger in very small orbits with periods of only a few days. A 2005 survey of radial-velocity-detected planets found that about 1.2% of Sun-like stars have a hot jupiter, where "Sun-like star" refers to any main-sequence star of spectral classes late-F, G, or early-K without a close stellar companion. This 1.2% is more than double the frequency of hot jupiters detected by the Kepler spacecraft, which may be because the Kepler field of view covers a different region of the Milky Way where the metallicity of stars is different. It is further estimated that 3% to 4.5% of Sun-like stars possess a giant planet with an orbital period of 100 days or less, where "giant planet" means a planet of at least 30 Earth masses.
It is known that small planets (of roughly Earth-like mass or somewhat larger) are more common than giant planets. It also appears that there are more planets in large orbits than in small orbits. Based on this, it is estimated that perhaps 20% of Sun-like stars have at least one giant planet whereas at least 40% may have planets of lower mass. A 2012 study of gravitational microlensing data collected between 2002 and 2007 concludes the proportion of stars with planets is much higher and estimates an average of 1.6 planets orbiting between 0.5–10 AU per star in the Milky Way, the authors of this study conclude that "stars are orbited by planets as a rule, rather than the exception". In November 2013 it was announced that 22±8% of Sun-like[a] stars have an Earth-sized[b] planet in the habitable[c] zone.
Whatever the proportion of stars with planets, the total number of exoplanets must be very large. Because the Milky Way has at least 200 billion stars, it must also contain tens or hundreds of billions of planets.
Most known exoplanets orbit stars roughly similar to the Sun, that is, main-sequence stars of spectral categories F, G, or K. One reason is that planet search programs have tended to concentrate on such stars. But in addition, statistical analysis indicates that lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to detect by the radial-velocity method. Although several tens of planets around red dwarfs have been discovered by the Kepler spacecraft which uses the transit method which can detect smaller planets.
Stars of spectral category A typically rotate very quickly, which makes it very difficult to measure the small Doppler shifts induced by orbiting planets because the spectral lines are very broad. However, this type of massive star eventually evolves into a cooler red giant that rotates more slowly and thus can be measured using the radial-velocity method. A few tens of planets have been found around red giants.
Observations using the Spitzer Space Telescope indicate that extremely massive stars of spectral category O, which are much hotter than the Sun, produce a photo-evaporation effect that inhibits planetary formation. When the O-type star goes supernova any planets that had formed would become free floating due to the loss of stellar mass unless the natal kick of the resulting remnant pushes it in the same direction as an escaping planet.
Doppler surveys around a wide variety of stars indicate about 1 in 6 stars having twice the mass of the Sun are orbited by one or more Jupiter-sized planets, vs. 1 in 16 for Sun-like stars and only 1 in 50 for red dwarfs. On the other hand, microlensing surveys indicate that long-period Neptune-mass planets are found around 1 in 3 red dwarfs.  Kepler Space Telescope observations of planets with up to one year periods show that occurrence rates of Earth to Neptune-sized planets (1 to 4 Earth radii) around M, K, G, and F stars are successively higher towards cooler, less massive stars.
Ordinary stars are composed mainly of the light elements hydrogen and helium. They also contain a small proportion of heavier elements, and this fraction is referred to as a star's metallicity (even if the elements are not metals in the traditional sense), denoted [m/H] and expressed on a logarithmic scale where zero is the Sun's metallicity.
A 2012 study of the Kepler spacecraft data found that the smaller planets with radii smaller than that of Neptune were found around stars with metallicities in the range −0.6 < [m/H] < +0.5 (about four times less than the Sun to three times more than the Sun),[d] whereas the larger planets were found mostly around stars with metallicity at the higher end of this range (at solar metallicity and above) In this study small planets occurred about three times as frequently as large planets around stars of metallicity greater than that of the Sun, but they occurred around six times as frequently for stars of metallicity less than that of the Sun. The lack of gas giants around low-metallicity stars could be because the metallicity of protoplanetary disks affects how quickly planetary cores can form and whether they accrete a gaseous envelope before the gas dissipates. However, Kepler can only observe planets very close to their star and the detected gas giants probably migrated from further out, so a decreased efficiency of migration in low-metallicity disks could also partly explain these findings.
Most known planets orbit single stars, but some orbit one member of a binary star system, and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known and one in the quadruple system Kepler 64. The Kepler results indicate circumbinary planetary systems are relatively common (as of October 2013 the spacecraft had found seven planets out of roughly 1000 eclipsing binaries searched). One puzzling finding is none of the close binary stars surveyed appear to have eclipsing planets. Half of the stars have an orbital period of 2.7 days or less, but none of the binaries with planets have a period less than 7.4 days. Another surprising Kepler finding is circumbinary planets tend to orbit their stars close to the critical instability radius (theoretical calculations indicate the minimum stable separation is roughly two to three times the size of the stars' separation).
Most stars form in open clusters, but very few planets have been found in open clusters and this led to the hypothesis that the open-cluster environment hinders planet formation. However, a 2011 study concluded that there have been an insufficient number of surveys of clusters to make such a hypothesis. The lack of surveys was because there are relatively few suitable open clusters in the Milky Way. Recent discoveries of both giant planets and low-mass planets in open clusters are consistent with there being similar planet occurrence rates in open clusters as around field stars.
Free-floating planets in open clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and 105 AU. The capture efficiency decreases with increasing cluster size, and for a given cluster size it increases with the host/primary mass. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system. Some planet–host metallicity correlation may still exist due to the common origin of the stars from the same cluster. Planets would be unlikely to be captured around neutron stars because these are likely to be ejected from the cluster by a pulsar kick when they form. Planets could even be captured around other planets to form free-floating planet binaries. After the cluster has dispersed some of the captured planets with orbits larger than 106 AU would be slowly disrupted by the galactic tide and likely become free floating again through encounters with other field stars or giant molecular clouds.
Galactic distribution of planets
The Milky Way is 100,000 light-years across, but 90% of planets with known distances lie within about 2000 light years of Earth, as of July 2014. One method that can detect planets much further away is microlensing. The WFIRST spacecraft could use microlensing to measure the relative frequency of planets in the galactic bulge vs. galactic disk. So far, the indications are that planets are more common in the disk than the bulge. Estimates of the distance of microlensing events is difficult: the first planet considered with high probability of being in the bulge is MOA-2011-BLG-293Lb at a distance of 7.7 kiloparsecs (about 25,000 light years).
Population I, or metal-rich stars, are those young stars whose metallicity is highest. The high metallicity of population I stars makes them more likely to possess planetary systems than older populations, because planets form by the accretion of metals. The Sun is an example of a metal-rich star. These are common in the spiral arms of the Milky Way. Generally, the youngest stars, the extreme population I, are found farther in and intermediate population I stars are farther out, etc. The Sun is considered an intermediate population I star. Population I stars have regular elliptical orbits around the Galactic Center, with a low relative velocity.
Population II, or metal-poor stars, are those with relatively low metallicity which can have hundreds (e.g. BD +17° 3248) or thousands (e.g. Sneden's Star) times less metallicity than the Sun. These objects formed during an earlier time of the universe. Intermediate population II stars are common in the bulge near the center of the Milky Way, whereas Population II stars found in the galactic halo are older and thus more metal-poor. Globular clusters also contain high numbers of population II stars. In 2014 the first planets around a halo star were discovered around Kapteyn's star, the nearest halo star to Earth, around 13 light years away. With an age greater than 10 billion years the planet Kapteyn b is the oldest known planet in a habitable zone. The metallicity of Kapteyn's star is estimated to be about 8[e] times less than the Sun.
Different types of galaxies have different histories of star formation and hence planet formation. Planet formation is affected by the ages, metallicities, and orbits of stellar populations within a galaxy and vary between the different types of galaxies. The distribution of the different types of galaxies in the universe depends on their location within galaxy clusters.
- The Ages of Stars, David R. Soderblom, 31 Mar 2010
- Towards asteroseismically calibrated age-rotation-activity relations for Kepler solar-like stars, R.A. Garcia et al. 27 Mar 2014
- Accurate parameters of the oldest known rocky-exoplanet hosting system: Kepler-10 revisited, Alexandra Fogtmann-Schulz et al. 5 Dec 2013
- The importance of asteroseismology in exoplanetary science, F Borsa, E Poretti - sait.oat.ts.astro.it
- What asteroseismology can do for exoplanets: Kepler-410A b is a Small Neptune around a bright star, in an eccentric orbit consistent with low obliquity, Vincent Van Eylen et al. 17 Dec 2013
- Pulsations and planets: the asteroseismology-extrasolar-planet connection, Sonja Schuh, 19 May 2010
- How stellar activity affects the size estimates of extrasolar planets, S. Czesla, K. F. Huber, U. Wolter, S. Schröter, J. H. M. M. Schmitt, 19 Jun 2009
- Hot Jupiters and stellar magnetic activity, A. F. Lanza, 20 May 2008
- Extrasolar Giant Planets and X-ray Activity, Vinay L. Kashyap, Jeremy J. Drake, Steven H. Saar, 21 Jul 2008
- Mass loss of "Hot Jupiters"—Implications for CoRoT discoveries. Part I: The importance of magnetospheric protection of a planet against ion loss caused by coronal mass ejections, Khodachenko et al. April 2007
Most known extrasolar planet candidates have been discovered using indirect methods and therefore only some of their physical and orbital parameters can be determined. For example, out of the six independent parameters that define an orbit, the radial-velocity method can determine four: semi-major axis, eccentricity, longitude of periastron, and time of periastron. Two parameters remain unknown: inclination and longitude of the ascending node.
Distance from star, semi-major axis and orbital period
The orbit of a planet is not centered on the star but on their common center of mass (see diagram on right). For circular orbits, the semi-major axis is the distance between the planet and the center of mass of the system. For elliptical orbits, the planet–star distance varies over the course of the orbit, in which case the semi-major axis is the average of the largest and smallest distances between the planet and the center of mass of the system. If the sizes of the star and planet are relatively small compared to the size of the orbit and the orbit is nearly circular and the center of mass is not too far from the star's center, such as in the Earth-Sun system, then the distance from any point on the star to any point on the planet is approximately the same as the semi-major axis. However, when a star's radius expands when it turns into a red giant, then the distance between the planet and the star's surface can become close to zero, or even less than zero if the planet has been engulfed by the expanding red giant, whereas the center of mass from which the semi-major axis is measured will still be near the center of the red giant.
Orbital period is the time taken to complete one orbit. For any given star, the shorter the semi-major axis of a planet, the shorter the orbital period. Also comparing planets around different stars but with the same semi-major axis, the more massive the star, the shorter the orbital period.
There are exoplanets that are much closer to their parent star than any planet in the Solar System is to the Sun, and there are also exoplanets that are much further from their star. Mercury, the closest planet to the Sun at 0.4 AU, takes 88 days for an orbit, but the shortest known orbits for exoplanets take only a few hours, e.g. Kepler-70b. The Kepler-11 system has five of its planets in shorter orbits than Mercury. Neptune is 30 AU from the Sun and takes 165 years to orbit, but there are exoplanets that are hundreds of AU from their star and take more than a thousand years to orbit, e.g. 1RXS1609 b.
Over the lifetime of a star, the semi-major axes of its planets changes. This planetary migration happens especially during the formation of the planetary system when planets interact with the protoplanetary disk and each other until a relatively stable position is reached, and later in the red-giant and asymptotic-giant-branch phases when the star expands and engulfs the nearest planets that can cause them to move inwards, and when the giant phases lose mass as the outer layers dissipate causing planets to move outwards as a result of the star's reduced gravitational field.
The radial-velocity and transit methods are most sensitive to planets with small orbits. The earliest discoveries such as 51 Peg b were gas giants with orbits of a few days. These "hot Jupiters" likely formed further out and migrated inwards. The Kepler spacecraft has found planets with even shorter orbits of only a few hours, which places them within the star's upper atmosphere or corona, and these planets are Earth-sized or smaller and are probably the left-over solid cores of giant planets that have evaporated due to being so close to the star, or even being engulfed by the star in its red giant phase in the case of Kepler-70b. As well as evaporation, other reasons why larger planets are unlikely to survive orbits only a few hours long include orbital decay caused by tidal force, tidal-inflation instability, and Roche-lobe overflow. The Roche limit implies that small planets with orbits of a few hours are likely made mostly of iron.
The direct imaging method is most sensitive to planets with large orbits, and has discovered some planets that have planet–star separations of hundreds of AU. However, protoplanetary disks are usually only around 100 AU in radius, and core accretion models predict giant planet formation to be within 10 AU, where the planets can coalesce quickly enough before the disk evaporates. Very long-period giant planets may have been free floating planets that were captured, or formed close-in and gravitationally scattered outwards, or the planet and star could be a mass-imbalanced wide binary system with the planet being the primary object of its own separate protoplanetary disk. Gravitational instability models might produce planets at multi-hundred AU separations but this would require unusually large disks. For planets with very wide orbits up to several hundred thousand AU it may be difficult to observationally determine whether the planet is gravitationally bound to the star.
Most planets that have been discovered are within a couple of AU of their star because the most used methods (radial-velocity and transit) require observation of several orbits to confirm that the planet exists and there has only been enough time since these methods were first used to cover small separations. Some planets with larger orbits have been discovered by direct imaging but there is a middle range of distances, roughly equivalent to the Solar System's gas giant region, which is largely unexplored. Direct imaging equipment for exploring that region is being installed on the world's largest telescopes and should begin operation in 2014. e.g. Gemini Planet Imager and VLT-SPHERE. The microlensing method has detected a few planets in the 1-10AU range. It appears plausible that in most exoplanetary systems, there are one or two giant planets with orbits comparable in size to those of Jupiter and Saturn in the Solar System. Giant planets with substantially larger orbits are now known to be rare, at least around Sun-like stars.
The distance of the habitable zone from a star depends on the type of star and this distance changes during the star's lifetime as the size and temperature of the star changes.
The eccentricity of an orbit is a measure of how elliptical (elongated) it is. All the planets of the Solar System except for Mercury have near-circular orbits (e<0.1). Most exoplanets with orbital periods of 20 days or less have near-circular orbits, i.e. very low eccentricity. That is believed to be due to tidal circularization: reduction of eccentricity over time due to gravitational interaction between two bodies. The mostly sub-Neptune-sized planets found by the Kepler spacecraft with short orbital periods have very circular orbits. By contrast, the giant planets with longer orbital periods discovered by radial-velocity methods have quite eccentric orbits. (As of July 2010, 55% of such exoplanets have eccentricities greater than 0.2, whereas 17% have eccentricities greater than 0.5.) Moderate to high eccentricities (e>0.2) of giant planets are not an observational selection effect, because a planet can be detected about equally well regardless of the eccentricity of its orbit. The prevalence of elliptical orbits for giant planets is a major puzzle, because current theories of planetary formation strongly suggest planets should form with circular (that is, non-eccentric) orbits.
However, for weak Doppler signals near the limits of the current detection ability the eccentricity becomes poorly constrained and biased towards higher values. It is suggested that some of the high eccentricities reported for low-mass exoplanets may be overestimates, because simulations show that many observations are also consistent with two planets on circular orbits. Reported observations of single planets in moderately eccentric orbits have about a 15% chance of being a pair of planets. This misinterpretation is especially likely if the two planets orbit with a 2:1 resonance. With the exoplanet sample known in 2009, a group of astronomers has concluded that "(1) around 35% of the published eccentric one-planet solutions are statistically indistinguishable from planetary systems in 2:1 orbital resonance, (2) another 40% cannot be statistically distinguished from a circular orbital solution" and "(3) planets with masses comparable to Earth could be hidden in known orbital solutions of eccentric super-Earths and Neptune mass planets".
Radial velocity surveys found exoplanet orbits beyond 0.1 AU to be eccentric, particularly for large planets. Kepler spacecraft transit data is consistent with the RV surveys and also revealed that smaller planets tend to have less eccentric orbits. 
- High Orbital Eccentricities of Extrasolar Planets Induced by the Kozai Mechanism, G. Takeda, F.A. Rasio, last revised 9 Jun 2005
- Extreme Climate Variations from Milankovitch-like Eccentricity Oscillations in Extrasolar Planetary Systems, David S. Spiegel, 11 Oct 2010
- Orbital Dynamics of Multi-Planet Systems with Eccentricity Diversity, Stephen R. Kane, Sean N. Raymond, 8 Feb 2014
- Type II migration of planets on eccentric orbits, Althea V. Moorhead, Eric B. Ford, 21 Apr 2009
Inclination vs spin-orbit angle
Orbital inclination is the angle between a planet's orbital plane and another plane of reference. For exoplanets the inclination is usually stated with respect to an observer on Earth: the angle used is that between the normal to the planet's orbital plane and the line of sight from Earth to the star. Therefore most planets observed by the transit method are close to 90 degrees. Since the word 'inclination' is used in exoplanet studies for this line-of-sight inclination then the angle between the planet's orbit and the star's rotation must use a different word and is termed the spin-orbit angle or spin-orbit alignment. In most cases the orientation of the star's rotational axis is unknown. The Kepler spacecraft has found a few hundred multi-planet systems and in most of these systems the planets all orbit in nearly the same plane, much like the Solar System. However, a combination of astrometric and radial-velocity measurements has shown that some planetary systems contain planets whose orbital planes are significantly tilted relative to each other. More than half of hot Jupiters have orbital planes substantially misaligned with their parent star's rotation. A substantial fraction of hot-Jupiters even have retrograde orbits, meaning that they orbit in the opposite direction from the star's rotation. Rather than a planet's orbit having been disturbed, it may be that the star itself flipped early in their system's formation due to interactions between the star's magnetic field and the planet-forming disc.
- A Window on Exoplanet Dynamical Histories: Rossiter-McLaughlin Observations of WASP-13b and WASP-32b, R. D. Brothwell et al. 17 Mar 2014
- Cyclic Transit Probabilities of Long-Period Eccentric Planets Due to Periastron Precession, Stephen R. Kane, Jonathan Horner, Kaspar von Braun, 7 Sep 2012
- Observability of the General Relativistic Precession of Periastra in Exoplanets, Andres Jordan, Gaspar A. Bakos, 3 Jun 2008
- Classical and relativistic node precessional effects in WASP-33b and perspectives for detecting them, Lorenzo Iorio, 25 Aug 2010
Rotation and axial tilt
In April 2014 the first measurement of a planet's rotation period was announced: the length of day for the super-Jupiter gas giant Beta Pictoris b is 8 hours (based on the assumption that the axial tilt of the planet is small.) With an equatorial rotational velocity of 25 km per second, this spin is faster than the gas giants of the solar system in line with expectation that the more mass a gas giant has the faster it spins. (Dwarf planet Ceres rotates in 5 hours but the smaller radius of Ceres means that a 5-hour rotation period corresponds to an equatorial rotational velocity that is much slower than Beta Pictoris b's velocity.) Beta Pictoris b's distance from its star is 9AU. At such distances the rotation of Jovian planets is not slowed by tidal effects. Beta Pictoris b is still warm and young and over the next hundreds of millions of years, it will cool down and shrink to about the size of Jupiter, and if its angular momentum is preserved then as it shrinks the length of its day will decrease to about 3 hours and its equatorial rotation velocity will speed up to about 40 km per second. The images of Beta Pictoris b do not have high enough resolution to directly see details but doppler spectroscopy techniques were used to show that different parts of the planet were moving at different speeds and in opposite directions from which it was inferred that the planet is rotating. With the next generation of large ground-based telescopes it will be possible to use doppler imaging techniques to make a global map of the planet, like the recent mapping of the brown dwarf Luhman 16B.
Origin of spin and tilt of terrestrial planets
Giant impacts have a large effect on the spin of terrestrial planets. The last few giant impacts during planetary formation tend to be the main determiner of a terrestrial planet's rotation rate. On average the spin angular velocity will be about 70% of the velocity that would cause the planet to break up and fly apart; the natural outcome of planetary embryo impacts at speeds slightly larger than escape velocity. In later stages terrestrial planet spin is also affected by impacts with planetesimals. During the giant impact stage, the thickness of a protoplanetary disk is far larger than the size of planetary embryos so collisions are equally likely to come from any direction in three-dimensions. This results in the axial tilt of accreted planets ranging from 0 to 180 degrees with any direction as likely as any other with both prograde and retrograde spins equally probable. Therefore prograde spin with small axial tilt, common for the solar system's terrestrial planets except for Venus, is not common in general for terrestrial planets built by giant impacts. The initial axial tilt of a planet determined by giant impacts can be substantially changed by stellar tides if the planet is close to its star and by satellite tides if the planet has a large satellite.
For most planets the rotation period and axial tilt (also called obliquity) are not known, but a large number of planets have been detected with very short orbits (where tidal effects are greater) and will probably have reached an equilibrium rotation that can be predicted.
Tidal effects are the result of forces acting on a body differing from one part of the body to another. For example the gravitational effect of a star varies with distance from one side of a planet to another. Also heat from a star creates a temperature gradient between the day and nightsides which is another source of tides. For example, on Earth, air pressure variations on the ground are affected more by temperature differences than gravitational ones.
Tides modify the rotation and orbit of planets until an equilibrium is reached. Whenever the rotation rate is slowed, there is an increase of the orbit semi-major axis due to the conservation of angular momentum. Most of the large moons in the Solar System, including the Moon, are tidally locked to their host planet; the same side of the moon is always facing the planet. This means the moons' rotation periods are synchronous with their orbital period. However when an orbit is eccentric, as is the case with many exoplanets' orbits of their host stars, there are equilibrium states such as spin-orbit resonances that are far more likely than synchronous rotation. A spin–orbit resonance is when the rotation period and the orbital period are in an integer ratio - this is called a commensurability. Non-resonant equilibriums such as the retrograde rotation of Venus can also occur when both gravitational and thermal atmospheric tides are both significant.
A synchronous tidal lock isn't necessarily particularly slow - there are planets with orbits that take only a few hours.
Gravitational tides tend to reduce the axial tilt to zero but over a longer time-scale than the rotation rate reaches equilibrium. However, the presence of multiple planets in a system can cause axial tilt to be captured in a resonance called a Cassini state. There are small oscillations around this state and in the case of Mars these axial tilt variations are chaotic.
Hot Jupiters' close proximity to their host star means that their spin-orbit evolution is mostly due to the star's gravity and not the other effects. Hot Jupiters rotation rate is not thought to be captured into spin-orbit resonance due to way fluid-body reacts to tides, and therefore slows down to synchronous rotation if it is on a circular orbit or slows to a non-synchronous rotation if on an eccentric orbit. Hot Jupiters are likely to evolve towards zero axial tilt even if they had been in a Cassini state during planetary migration when they were further from their star. Hot Jupiters' orbits will become more circular over time, however the presence of other planets in the system on eccentric orbits, even ones as small as Earth and as far away as the habitable zone, can continue to maintain the eccentricity of the Hot Jupiter so that the length of time for tidal circularization can be billions instead of millions of years.
The rotation rate of planet HD 80606 b is predicted to be about 1.9 days. HD 80606 b avoids spin-orbit resonance because it is a gas giant. The eccentricity of its orbit means that it avoids becoming tidally locked.
The super-Earth Gliese 581 d would most probably be in a spin-orbit resonance of 2:1, performing two rotations about its axis during each orbit of its parent star. Therefore the day on Gliese 581 d should approximately be 67 Earth’s days long. The second likeliest resonant state for that planet is 3:2.
When a planet is found by the radial-velocity method, its orbital inclination i is unknown and can range from 0 to 90 degrees. The method is unable to determine the true mass (M) of the planet, but rather gives a lower limit for its mass, M sini. In a few cases an apparent exoplanet may be a more massive object such as a brown dwarf or red dwarf. However, the probability of a small value of i (say less than 30 degrees, which would give a true mass at least double the observed lower limit) is relatively low (1−(√3)/2 ≈ 13%) and hence most planets will have true masses fairly close to the observed lower limit.
If a planet's orbit is nearly perpendicular to the line of vision (i.e. i close to 90°), a planet can be detected through the transit method. The inclination will then be known, and the inclination combined with M sini from radial-velocity will give the planet's true mass.
Also, astrometric observations and dynamical considerations in multiple-planet systems can sometimes provide an upper limit to the planet's true mass.
The mass of a transiting exoplanet can also be determined from the transmission spectrum of its atmosphere, as it can be used to constrain independently the atmospheric composition, temperature, pressure, and scale height.
Radius, density and bulk composition
Prior to recent results from the Kepler spacecraft most confirmed planets were gas giants comparable in size to Jupiter or larger because they are most easily detected. However, the planets detected by Kepler are mostly between the size of Neptune and the size of Earth.
If a planet is detectable by both the radial-velocity and the transit methods, then both its true mass and its radius can be found. The planet's density can then be calculated. Planets with low density are inferred to be composed mainly of hydrogen and helium, whereas planets of intermediate density are inferred to have water as a major constituent. A planet of high density is inferred to be rocky, like Earth and the other terrestrial planets of the Solar System.
Gaseous planets that are hot because they are close to their star or because they are still hot from their formation are expanded by the heat. For colder gas planets there is a maximum radius which is slightly larger than Jupiter which occurs when the mass reaches a few Jupiter-masses. Adding mass beyond this point causes the radius to shrink.
Even when taking heating from the star into account, many transiting exoplanets are much larger than expected given their mass, meaning that they have surprisingly low density. See the magnetic field section for one possible explanation.
Super-Earths, mini-Neptunes, and gas dwarfs
If a planet has a radius and/or mass between that of Earth and Neptune then there is a question about whether the planet is rocky like Earth, a mixture of volatiles and gas like Neptune, a small planet with a hydrogen/helium envelope (mini-Jupiter), or some other composition.
Some of the Kepler transiting planets with radii in the range 1-4 Earth-radii have had their masses measured by radial-velocity or transit-timing methods. The calculated densities show that up to 1.5 Earth-radii these planets are rocky and that density increases with increasing radius due to gravitational compression. However between 1.5 and 4 Earth-radii the density decreases with increasing radius. This indicates that above 1.5 Earth-radii planets tend to have increasing amounts of volatiles and gas. Despite this general trend there is a wide range of masses at a given radius which could be because gas planets can have rocky cores of different masses or compositions and could also be due to photoevaporation of volatiles. Thermal evolutionary atmosphere models suggest a radius of 1.75 times that of Earth as a dividing line between rocky and gaseous planets. Excluding close-in planets that have lost their gas envelope due to stellar irradiation, studies of the metallicity of stars suggest a dividing line of 1.7 Earth radii between rocky planets and gas dwarfs; then another dividing line at 3.9 Earth radii between gas dwarfs and gas giants. These dividing lines are statistical trends and do not necessarily apply to specific planets because there are many other factors besides metallicity that affect planet formation including distance from star - there may be larger rocky planets formed at larger distances.
The discovery of the low-density Earth-mass planet KOI-314c shows that there is an overlapping range of masses in which both rocky planets and low-density planets occur. Low-mass low-density planets could be ocean planets or super-Earths with a remnant hydrogen atmosphere, or hot planets with a steam atmosphere, or mini-Neptunes with a hydrogen-helium atmosphere. Other possibilities for low-mass low-density planets are large atmospheres of carbon monoxide, carbon dioxide, methane, or nitrogen.
Massive solid planets
In 2014 new measurements of Kepler-10c found that it was a Neptune mass planet (17 Earth masses) with a density higher than the Earth, indicating that Kepler-10c is made mostly of rock with possibly up to 20% high-pressure water-ice but without a hydrogen-dominated envelope. As it is well above the 10 Earth mass upper limit that is commonly used for the term 'super-Earth', the term mega-Earth has been proposed. A similarly massive and dense planet could be Kepler-131b although its density is not as well measured as Kepler 10c. The next most massive known solid planets are half this mass: 55 Cancri e and Kepler-20b.
Transit-timing variation measurements indicate that Kepler-52b, Kepler-52c and Kepler-57b have maximum-masses between 30 and 100 times the mass of the Earth although the actual masses could be much lower. With radii about 2 Earth radii in size they might have densities larger than an iron planet of the same size. They orbit very close to their stars so they could be the remnant cores (chthonian planets) of evaporated gas giants or brown dwarfs. If cores are massive enough they could remain compressed for billions of years despite losing the atmospheric mass.
Solid planets up to thousands of Earth masses may be able to form around massive stars (B-type and O-type stars; 5–120 solar masses), where the protoplanetary disk would contain enough heavy elements. Also, these stars have high UV radiation and winds that could photoevaporate the gas in the disk leaving just the heavy elements. For comparison, Neptune's mass equals 17 Earth masses, Jupiter has 318 Earth masses, and the 13 Jupiter-mass limit used in the IAU's working definition of an exoplanet equals approximately 4000 Earth masses.
Another way of forming massive solid planets is when a white dwarf in a close binary system loses material to a companion neutron star. The white dwarf can be reduced to planetary-mass, leaving just its crystallised carbon–oxygen core. A likely example of this is PSR J1719-1438 b.
Cold planets have a maximum radius because adding more mass at that point causes the planet to compress under the weight instead of increasing the radius. The maximum radius for solid planets is smaller than the maximum radius for gas planets.
When the size of a planet is described using its radius this is approximating the shape by a sphere. However the rotation of a planet causes it to be flattened at the poles so that the equatorial radius is larger than the polar radius making it closer to an oblate spheroid. The oblateness of transiting exoplanets will affect the transit light curves. At the limits of current technology it has been possible to show that HD 189733b is less oblate than Saturn. If the planet is close to its star, then gravitational tides will elongate the planet in the direction of the star, so that the planet will be closer to a triaxial ellipsoid. Because tidal deformation is along a line between the planet and the star, it is difficult to detect from transit photometry—it will have an order of magnitude less effect on the transit light curves than that caused by rotational deformation even in cases where the tidal deformation is larger than rotational deformation (such as is the case for tidally locked hot Jupiters). Material rigidity of rocky planets and rocky cores of gas planets will cause further deviations from the aforementioned shapes. Thermal tides caused by unevenly irradiated surfaces are another factor.
As of February 2014, more than fifty transiting and five directly imaged exoplanet atmospheres have been observed, resulting in detection of molecular spectral features; observation of day–night temperature gradients; and constraints on vertical atmospheric structure. Also, an atmosphere has been detected on the non-transiting hot Jupiter Tau Boötis b.
The atmospheric circulation of planets that rotate more slowly or have a thicker atmosphere allows more heat to flow to the poles which reduces the temperature differences between the poles and the equator.
Precipitation in the form of liquid (rain) or solid (snow) varies in composition depending on atmospheric temperature, pressure, composition, and altitude. Hot atmospheres could have iron rain, molten-glass rain, and rain made from rocky minerals such as enstatite, corundum, spinel, and wollastonite. Deep in the atmospheres of gas giants it could rain diamonds and helium containing dissolved neon.
The processes of life result in a mixture of chemicals that are not in chemical equilibrium but there are also abiotic disequilibrium processes that need to be considered. The most robust atmospheric biosignature is often considered to be molecular oxygen O2 and its photochemical byproduct ozone O3. The photolysis of water H2O by UV rays followed by hydrodynamic escape of hydrogen can lead to a build-up of oxygen in planets close to their star undergoing runaway greenhouse effect. For planets in the habitable zone it was believed that water photolysis would be strongly limited by cold-trapping of water vapour in the lower atmosphere. However the extent of H2O cold-trapping depends strongly on the amount of non-condensible gases in the atmosphere such as nitrogen N2 and argon. In the absence of such gases the likelihood of build-up of oxygen also depends in complex ways on the planet’s accretion history, internal chemistry, atmospheric dynamics and orbital state. Therefore, oxygen on its own cannot be considered a robust biosignature. The ratio of nitrogen and argon to oxygen could be detected by studying thermal phase curves or by transit transmission spectroscopy measurement of the spectral Rayleigh scattering slope in a clear-sky (i.e. aerosol-free) atmosphere.
Climate and weather
- Patterns of Sunlight on Extra-Solar Planets, Tony Dobrovolskis, March 18, 2014
- Possible climates on terrestrial exoplanets, Francois Forget, Jeremy Leconte, 18 Nov 2013
- Indication of insensitivity of planetary weathering behavior and habitable zone to surface land fraction, Dorian S. Abbot, Nicolas B. Cowan, Fred J. Ciesla, 8 Aug 2012
- Clouds and Hazes in Exoplanet Atmospheres, Mark S. Marley, Andrew S. Ackerman, Jeffrey N. Cuzzi, Daniel Kitzmann, 23 Jan 2013
- Atmospheric Circulation of Exoplanets, Adam P. Showman, James Y-K. Cho, Kristen Menou, 16 Nov 2009
- New Technique Could Measure Exoplanet Atmospheric Pressure, an Indicator of Habitability, Shannon Hall on March 6, 2014, www.universetoday.com
Surface features can be distinguished from atmospheric features by comparing emission and reflection spectroscopy with transmission spectroscopy. Mid-infrared spectroscopy of exoplanets may detect rocky surfaces, and near-infrared may identify magma oceans or high-temperature lavas, hydrated silicate surfaces and water ice, giving an unambiguous method to distinguish between rocky and gaseous exoplanets.
One can estimate the temperature of an exoplanet based on the intensity of the light it receives from its parent star. For example, the planet OGLE-2005-BLG-390Lb is estimated to have a surface temperature of roughly −220 °C (50 K). However, such estimates may be substantially in error because they depend on the planet's usually unknown albedo, and because factors such as the greenhouse effect may introduce unknown complications. A few planets have had their temperature measured by observing the variation in infrared radiation as the planet moves around in its orbit and is eclipsed by its parent star. For example, the planet HD 189733b has been found to have an average temperature of 1205±9 K (932±9 °C) on its dayside and 973±33 K (700±33 °C) on its nightside.
- Global Mapping of Earth-like Exoplanets from Scattered Light Curves, Hajime Kawahara, Yuka Fujii, 16 Jul 2010
- A Two-Dimensional Infrared Map of the Extrasolar Planet HD 189733b, C. Majeau, E. Agol, N. Cowan, 19 Sep 2012
- Water: from clouds to planets, Ewine F. van Dishoeck, Edwin A. Bergin, Dariusz C. Lis, Jonathan I. Lunine, 25 Feb 2014
- Are Exoplanets Orbiting Red Dwarf Stars too Dry for Life?, Michael Schirber, Astrobiology Magazine, August 27, 2013
- Carbon-Rich Exoplanets May Lack Surface Water, October 26, 2013
- 'Water-Trapped' Worlds, Adam Hadhazy, Astrobiology Magazine, 07/18/13
- Lobster-Shaped Extrasolar Oceans, 03/10/14, Charles Q. Choi, Astrobiology Magazine
- Alien Moons Could Bake Dry from Young Gas Giants' Hot Glow, Adam Hadhazy, Astrobiology Magazine March 25, 2014
- The Longevity of Oceans on Terrestrial Exoplanets, Bullock, Mark Alan; Grinspoon, D. H.
- False Positive For Ocean Glint on Exoplanets: the Latitude-Albedo Effect, Nicolas B. Cowan, Dorian S. Abbot, Aiko Voigt, 4 May 2012
Color and brightness
The darkest planet discovered is TrES-2b, a hot Jupiter, which reflects less than 1% of the light from its star making it darker than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark - it could be due to an unknown chemical. TrES-2b does emit a faint red glow because it is so hot.
Interaction between a close-in planet's magnetic field and a star can produce spots on the star in a similar way to how the Galilean moons produce aurorae on Jupiter. Auroral radio emissions could be detected with radio telescopes such as LOFAR. The radio emissions could enable determination of the rotation rate of a planet which is difficult to detect otherwise.
Earth's magnetic field results from its flowing liquid metallic core, but in super-Earths the mass can produce high pressures with large viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Magnesium oxide, which is rocky on Earth, can be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.
Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.
On Earth-sized planets, plate tectonics is more likely if there are oceans of water; however, in 2007 two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-earths with one team saying that plate tectonics would be episodic or stagnant and the other team saying that plate tectonics is very likely on super-earths even if the planet is dry.
If super-earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.
The star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.
The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 to 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.
The rings of the Solar System's gas giants are aligned with their planet's equator. However for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.
KIC 12557548 b is a small rocky planet, very close to it star, that is evaporating and leaving a trailing tail of cloud and dust like a comet. The dust could be ash erupting from volcanoes and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.
- Detecting Volcanism on Extrasolar Planets, L. Kaltenegger, W. G. Henning, D. D. Sasselov, 7 Sep 2010
- Detecting planetary geochemical cycles on exoplanets: Atmospheric signatures and the case of SO2, L. Kaltenegger, D. Sasselov, 17 Nov 2009
- Geodynamics and Rate of Volcanism on Massive Earth-like Planets, Edwin S. Kite, Michael Manga, Eric Gaidos, 31 May 2009
- Tidal Heating of Terrestrial Extra-Solar Planets and Implications for their Habitability, Brian Jackson, Rory Barnes, Richard Greenberg, 20 Aug 2008
- Planetary internal structures, I. Baraffe, G. Chabrier, J. Fortney, C. Sotin, 19 Jan 2014
The habitable zone around a star is the region where the temperature is just right to allow liquid water to exist on a planet; that is, not too close to the star for the water to evaporate and not too far away from the star for the water to freeze. The heat produced by stars varies depending on the size and age of the star so that the habitable zone can be at different distances. Also, the atmospheric conditions on the planet influence the planet's ability to retain heat so that the location of the habitable zone is also specific to each type of planet: desert planets (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect, meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out. Rocky planets with a thick hydrogen atmosphere could maintain surface water much further out than the Earth–Sun distance. Habitable zones have usually been defined in terms of surface temperature, however over half of Earth's biomass is from subsurface microbes, and the temperature increases as you go deeper underground, so the subsurface can be habitable when the surface is frozen and if this is considered then the habitable zone extends much further from the star, even rogue planets (those without a star) could have liquid water at sufficient depths underground. In an earlier era of the universe the temperature of the cosmic microwave background would allow any rocky planets that existed to have liquid water on their surface regardless of their distance from a star. Jupiter-like planets might not be habitable, but they could have habitable moons.
- Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate, Jun Yang, Gwenael Boue, Daniel C. Fabrycky, Dorian S. Abbot, 19 Apr 2014
- Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass, Ravi kumar Kopparapu, Ramses M. Ramirez, James SchottelKotte, James F. Kasting, Shawn Domagal-Goldman, Vincent Eymet, 21 Apr 2014
Ice ages and snowball states
The outer edge of the habitable zone is where planets will be completely frozen but even planets well inside the HZ can periodically become frozen. If orbital fluctuations or other causes produce cooling then this creates more ice but ice reflects sunlight causing even more cooling creating a feedback loop until the planet is completely or nearly completely frozen. When the surface is frozen this stops carbon dioxide weathering resulting in a build-up of carbon dioxide in the atmosphere from volcanic emissions. This creates a greenhouse effect which unfreezes the planet again. Planets with a large axial tilt are less likely to enter snowball states and can retain liquid water further from their star. Large fluctuations of axial tilt can have even more of a warming effect than a fixed large tilt. Paradoxically planets around cooler stars, such as red dwarfs, are less likely to enter snowball states because the infrared radiation emitted by cooler stars is mostly at wavelengths that are absorbed by ice which heats it up.
If a planet has an eccentric orbit then tidal heating can provide another source of energy besides stellar irradiation. This means that eccentric planets in the radiative habitable zone can be too hot for liquid water (Tidal Venus). Tides also circularize orbits over time so there could be planets in the habitable zone with circular orbits that have no water because they used to have eccentric orbits. Eccentric planets further out than the radiative habitable zone would still have frozen surfaces but the tidal heating could create a subsurface ocean similar to Europa's. In some planetary systems, such as in the Upsilon Andromedae system, the eccentricity of orbits is maintained or even periodically varied by perturbations from other planets in the system. Tidal heating can cause outgassing from the mantle, contributing to the formation and replenishment of an atmosphere.
Potentially habitable planets
Confirmed planet discoveries in the habitable zone include the Kepler-22b, the first super-Earth located in the habitable zone of a Sun-like star. In September 2012, the discovery of two planets orbiting the red dwarf Gliese 163 was announced. One of the planets, Gliese 163 c, about 6.9 times the mass of Earth and somewhat hotter, was considered to be within the habitable zone. In 2013, three more potentially habitable planets, Kepler-62 e, Kepler-62 f, and Kepler-69 c, orbiting Kepler-62 and Kepler-69 respectively, were discovered. All three planets were super-Earths and may be covered by oceans thousands of kilometers deep. In June 2013, a dynamically packed planetary system around the nearby red dwarf Gliese 667C was announced. The system was found to contain at least three super-Earths in its habitable zone (Gliese 667 Cc, Gliese 667 Ce and Gliese 667 Cf), establishing the new record in the number of potentially habitable worlds around a single star. The system contains two other planet candidates (Gliese 667 Cd and Gliese 667 Ch) which would lie in the cold/hot edges of the star's habitable zone. This later result highlights the prevalence of low mass stars as hosts of potentially habitable worlds.
In November 2013 it was announced that 22±8% of Sun-like[a] stars have an Earth-sized[b] planet in the habitable[c] zone. Assuming 200 billion stars in the Milky Way,[d] that would be 11 billion potentially habitable Earths, rising to 40 billion if red dwarf stars are included.
In February 2013, researchers calculated that up to 6% of small red dwarfs may have planets with Earth-like properties. This suggests that the closest "alien Earth" to the Solar System could be 13 light-years away. The estimated distance increases to 21 light-years when a 95 percent confidence interval is used. In March 2013 a revised estimate based on a more accurate consideration of the size of the habitable zone around red dwarfs gave an occurrence rate of 50% for Earth-size planets in the HZ of red dwarfs.
Planetary systems can be categorized according to their orbital dynamics as resonant, non-resonant-interacting, hierarchical, or some combination of these. In resonant systems the orbital periods of the planets are in integer ratios. The Kepler-223 system contains four planets in an 8:6:4:3 orbital resonance. In interacting systems the planets orbits are close enough together that they perturb the orbital parameters. The Solar System could be described as weakly interacting. In strongly interacting systems Kepler's laws do not hold. In hierarchical systems the planets are arranged so that the system can be gravitationally considered as a nested system of two-bodies, e.g. in a star with a close-in hot jupiter with another gas giant much further out, the star and hot jupiter form a pair that appears as a single object to another planet that is far enough out. A system can contain bodies of different dynamical types, e.g. the Galilean moons of Jupiter where Io, Europa, and Ganymede are in resonance but Callisto is too distant to be part of this resonance.
Other, as yet unobserved, orbital possibilities include:
- double planets
- various co-orbital planets such as quasi-satellites, trojans and exchange orbits
- interlocking orbits maintained by precessing orbital planes
Number of planets in a system and their relative masses, radii, orbital spacings and parameters
- On The Relative Sizes of Planets Within Kepler Multiple Candidate Systems, David R. Ciardi et al. 9 Dec 2012
- Architecture of Planetary Systems Based on Kepler Data: Number of Planets and Coplanarity, Julia Fang, Jean-Luc Margot, 30 Oct 2012
- Exoplanet Predictions Based on the Generalised Titius-Bode Relation, Timothy Bovaird, Charles H. Lineweaver, 1 Aug 2013
- The Solar System and the Exoplanet Orbital Eccentricity - Multiplicity Relation, Mary Anne Limbach, Edwin L. Turner, 9 Apr 2014
- On the Relationship Between Debris Disks and Planets, Ágnes Kóspál, David R. Ardila, Attila Moór, Péter Ábrahám, 30 Jun 2009
- Signatures of exosolar planets in dust debris disks, Leonid M. Ozernoy, Nick N. Gorkavyi, John C. Mather, Tanya Taidakova, 4 Jul 2000
Second- and third-generation planets
- Planets in evolved binary systems, Hagai B. Perets, 13 Jan 2011
- Second generation planet formation in NN Serpentis?, M. Völschow, R. Banerjee, F.V. Hessman, 29 Dec 2013
Engulfment by red giants, asymptotic-giant-branch stars and planetary nebulae
- Can Planets survive Stellar Evolution?, Eva Villaver, Mario Livio, Feb 2007
- The Orbital Evolution of Gas Giant Planets around Giant Stars, Eva Villaver, Mario Livio, 13 Oct 2009
- On the survival of brown dwarfs and planets engulfed by their giant host star, Jean-Claude Passy, Mordecai-Mark Mac Low, Orsola De Marco, 2 Oct 2012
- Foretellings of Ragnarök: World-engulfing Asymptotic Giants and the Inheritance of White Dwarfs, Alexander James Mustill, Eva Villaver, 5 Dec 2012
On May 9, 2013, a congressional hearing by two United States House of Representatives subcommittees discussed "Exoplanet Discoveries: Have We Found Other Earths?", prompted by the discovery of exoplanet Kepler-62f, along with Kepler-62e and Kepler-62c. A related special issue of the journal Science, published earlier, described the discovery of the exoplanets.
- Extragalactic planet
- List of exoplanet research projects
- Lists of extrasolar planets
- For the purpose of this 1 in 5 statistic, "Sun-like" means G-type star. Data for Sun-like stars wasn't available so this statistic is an extrapolation from data about K-type stars
- For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii
- For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).
- About 1/4 of stars are GK sun-like stars. The number of stars in the galaxy is not accurately known but assuming 200 billion stars in total, the Milky Way would have about 50 billion sun-like (GK) stars, of which about 1 in 5 (22%) or 11 billion would be earth-sized in the habitable zone. Including red dwarf stars would increase this to 40 billion.
- Metallicity of Kapteyn's star estimated at [Fe/H]= -0.89. 10-0.89 ≈ 1/8
- Dorminey, Bruce (2001) Distant Wanderers Springer-Verlag ISBN 978-0-387-95074-7 (Hardback) ISBN 978-1-4419-2872-6 (Paperback)
- Villard, Ray & Cook, Lynette R (2005) Infinite Worlds: An Illustrated Voyage to Planets Beyond Our Sun University of California Press ISBN 978-0-520-23710-0
- Boss, Alan (2009) The Crowded Universe: The Search for Living Planets Basic Books ISBN 978-0-465-00936-7 (Hardback) ISBN 978-0-465-02039-3 (Paperback)
- Seager, Sara (2010) Exoplanet Atmospheres: Physical Processes Princeton University Press ISBN 978-0-691-11914-4 (Hardback) ISBN 978-0-691-14645-4 (Paperback)
- Seager, Sara (Editor) (2011) Exoplanets University of Arizona Press ISBN 978-0-8165-2945-2
- Perryman, Michael (2011) The Exoplanet Handbook Cambridge University Press ISBN 978-0-521-76559-6
- Yaqoob, Tahir (2011) "Exoplanets and Alien Solar Systems" New Earth Labs (Education and Outreach) ISBN 978-0-974-16892-0 (Paperback)
- Claven, W. (3 January 2013). "Billions and Billions of Planets". NASA. Retrieved 3 January 2013.
- Staff (2 January 2013). "100 Billion Alien Planets Fill Our Milky Way Galaxy: Study". Space.com. Retrieved 3 January 2013.
- Cassan, A.; et al. (2012). "One or more bound planets per Milky Way star from microlensing observations". Nature 481 (7380): 167–169. arXiv:1202.0903. Bibcode:2012Natur.481..167C. doi:10.1038/nature10684. PMID 22237108.
- "Planet Population is Plentiful". ESO. 11 January 2012. Retrieved 13 January 2012.
- Schneider, J. "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopedia.
- Beichman, C.; Gelino, Christopher R.; Kirkpatrick, J. Davy; Cushing, Michael C.; Dodson-Robinson, Sally et al. (2014). "WISE Y Dwarfs As Probes of the Brown Dwarf-Exoplanet Connection". The Astrophysical Journal 783 (2): 68. arXiv:1401.1194v2. Bibcode:2014ApJ...783...68B. doi:10.1088/0004-637X/783/2/68.
- "NASA - Kepler". Retrieved 4 November 2013.
- Harrington, J. D.; Johnson, M. (4 November 2013). "NASA Kepler Results Usher in a New Era of Astronomy".
- Tenenbaum, P.; et al. (2012). "Detection of Potential Transit Signals in the First Twelve Quarters of Kepler Mission Data". arXiv:1212.2915 [astro-ph.EP].
- "My God, it's full of planets! They should have sent a poet." (Press release). Planetary Habitability Laboratory, University of Puerto Rico at Arecibo. 3 January 2012. Retrieved 4 January 2013.
- Santerne, A.; Díaz, R. F.; Almenara, J.-M.; Lethuillier, A.; Deleuil, M.; Moutou, C. (2013). "Astrophysical false positives in exoplanet transit surveys: Why do we need bright stars?". arXiv:1310.2133 [astro-ph.EP].
- Sanders, R. (4 November 2013). "Astronomers answer key question: How common are habitable planets?". newscenter.berkeley.edu.
- Petigura, E. A.; Howard, A. W.; Marcy, G. W. (2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences 110 (48): 19273. arXiv:1311.6806. Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110.
- Khan, Amina (4 November 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times. Retrieved 5 November 2013.
- Strigari, L. E.; Barnabè, M.; Marshall, P. J.; Blandford, R. D. (2012). "Nomads of the Galaxy". Monthly Notices of the Royal Astronomical Society 423 (2): 1856–1865. arXiv:1201.2687. Bibcode:2012MNRAS.423.1856S. doi:10.1111/j.1365-2966.2012.21009.x. estimates 700 objects >10−6 solar masses (roughly the mass of Mars) per main-sequence star between 0.08 and 1 Solar mass, of which there are billions in the Milky Way.
- Johnson, Michele; Harrington, J.D. (26 February 2014). "NASA's Kepler Mission Announces a Planet Bonanza, 715 New Worlds". NASA. Retrieved 26 February 2014.
- Wall, Mike. "Population of Known Alien Planets Nearly Doubles as NASA Discovers 715 New Worlds". Retrieved 26 February 2014.
- "Kepler telescope bags huge haul of planets". Retrieved 27 February 2014.
- Staff. "DENIS-P J082303.1-491201 b". Caltech. Retrieved 8 March 2014.
- Sahlmann, J.; Lazorenko, P. F.; Ségransan, D.; Martín, E. L.; Queloz, D.; Mayor, M.; Udry, S. (August 2013). "Astrometric orbit of a low-mass companion to an ultracool dwarf". Harvard University. arXiv:1306.3225. Retrieved 8 March 2014.
- Wolszczan, A.; Frail, D. A. (1992). "A planetary system around the millisecond pulsar PSR1257 + 12". Nature 355 (6356): 145. doi:10.1038/355145a0.
- "IAU 2006 General Assembly: Result of the IAU Resolution votes". 2006. Retrieved 2010-04-25.
- R. R. Brit (2006). "Why Planets Will Never Be Defined". Space.com. Retrieved 2008-02-13.
- "Working Group on Extrasolar Planets: Definition of a "Planet"". IAU position statement. 28 February 2003. Retrieved 2006-09-09.[dead link]
- Mordasini, C. et al. (2007). "Giant Planet Formation by Core Accretion". arXiv:0710.5667v1 [astro-ph].
- Baraffe, I.; Chabrier, G.; Barman, T. (2008). "Structure and evolution of super-Earth to super-Jupiter exoplanets. I. Heavy element enrichment in the interior". Astronomy and Astrophysics 482 (1): 315–332. arXiv:0802.1810. Bibcode:2008A&A...482..315B. doi:10.1051/0004-6361:20079321.
- Bouchy, F.; et al. (2009). "The SOPHIE search for northern extrasolar planets. I. A companion around HD 16760 with mass close to the planet–brown-dwarf transition". Astronomy and Astrophysics 505 (2): 853–858. arXiv:0907.3559. Bibcode:2009A&A...505..853B. doi:10.1051/0004-6361/200912427.
- Boss, Alan P.; Basri, Gibor; Kumar, Shiv S.; Liebert, James; Martín, Eduardo L.; Reipurth, Bo; Zinnecker, Hans (2003), "Nomenclature: Brown Dwarfs, Gas Giant Planets, and ?", Brown Dwarfs 211: 529, Bibcode:2003IAUS..211..529B
- An Analysis of the SEEDS High-Contrast Exoplanet Survey: Massive Planets or Low-Mass Brown Dwarfs?, Timothy D. Brandt, Michael W. McElwain, Edwin L. Turner, Kyle Mede, David S. Spiegel, Masayuki Kuzuhara, Joshua E. Schlieder, John P. Wisniewski, L. Abe, W. Brandner, J. Carson, T. Currie, S. Egner, M. Feldt, T. Golota, M. Goto, C. A. Grady, O. Guyon, J. Hashimoto, Y. Hayano, M. Hayashi, S. Hayashi, T. Henning, K. W. Hodapp, S. Inutsuka, M. Ishii, M. Iye, M. Janson, R. Kandori, G. R. Knapp, T. Kudo, N. Kusakabe, J. Kwon, T. Matsuo, S. Miyama, J.-I. Morino, A. Moro-Martín, T. Nishimura, T.-S. Pyo, E. Serabyn, H. Suto, R. Suzuki, M. Takami, N. Takato, H. Terada, C. Thalmann, D. Tomono, M. Watanabe, T. Yamada, H. Takami, T. Usuda, M. Tamura, (Submitted on 21 Apr 2014)
- Spiegel; Adam Burrows; Milsom (2010). "The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets". arXiv:1008.5150 [astro-ph.EP].
- Schneider, J.; Dedieu, C.; Le Sidaner, P.; Savalle, R.; Zolotukhin, I. (2011). "Defining and cataloging exoplanets: The exoplanet.eu database". Astronomy & Astrophysics 532 (79): A79. arXiv:1106.0586. Bibcode:2011A&A...532A..79S. doi:10.1051/0004-6361/201116713.
- Wright, J. T.; et al. (2010). "The Exoplanet Orbit Database". arXiv:1012.5676 [astro-ph.SR].
- Exoplanet Criteria for Inclusion in the Archive, NASA Exoplanet Archive
- "Planetesimals To Brown Dwarfs: What is a Planet?". Ann. Rev. Earth Planet. Sci. 34: 193–216. 2006. arXiv:astro-ph/0608417. Bibcode:2006AREPS..34..193B. doi:10.1146/annurev.earth.34.031405.125058.
- Boss, Alan P.; Basri, Gibor; Kumar, Shiv S.; Liebert, James; Martín, Eduardo L.; Reipurth, Bo; Zinnecker, Hans (2003). "Nomenclature: Brown Dwarfs, Gas Giant Planets, and ?". Brown Dwarfs 211: 529. Bibcode:2003IAUS..211..529B.
- Giordano Bruno. On the Infinite Universe and Worlds (1584).
- Newton, Isaac; I. Bernard Cohen and Anne Whitman (1999 ). The Principia: A New Translation and Guide. University of California Press. p. 940. ISBN 0-520-20217-1.
- W. S. Jacob (1855). "On Certain Anomalies presented by the Binary Star 70 Ophiuchi". Monthly Notices of the Royal Astronomical Society 15: 228. Bibcode:1855MNRAS..15..228J.
- T. J. J. See (1896). "Researches on the Orbit of F.70 Ophiuchi, and on a Periodic Perturbation in the Motion of the System Arising from the Action of an Unseen Body". Astronomical Journal 16: 17. Bibcode:1896AJ.....16...17S. doi:10.1086/102368.
- T. J. Sherrill (1999). "A Career of Controversy: The Anomaly of T. J. J. See". Journal for the History of Astronomy 30 (98): 25–50. Bibcode:1999JHA....30...25S.
- P. van de Kamp (1969). "Alternate dynamical analysis of Barnard's star". Astronomical Journal 74: 757–759. Bibcode:1969AJ.....74..757V. doi:10.1086/110852.
- Boss, Alan (2009). The Crowded Universe: The Search for Living Planets. Basic Books. pp. 31–32. ISBN 978-0-465-00936-7.
- M. Bailes, A. G. Lyne, S. L. Shemar (1991). "A planet orbiting the neutron star PSR1829-10". Nature 352 (6333): 311–313. Bibcode:1991Natur.352..311B. doi:10.1038/352311a0.
- A. G. Lyne, M. Bailes (1992). "No planet orbiting PS R1829-10". Nature 355 (6357): 213. Bibcode:1992Natur.355..213L. doi:10.1038/355213b0.
- Campbell, B.; Walker, G. A. H.; Yang, S. (1988). "A search for substellar companions to solar-type stars". The Astrophysical Journal 331: 902. doi:10.1086/166608.
- Lawton, A. T.; Wright, P. (1989). "A planetary system for Gamma Cephei?". Journal of the British Interplanetary Society 42: 335–336. Bibcode:1989JBIS...42..335L.
- Walker, G. A. H; Bohlender, D. A.; Walker, A. R.; Irwin, A. W.; Yang, S. L. S.; Larson, A. (1992). "Gamma Cephei – Rotation or planetary companion?". Astrophysical Journal Letters 396 (2): L91–L94. Bibcode:1992ApJ...396L..91W. doi:10.1086/186524.
- Hatzes, A. P.; et al. (2003). "A Planetary Companion to Gamma Cephei A". Astrophysical Journal 599 (2): 1383–1394. arXiv:astro-ph/0305110. Bibcode:2003ApJ...599.1383H. doi:10.1086/379281.
- Holtz, Robert (22 April 1994). "Scientists Uncover Evidence of New Planets Orbiting Star". Los Angeles Times via The Tech Online.
- M. Mayor, D. Queloz (1995). "A Jupiter-mass companion to a solar-type star". Nature 378 (6555): 355–359. Bibcode:1995Natur.378..355M. doi:10.1038/378355a0.
- Jack J. Lissauer (1999). "Three Planets for Upsilon Andromedae". Nature 398 (659): 659. Bibcode:1999Natur.398..659L. doi:10.1038/19409.
- Doyle, L. R.; et al. (16 September 2011). "Kepler-16: A Transiting Circumbinary Planet". Science 333 (6049): 1602–6. arXiv:1109.3432. Bibcode:2011Sci...333.1602D. doi:10.1126/science.1210923. PMID 21921192.
- A Family Portrait of the Alpha Centauri Star System
- Whoa! Earth-size planet in Alpha Centauri system
- "NASA's Exoplanet Archive KOI table". NASA. Retrieved 28 February 2014.
- Perryman, Michael (2011). The Exoplanet Handbook. Cambridge University Press. p. 149. ISBN 978-0-521-76559-6.
- Discovery of H-alpha Emission from the Close Companion Inside the Gap of Transitional Disk HD142527, L.M. Close, K.B. Follette, J.R. Males, A. Puglisi, M. Xompero, D. Apai, J. Najita, A.J. Weinberger, K. Morzinski, T.J. Rodigas, P. Hinz, V. Bailey, R. Briguglio, (Submitted on 7 Jan 2014)
- F. Pepe, C. Lovis, D. Segransan et al. (2011). "The HARPS search for Earth-like planets in the habitable zone". Astronomy & Astrophysics 534: A58. arXiv:1108.3447. Bibcode:2011A&A...534A..58P. doi:10.1051/0004-6361/201117055.
- Rodler, F.; Lopez-Morales, M.; Ribas, I. (2012). "Weighing the Non-Transiting Hot Jupiter Tau BOO b". The Astrophysical Journal Letters 753 (25): L25. arXiv:1206.6197. Bibcode:2012ApJ...753L..25R. doi:10.1088/2041-8205/753/1/L25.
- Planet Hunting: Finding Earth-like Planets, www.scientificcomputing.com, Mon, 07/19/2010 - 11:58am
- Ballard, S.; et al. (2011). "The Kepler-19 System: A Transiting 2.2 R_Earth Planet and a Second Planet Detected via Transit Timing Variations". arXiv:1109.1561 [astro-ph.EP].
- Jack J. Lissauer, Daniel C. Fabrycky, Eric B. Ford, et al. (2011). "A closely packed system of low-mass, low-density planets transiting Kepler-11". Nature 470 (7332): 53. arXiv:1102.0291. Bibcode:2011Natur.470...53L. doi:10.1038/nature09760.
- Pál, A.; Kocsis, B. (2008). "Periastron Precession Measurements in Transiting Extrasolar Planetary Systems at the Level of General Relativity". Monthly Notices of the Royal Astronomical Society 389: 191–198. arXiv:0806.0629. Bibcode:2008MNRAS.389..191P. doi:10.1111/j.1365-2966.2008.13512.x.
- Silvotti, R.; et al. (2007). "A giant planet orbiting the 'extreme horizontal branch' star V391 Pegasi". Nature 449 (7159): 189–91. Bibcode:2007Natur.449..189S. doi:10.1038/nature06143. PMID 17851517.
- Jenkins, J.M.; Laurance R. Doyle (2003-09-20). "Detecting reflected light from close-in giant planets using space-based photometers" (PDF). Astrophysical Journal 1 (595): 429–445. arXiv:astro-ph/0305473. Bibcode:2003ApJ...595..429J. doi:10.1086/377165.
- Loeb, A.; Gaudi, B. S. (2003). "Periodic Flux Variability of Stars due to the Reflex Doppler Effect Induced by Planetary Companions". The Astrophysical Journal Letters 588 (2): L117. arXiv:astro-ph/0303212. Bibcode:2003ApJ...588L.117L. doi:10.1086/375551.
- Using the Theory of Relativity and BEER to Find Exoplanets, www.universetoday.com, by Nancy Atkinson on May 13, 2013
- Schmid, H. M.; Beuzit, J.-L.; Feldt, M. et al. (2006). "Search and investigation of extra-solar planets with polarimetry". Direct Imaging of Exoplanets: Science & Techniques. Proceedings of the IAU Colloquium #200 1 (C200): 165–170. Bibcode:2006dies.conf..165S. doi:10.1017/S1743921306009252.
- Berdyugina, Svetlana V.; Andrei V. Berdyugin; Dominique M. Fluri; Vilppu Piirola (20 January 2008). "First detection of polarized scattered light from an exoplanetary atmosphere". The Astrophysical Journal 673: L83. arXiv:0712.0193. Bibcode:2008ApJ...673L..83B. doi:10.1086/527320.
- Gaia Science Homepage
- Staff (19 November 2012). "Announcement of Opportunity for the Gaia Data Processing Archive Access Co-Ordination Unit". ESA. Retrieved 17 March 2013.
- Staff (30 January 2012). "DPAC Newsletter no. 15" (PDF). European Space Agency. Retrieved 16 March 2013.
- Space eye with 34 telescopes will investigate one million stars
- Spectroscopic Coronagraphy for Planetary Radial Velocimetry of Exoplanets, Hajime Kawahara, Naoshi Murakami, Taro Matsuo, Takayuki Kotani, 23 Apr 2014
- Characterizing Extra-Solar Planets with Color Differential Astrometry on SPICA, L. Abe1, M. Vannier1, R. Petrov1, K. Enya2 and H. Kataza2, SPICA Workshop 2009
- Confirmation of an exoplanet using the transit color signature: Kepler-418b, a blended giant planet in a multiplanet system, B. Tingley, H. Parviainen, D. Gandolfi, H. J. Deeg, E. Pallé, P. Montañés Rodriguez, F. Murgas, R. Alonso, H. Bruntt, M. Fridlund, (Submitted on 21 May 2014 (v1), last revised 22 May 2014 (this version, v2))
- Doppler tomographic observations of exoplanetary transits, Johnson, Marshall Caleb, 2013
- Stromberg, Joseph (10 July 2014). "We've found hundreds of new planets. And now they're going to get cool names". Vox. Retrieved 10 July 2014.
- Hessman, F. V.; et al. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
- William I. Hartkopf & Brian D. Mason. "Addressing confusion in double star nomenclature: The Washington Multiplicity Catalog". United States Naval Observatory. Retrieved 2008-09-12.
- Jean Schneider (2011). "Notes for star 55 Cnc". Extrasolar Planets Encyclopaedia. Retrieved 26 September 2011.
- Jean Schneider (2011). "Notes for Planet 16 Cyg B b". Extrasolar Planets Encyclopaedia. Retrieved 26 September 2011.
- Jean Schneider (2011). "Notes for Planet HD 178911 B b". Extrasolar Planets Encyclopaedia. Retrieved 26 September 2011.
- Jean Schneider (2011). "Notes for Planet HD 41004 A b". Extrasolar Planets Encyclopaedia. Retrieved 26 September 2011.
- Jean Schneider (2011). "Notes for Planet Tau Boo b". Extrasolar Planets Encyclopaedia. Retrieved 26 September 2011.
- Doyle, L. R.; et al. (2011). "Kepler-16: A Transiting Circumbinary Planet". Science 333 (6049): 1602–6. arXiv:1109.3432. Bibcode:2011Sci...333.1602D. doi:10.1126/science.1210923. PMID 21921192.
- Marcy, G.; et al. (2005). "Observed Properties of Exoplanets: Masses, Orbits and Metallicities". Progress of Theoretical Physics Supplement 158: 24–42. arXiv:astro-ph/0505003. Bibcode:2005PThPS.158...24M. doi:10.1143/PTPS.158.24.
- The Frequency of Hot Jupiters Orbiting Nearby Solar-Type Stars, J. T. Wright, G. W. Marcy, A. W. Howard, John Asher Johnson, T. Morton, D. A. Fischer, (Submitted on 10 May 2012)
- Andrew Cumming; R. Paul Butler; Geoffrey W. Marcy et al. et al. (2008). "The Keck Planet Search: Detectability and the Minimum Mass and Orbital Period Distribution of Extrasolar Planets". Publications of the Astronomical Society of the Pacific 120 (867): 531–554. arXiv:0803.3357. Bibcode:2008PASP..120..531C. doi:10.1086/588487.
- Planet Occurrence within 0.25 AU of Solar-type Stars from Kepler, Andrew W. Howard et al. (Submitted on 13 Mar 2011)
- Amos, Jonathan (19 October 2009). "Scientists announce planet bounty". BBC News. Retrieved 2010-03-31.
- David P. Bennett, Jay Anderson, Ian A. Bond, Andrzej Udalski, Andrew Gould (2006). "Identification of the OGLE-2003-BLG-235/MOA-2003-BLG-53 Planetary Host Star". Astrophysical Journal Letters 647 (2): L171–L174. arXiv:astro-ph/0606038. Bibcode:2006ApJ...647L.171B. doi:10.1086/507585.
- Bonfils, X.; et al. (2005). "The HARPS search for southern extra-solar planets: VI. A Neptune-mass planet around the nearby M dwarf Gl 581". Astronomy & Astrophysics 443 (3): L15–L18. arXiv:astro-ph/0509211. Bibcode:2005A&A...443L..15B. doi:10.1051/0004-6361:200500193.
- L. Vu (3 October 2006). "Planets Prefer Safe Neighborhoods". Spitzer Science Center. Archived from the original on 13 July 2007. Retrieved 2007-09-01.
- Limits on Planets Orbiting Massive Stars from Radio Pulsar Timing, Thorsett, S.E. Dewey, R.J. 16-Sep-1993
- J. A. Johnson (2011). "The Stars that Host Planets". Sky & Telescope (April): 22–27.
- A stellar-mass-dependent drop in planet occurrence rates, Gijs D. Mulders, Ilaria Pascucci, Daniel Apai, (Submitted on 28 Jun 2014)
- Buchhave, L. A.; et al. (2012). "An abundance of small exoplanets around stars with a wide range of metallicities". Nature. Bibcode:2012Natur.486..375B. doi:10.1038/nature11121.
- Israelian, G.; et al. (2009). "Enhanced lithium depletion in Sun-like stars with orbiting planets". Nature 462 (7270): 189–191. arXiv:0911.4198. Bibcode:2009Natur.462..189I. doi:10.1038/nature08483. PMID 19907489.
- BINARY CATALOGUE OF EXOPLANETS, Maintained by Richard Schwarz], retrieved 28 Sept 2013
- Welsh, William F.; Doyle, Laurance R. (2013). "Worlds with Two Suns". Scientific American 309 (5): 40. doi:10.1038/scientificamerican1113-40.
- Ensemble analysis of open cluster transit surveys: upper limits on the frequency of short-period planets consistent with the field, Jennifer L. van Saders, B. Scott Gaudi, (Submitted on 15 Sep 2010)
- Three planetary companions around M67 stars, A. Brucalassi (1,2), L. Pasquini (3), R. Saglia (1,2), M. T. Ruiz (4), P. Bonifacio (5), L. R. Bedin (6), K. Biazzo (7), C. Melo (8), C. Lovis (9), S. Randich (10) ((1) MPI Munich, (2) UOM-LMU Munchen, (3) ESO Garching, (4) Astron. Dpt. Univ. de Chile, (5) GEPI Paris, (6) INAF-OAPD, (7) INAF-OACT, (8) ESO Santiago, (9) Obs. de Geneve, (10) INAF-OAFI) (Submitted on 20 Jan 2014)
- The same frequency of planets inside and outside open clusters of stars, Søren Meibom, Guillermo Torres, Francois Fressin, David W. Latham, Jason F. Rowe, David R. Ciardi, Steven T. Bryson, Leslie A. Rogers, Christopher E. Henze, Kenneth Janes, Sydney A. Barnes, Geoffrey W. Marcy, Howard Isaacson, Debra A. Fischer, Steve B. Howell, Elliott P. Horch, Jon M. Jenkins, Simon C. Schuler & Justin Crepp Nature 499, 55–58 (04 July 2013) doi:10.1038/nature12279 Received 06 November 2012 Accepted 02 May 2013 Published online 26 June 2013
- On the origin of planets at very wide orbits from the re-capture of free floating planets, Hagai B. Perets, M. B. N. Kouwenhoven, 2012
- SAG 11: Preparing for the WFIRST Microlensing Survey, Jennifer Yee
- Toward a New Era in Planetary Microlensing, Andy Gould, September 21, 2010
- MOA-2011-BLG-293Lb: First Microlensing Planet possibly in the Habitable Zone, V. Batista, J.-P. Beaulieu, A. Gould, D.P. Bennett, J.C Yee, A. Fukui, B.S. Gaudi, T. Sumi, A. Udalski, (Submitted on 14 Oct 2013 (v1), last revised 30 Oct 2013 (this version, v3))
- Charles H. Lineweaver (2000). "An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect". Icarus 151 (2): 307–313. arXiv:astro-ph/0012399. Bibcode:2001Icar..151..307L. doi:10.1006/icar.2001.6607.
- T. S. van Albada, Norman Baker (1973). "On the Two Oosterhoff Groups of Globular Clusters". Astrophysical Journal 185: 477–498. Bibcode:1973ApJ...185..477V. doi:10.1086/152434.
- Two planets around Kapteyn's star : a cold and a temperate super-Earth orbiting the nearest halo red-dwarf, Guillem Anglada-Escudé, Pamela Arriagada, Mikko Tuomi, Mathias Zechmeister, James S. Jenkins, Aviv Ofir, Stefan Dreizler, Enrico Gerlach, Chris J. Marvin, Ansgar Reiners, Sandra V. Jeffers, R. Paul Butler, Steven S. Vogt, Pedro J. Amado, Cristina Rodríguez-López, Zaira M. Berdiñas, Julian Morin, Jeff D. Crane, Stephen A. Shectman, Ian B. Thompson, Mateo Díaz, Eugenio Rivera, Luis F. Sarmiento, Hugh R.A. Jones, (Submitted on 3 Jun 2014)
- Habitable Zones in the Universe, G. Gonzalez, (Submitted on 14 Mar 2005 (v1), last revised 21 Mar 2005 (this version, v2))
- Time Really Flies on These Kepler Planets
- Saul Rappaport; Roberto Sanchis-Ojeda; Rogers, Leslie A.; Alan Levine; Winn, Joshua N. (2013). "The Roche limit for close-orbiting planets: Minimum density, composition constraints, and application to the 4.2-hour planet KOI 1843.03". arXiv:1307.4080 [astro-ph.EP].
- Scharf, Caleb; Menou, Kristen (2009). "Long-Period Exoplanets from Dynamical Relaxation". The Astrophysical Journal 693 (2): L113. arXiv:0811.1981. Bibcode:2009ApJ...693L.113S. doi:10.1088/0004-637X/693/2/L113.
- The Extrasolar Planet Encyclopaedia — Catalog Listing
- Eric L. Nielsen and Laird M. Close (2010). "A Uniform Analysis of 118 Stars with High-Contrast Imaging: Long-Period Extrasolar Giant Planets are Rare around Sun-like Stars". Astrophysical Journal 717 (2): 878. arXiv:0909.4531. Bibcode:2010ApJ...717..878N. doi:10.1088/0004-637X/717/2/878.
- T. Rodigas; Hinz (2009). "Which Radial Velocity Exoplanets Have Undetected Outer Companions?". Astrophys.J. 702: 716–723. arXiv:0907.0020. Bibcode:2009ApJ...702..716R. doi:10.1088/0004-637X/702/1/716.
- Guillem Anglada-Escudé, Mercedes López-Morales and John E. Chambers (2010). "How Eccentric Orbital Solutions Can Hide Planetary Systems in 2:1 Resonant Orbits". Astrophysical Journal 709 (1): 168. arXiv:0809.1275. Bibcode:2010ApJ...709..168A. doi:10.1088/0004-637X/709/1/168.
- The Exoplanet Eccentricity Distribution from Kepler Planet Candidates, Stephen R. Kane, David R. Ciardi, Dawn M. Gelino, Kaspar von Braun (Submitted on 7 Mar 2012 (v1), last revised 2 Jul 2012 (this version, v2))
- Page 2, Exoplanets: Detection, Formation, Properties, Habitability, John Mason, Springer, 2008
- Out of Flatland: Orbits Are Askew in a Nearby Planetary System, www.scientificamerican.com, 24 May 2010.
- "Turning planetary theory upside down". Astro.gla.ac.uk. Retrieved 2012-02-28.
- Tilting stars may explain backwards planets, New Scientist, 1 September 2010, Magazine issue 2776.
- Length of Exoplanet Day Measured for First Time, eso.org press release, 30 April 2014
- The fast spin-rotation of a young extra-solar planet, Ignas A. G. Snellen, Bernhard R. Brandl, Remco J. de Kok, Matteo Brogi, Jayne Birkby, and Henriette Schwarz, April 2014, http://arxiv.org/abs/1404.7506
- Newly Clocked Exoplanet Spins a Whole Day in 8 Hours, discovery.com Apr 30, 2014 01:00 PM ET // by Irene Klotz
- Tidal Evolution of Exoplanets, Alexandre C. M. Correia, Jacques Laskar, Chapter in Exoplanets, ed. S. Seager, published by University of Arizona Press, 2010
- Exoplanet Rotation Detected for the First Time, www.scientificamerican.com, Apr 30, 2014 |By Ron Cowen and Nature magazine
- Doppler Imaging of Exoplanets and Brown Dwarfs, Ian J. M. Crossfield, (Submitted on 30 Apr 2014)
- Terrestrial Planet Formation at Home and Abroad, Sean N. Raymond, Eiichiro Kokubo, Alessandro Morbidelli, Ryuji Morishima, Kevin J. Walsh, (Submitted on 5 Dec 2013 (v1), last revised 28 Jan 2014 (this version, v3))
- Makarov, Valeri V. et al. (2012). "Dynamical evolution and spin-orbit resonances of potentially habitable exoplanets. The case of GJ 581d". The Astrophysical Journal 761 (2): 83. arXiv:1208.0814. Bibcode:2012ApJ...761...83M. doi:10.1088/0004-637X/761/2/83.
- de Wit, Julien; Seager, S. (19 December 2013). "Constraining Exoplanet Mass from Transmission Spectroscopy". Science 342 (6165): 1473–1477. arXiv:1401.6181. Bibcode:2013Sci...342.1473D. doi:10.1126/science.1245450. PMID 24357312.
- Mass and Orbit Determination from Transit Timing Variations of Exoplanets, FREE ISSUE, The Astrophysical Journal Volume 688 Number 1, David Nesvorný and Alessandro Morbidelli, 2008, ApJ 688 636, doi:10.1086/592230
- Introduction to Exoplanets, Seager and Lissauer, Chapter in Exoplanets, edited by Sara Seager, University of Arizona Press, 2010
- Fundamental Planetary Science: Physics, Chemistry and Habitability, Jack J. Lissauer, Imke de Pater, Cambridge University Press, 16 Sep 2013, page 74
- Planetesimals To Brown Dwarfs: What is a Planet?, Gibor Basri, Michael E. Brown, (Submitted on 20 Aug 2006)
- I. Baraffe and G. Chabrier and T. Barman (2010). "The physical properties of extra-solar planets". Reports on Progress in Physics 73 (16901): 1. arXiv:1001.3577. Bibcode:2010RPPh...73a6901B. doi:10.1088/0034-4885/73/1/016901.
- The Mass-Radius Relation for 65 Exoplanets Smaller than 4 Earth Radii, Lauren M. Weiss, Geoffrey W. Marcy, (Submitted on 3 Dec 2013 (v1), last revised 15 Feb 2014 (this version, v4))
- Occurrence and core-envelope structure of 1--4x Earth-size planets around Sun-like stars, Geoffrey W. Marcy, Lauren M. Weiss, Erik A. Petigura, Howard Isaacson, Andrew W. Howard, Lars A. Buchhave, (Submitted on 10 Apr 2014)
- Lopez, E. D.; Fortney, J. J. (2013). "Understanding the Mass-Radius Relation for Sub-Neptunes: Radius as a Proxy for Composition". arXiv:1311.0329 [astro-ph.EP].
- Three regimes of extrasolar planets inferred from host star metallicities, Lars A. Buchhave, Martin Bizzarro, David W. Latham, Dimitar Sasselov, William D. Cochran, Michael Endl, Howard Isaacson, Diana Juncher, Geoffrey W. Marcy, (Submitted on 29 May 2014)
- Earth-mass exoplanet is no Earth twin, Nature News, Ron Cowen, 06 January 2014
- How to Distinguish between Cloudy Mini-Neptunes and Water/Volatile-Dominated Super-Earths, Björn Benneke, Sara Seager, 26 Jun 2013
- EXOPLANETS: FROM EXHILARATING TO EXASPERATING, 22:59, Kepler-10c: The "Mega-Earth", Dimitar Sasselov, June 2, 2014 YouTube
- The Kepler-10 planetary system revisited by HARPS-N: A hot rocky world and a solid Neptune-mass planet, Xavier Dumusque, Aldo S. Bonomo, Raphaelle D. Haywood, Luca Malavolta, Damien Segransan, Lars A. Buchhave, Andrew Collier Cameron, David W. Latham, Emilio Molinari, Francesco Pepe, Stephane Udry, David Charbonneau, Rosario Cosentino, Courtney D. Dressing, Pedro Figueira, Aldo F. M. Fiorenzano, Sara Gettel, Avet Harutyunyan, Keith Horne, Mercedes Lopez-Morales, Christophe Lovis, Michel Mayor, Giusi Micela, Fatemeh Motalebi, Valerio Nascimbeni, David F. Phillips, Giampaolo Piotto, Don Pollacco, Didier Queloz, Ken Rice, Dimitar Sasselov, Alessandro Sozzetti, Andrew Szentgyorgyi, Chris Watson, (Submitted on 30 May 2014)
- Super-dense remnants of gas giant exoplanets, A. Mocquet, O. Grasset and C. Sotin, EPSC Abstracts, Vol. 8, EPSC2013-986-1, 2013, European Planetary Science Congress 2013
- Very high-density planets: a possible remnant of gas giants, A. Mocquet, O. Grasset, C. Sotin, Published 24 March 2014, doi:10.1098/rsta.2013.0164 Phil. Trans. R. Soc. A, 28 April 2014, vol. 372, no. 2014
- MASS-RADIUS RELATIONSHIPS FOR SOLID EXOPLANETS, S. Seager, M. Kuchner, C. A. Hier-Majumder, B. Militzer, February 1, 2008
- Empirical Constraints on the Oblateness of an Exoplanet, Joshua A. Carter, Joshua N. Winn, (Submitted on 8 Dec 2009 (v1), last revised 30 Dec 2009 (this version, v2))
- Distorted, non-spherical transiting planets: impact on the transit depth and on the radius determination, Jérémy Leconte, Dong Lai, Gilles Chabrier,(Submitted on 14 Jan 2011 (v1), last revised 10 Oct 2011 (this version, v2))
- Thermal Tides in Short Period Exoplanets, Phil Arras, Aristotle Socrates, (Submitted on 7 Jan 2009)
- Exoplanetary Atmospheres, Nikku Madhusudhan (Cambridge), Heather Knutson (Caltech), Jonathan Fortney (UCSC), Travis Barman (U. Arizona), (Submitted on 5 Feb 2014)
- Seager, S.; Deming, D. (2010). "Exoplanet Atmospheres". arXiv:1005.4037 [astro-ph.EP].
- Brogi, Matteo; Snellen, Ignas A. G.; de Kok, Remco J.; Albrecht, Simon; Birkby, Jayne; de Mooij, Ernst J. W. (28 June 2012). "The signature of orbital motion from the dayside of the planet τ Boötis b". Nature 486 (7404): 502–504. arXiv:1206.6109. Bibcode:2012Natur.486..502B. doi:10.1038/nature11161.
- Rodler, F. et al. (2012). "Weighing the Non-transiting Hot Jupiter τ Boo b". The Astrophysical Journal Letters 753 (1). L25. arXiv:1206.6197. Bibcode:2012ApJ...753L..25R. doi:10.1088/2041-8205/753/1/L25.
- D. Charbonneau, T. Brown; A. Burrows; G. Laughlin (2006). "When Extrasolar Planets Transit Their Parent Stars". Protostars and Planets V. University of Arizona Press. arXiv:astro-ph/0603376.
- Detection of an Extrasolar Planet Atmosphere, David Charbonneau, Timothy M. Brown, Robert W. Noyes, Ronald L. Gilliland, (Submitted on 28 Nov 2001)
- Molecular Signatures in the Near Infrared Dayside Spectrum of HD 189733b, Swain MR, Vasisht G, Tinetti G, Bouwman J, Chen P et al. 2009. ApJ. 690:L114
- NASA - Hubble Finds First Organic Molecule on an Exoplanet, 03.19.08
- Staff (3 December 2013). "Hubble Traces Subtle Signals of Water on Hazy Worlds". NASA. Retrieved 4 December 2013.
- Deming, Drake et al. (10 September 2013). "Infrared Transmission Spectroscopy of the Exoplanets HD 209458b and XO-1b Using the Wide Field Camera-3 on the Hubble Space Telescope". Astrophysical Journal 774 (2): 95. arXiv:1302.1141. Bibcode:2013ApJ...774...95D. doi:10.1088/0004-637X/774/2/95. Retrieved 4 December 2013.
- Mandell, Avi M.; Haynes, Korey; Sinukoff, Evan; Madhusudhan, Nikku; Burrows, Adam; Deming, Drake (3 December 2013). "Exoplanet Transit Spectroscopy Using WFC3: WASP-12 b, WASP-17 b, and WASP-19 b". Astrophysical Journal 779 (2): 128. arXiv:1310.2949. Bibcode:2013ApJ...779..128M. doi:10.1088/0004-637X/779/2/128. Retrieved 4 December 2013.
- Harrington, J.D.; Villard, Ray (24 July 2014). "RELEASE 14-197 - Hubble Finds Three Surprisingly Dry Exoplanets". NASA. Retrieved 25 July 2014.
- Kawahara, H.; et al. (2012). "Can Ground-based Telescopes Detect the Oxygen 1.27 Micron Absorption Feature as a Biomarker in Exoplanets?". The Astrophysical Journal 758 (13): 13. arXiv:1206.0558. Bibcode:2012ApJ...758...13K. doi:10.1088/0004-637X/758/1/13.
- Atmospheric Circulation of Terrestrial Exoplanets, Adam P. Showman, Robin D. Wordsworth, Timothy M. Merlis, Yohai Kaspi, 11 Jun 2013
- Chu, Jennifer (October 2, 2013). "Scientists generate first map of clouds on an exoplanet". MIT. Retrieved January 2, 2014.
- Demory, Brice-Olivier et al. (September 30, 2013). "Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere". arXiv. arXiv:1309.7894. Bibcode:2013ApJ...776L..25D. doi:10.1088/2041-8205/776/2/L25. Retrieved January 2, 2014.
- Harrington, J.D.; Weaver, Donna; Villard, Ray (December 31, 2013). "Release 13-383 - NASA's Hubble Sees Cloudy Super-Worlds With Chance for More Clouds". NASA. Retrieved January 1, 2014.
- Moses, Julianne (January 1, 2014). "Extrasolar planets: Cloudy with a chance of dustballs". Nature (journal) 505 (7481): 31–32. Bibcode:2014Natur.505...31M. doi:10.1038/505031a. Retrieved January 1, 2014.
- Knutson, Heather et al. (January 1, 2014). "A featureless transmission spectrum for the Neptune-mass exoplanet GJ 436b". Nature (journal) 505 (7481): 66–68. arXiv:1401.3350. Bibcode:2014Natur.505...66K. doi:10.1038/nature12887. Retrieved January 1, 2014.
- Kreidberg, Laura et al. (January 1, 2014). "Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b". Nature (journal) 505 (7481): 69–72. arXiv:1401.0022. Bibcode:2014Natur.505...69K. doi:10.1038/nature12888. Retrieved January 1, 2014.
- New World of Iron Rain
- On Giant Blue Alien Planet, It Rains Molten Glass, by Elizabeth Howell, SPACE.com Contributor | August 30, 2013 04:15pm ET
- Raining Pebbles: Rocky Exoplanet Has Bizarre Atmosphere, Simulation Suggests, October 1, 2009, www.sciencedaily.com
- 'Diamond rain' falls on Saturn and Jupiter, 14 October 2013 Last updated at 12:04, By James Morgan Science reporter, BBC News
- Helium rain on Jupiter explains lack of neon in atmosphere, By Robert Sanders, Media Relations | March 22, 2010, newscenter.berkeley.edu
- Abiotic oxygen-dominated atmospheres on terrestrial habitable zone planets, Robin Wordsworth, Raymond Pierrehumbert, (Submitted on 11 Mar 2014)
- Thermal phase curves of nontransiting terrestrial exoplanets 1. Characterizing atmospheres, Franck Selsis, Robin Wordsworth, François Forget, (Submitted on 25 Apr 2011 (v1), last revised 30 May 2011 (this version, v3))
- Atmospheric Retrieval for Super-Earths: Uniquely Constraining the Atmospheric Composition with Transmission Spectroscopy, Bjoern Benneke, Sara Seager, (Submitted on 19 Mar 2012 (v1), last revised 27 Jun 2012 (this version, v2))
- Theoretical Spectra of Terrestrial Exoplanet Surfaces, Renyu Hu, Bethany L. Ehlmann, Sara Seager, (Submitted on 6 Apr 2012)
- Heather Knutson, David Charbonneau, Lori Allen, et al. (2007). "A map of the day-night contrast of the extrasolar planet HD 189733b". Nature 447 (7141): 183–186. arXiv:0705.0993. Bibcode:2007Natur.447..183K. doi:10.1038/nature05782. PMID 17495920.
- NASA Hubble Finds a True Blue Planet, July 11, 2013
- The Deep Blue Color of HD189733b: Albedo Measurements with Hubble Space Telescope/Space Telescope Imaging Spectrograph at Visible Wavelengths, Thomas M. Evans, Frédéric Pont, David K. Sing, Suzanne Aigrain, Joanna K. Barstow, Jean-Michel Désert, Neale Gibson, Kevin Heng, Heather A. Knutson, Alain Lecavelier des Etangs, (Submitted on 11 Jul 2013)
- "Coal-Black Alien Planet Is Darkest Ever Seen". Space.com. Retrieved 2011-08-12.
- Detection of visible light from the darkest world, David M. Kipping, David S. Spiegel, (Submitted on 10 Aug 2011 (v1), last revised 16 Aug 2011 (this version, v2))
- Footprint of a Magnetic Exoplanet, www.skyandtelescope.com, January 9, 2004, Robert Naeye
- Magnetosphere–ionosphere coupling at Jupiter-like exoplanets with internal plasma sources: implications for detectability of auroral radio emissions, J. D. Nichols, Monthly Notices of the Royal Astronomical Society, published online: 31 MAR 2011, arXiv version
- Radio Telescopes Could Help Find Exoplanets, RedOrbit – Apr 18, 2011
- "Radio Detection of Extrasolar Planets: Present and Future Prospects" (PDF). NRL, NASA/GSFC, NRAO, Observatoìre de Paris. Retrieved 2008-10-15.
- Super-Earths Get Magnetic 'Shield' from Liquid Metal, Charles Q. Choi, SPACE.com, November 22, 2012 02:01pm ET,
- Stellar Magnetic Fields as a Heating Source for Extrasolar Giant Planets, D. Buzasi, (Submitted on 6 Feb 2013)
- Valencia, Diana; O'Connell, Richard J. (2009). "Convection scaling and subduction on Earth and super-Earths". Earth and Planetary Science Letters 286 (3–4): 492. Bibcode:2009E&PSL.286..492V. doi:10.1016/j.epsl.2009.07.015.
- Van Heck, H.J.; Tackley, P.J. (2011). "Plate tectonics on super-Earths: Equally or more likely than on Earth". Earth and Planetary Science Letters 310 (3–4): 252. Bibcode:2011E&PSL.310..252V. doi:10.1016/j.epsl.2011.07.029.
- O'Neill, C.; Lenardic, A. (2007). "Geological consequences of super-sized Earths". Geophysical Research Letters 34 (19). Bibcode:2007GeoRL..3419204O. doi:10.1029/2007GL030598.
- Valencia, Diana; O'Connell, Richard J.; Sasselov, Dimitar D (November 2007). "Inevitability of Plate Tectonics on Super-Earths". Astrophysical Journal Letters 670 (1): L45–L48. arXiv:0710.0699. Bibcode:2007ApJ...670L..45V. doi:10.1086/524012.
- Super Earths Likely To Have Both Oceans and Continents, astrobiology.com, Source: Northwestern University, Posted January 7, 2014 11:55 AM
- Water Cycling Between Ocean and Mantle: Super-Earths Need Not be Waterworlds, Nicolas B. Cowan, Dorian S. Abbo, (Submitted on 3 Jan 2014
- Scientists Discover a Saturn-like Ring System Eclipsing a Sun-like Star, Space Daily, Jan 13, 2012
- Planetary Construction Zones in Occultation: Discovery of an Extrasolar Ring System Transiting a Young Sun-like Star and Future Prospects for Detecting Eclipses by Circumsecondary and Circumplanetary Disks, Eric E. Mamajek, Alice C. Quillen, Mark J. Pecaut, Fred Moolekamp, Erin L. Scott, Matthew A. Kenworthy, Andrew Collier Cameron, Neil R. Parley, (Submitted on 19 Aug 2011 (v1), last revised 10 Jan 2012 (this version, v2))
- Kalas, Paul; et al. (2008-11-13). "Optical Images of an Exosolar Planet 25 Light-Years from Earth". Science 322 (5906): 1345–8. arXiv:0811.1994. Bibcode:2008Sci...322.1345K. doi:10.1126/science.1166609. PMID 19008414.
- Warm Saturns: On the Nature of Rings around Extrasolar Planets that Reside Inside the Ice Line, Hilke E. Schlichting (UCLA), Philip Chang (CITA), (Submitted on 19 Apr 2011)
- A sub-Earth-mass moon orbiting a gas giant primary or a high-velocity planetary system in the galactic bulge
- exoplanet stirs up dust, Phys.org, Aug 28, 2012
- New-found exoplanet is evaporating away, Posted May 18, 2012 - 10:55 by Emma Woollacott
- Alien Life More Likely on 'Dune' Planets, 09/01/11, Charles Q. Choi, Astrobiology Magazine
- Habitable Zone Limits for Dry Planets, Yutaka Abe, Ayako Abe-Ouchi, Norman H. Sleep, and Kevin J. Zahnle. Astrobiology. June 2011, 11(5): 443–460. doi:10.1089/ast.2010.0545
- Exoplanet Habitability, DOI: 10.1126/science.1232226 Exoplanet Habitability, Science 340, 577 (2013); Sara Seager
- Amend, J. P., & Teske, A. (2005). Expanding frontiers in deep subsurface microbiology. Palaeogeography, Palaeoclimatology, Palaeoecology, 219(1-2), 131-155. Elsevier.
- Further away planets 'can support life' say researchers, BBC, 7 January 2014 Last updated at 12:40
- The Steppenwolf: A proposal for a habitable planet in interstellar space, Dorian S. Abbot, Eric R. Switzer, (Submitted on 5 Feb 2011 (v1), last revised 2 Jun 2011 (this version, v2))
- The Habitable Epoch of the Early Universe, Abraham Loeb (Harvard), (Submitted on 2 Dec 2013)
- Habitability of Earth-like planets with high obliquity and eccentric orbits: results from a general circulation model, Manuel Linsenmeier, Salvatore Pascale, Valerio Lucarini, 21 Jan 2014
- April 15, 2014, Astronomers: ‘Tilt-a-worlds’ could harbor life, Peter Kelley, www.washington.edu
- Effects of Extreme Obliquity Variations on the Habitability of Exoplanets, J. C. Armstrong, R. Barnes, S. Domagal-Goldman, J. Breiner, T. R. Quinn, V. S. Meadows 14 Apr 2014
- July 18, 2013, A warmer planetary haven around cool stars, as ice warms rather than cools, Peter Kelley, www.washington.edu
- Spectrum-driven Planetary Deglaciation Due to Increases in Stellar Luminosity, Aomawa L. Shields, Cecilia M. Bitz, Victoria S. Meadows, Manoj M. Joshi, Tyler D. Robinson, 14 Mar 2014
- Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating, Rory Barnes, Kristina Mullins, Colin Goldblatt, Victoria S. Meadows, James F. Kasting, Rene Heller, (Submitted on 22 Mar 2012 (v1), last revised 27 Nov 2012 (this version, v2))
- Superhabitable Worlds, René Heller (1), John Armstrong (2) ((1) McMaster University, Dept. of Physics & Astronomy, Hamilton (ON), Canada, (2) Weber State University, Dept. of Physics, Ogden (UT), USA), (Submitted on 10 Jan 2014)
- Tidal Heating of Terrestrial Extra-Solar Planets and Implications for their Habitability, Brian Jackson, Rory Barnes, Richard Greenberg, (Submitted on 20 Aug 2008)
- Anglada-Escudé, G.; et al. (2013). "A dynamically-packed planetary system around GJ 667C with three super-Earths in its habitable zone". Astronomy & Astrophysics 556: A126. arXiv:1306.6074. Bibcode:2013A&A...556A.126A. doi:10.1051/0004-6361/201321331. Retrieved 2013-06-25.
- [NULL] (5 December 2011). "Kepler-22b, our first planet in the habitable zone of a Sun-like Star". Kepler.nasa.gov. Retrieved 2012-02-28.
- Staff (20 September 2012). "LHS 188 – High proper-motion Star". Centre de données astronomiques de Strasbourg (Strasbourg astronomical Data Center). Retrieved 20 September 2012.
- Méndez, Abel (29 August 2012). "A Hot Potential Habitable Exoplanet around Gliese 163". University of Puerto Rico at Arecibo (Planetary Habitability Laboratory). Retrieved 20 September 2012.
- Redd, Nola Taylor (20 September 2012). "Newfound Alien Planet a Top Contender to Host Life". Space.com. Retrieved 20 September 2012.
- Borucki, W. J.; et al. (2013). "Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone". Science 340 (6132): 587. arXiv:1304.7387. Bibcode:2013Sci...340..587B. doi:10.1126/science.1234702.
- Johnson, Michele; Harrington, J.D. (18 April 2013). "NASA's Kepler Discovers Its Smallest 'Habitable Zone' Planets to Date". NASA. Retrieved 18 April 2013.
- Kaltenegger, L.; Sasselov, D; Rugheimer, S. (2013). "Water Planets in the Habitable Zone: Atmospheric Chemistry, Observable Features, and the case of Kepler-62e and -62f". arXiv:1304.5058 [astro-ph.EP].
- Howell, Elizabeth (6 February 2013). "Closest 'Alien Earth' May Be 13 Light-Years Away". Space.com. TechMediaNetwork. Retrieved 7 February 2013.
- Kopparapu, Ravi kumar (March 2013). "A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs". The Astrophysical Journal Letters 767: L8. arXiv:1303.2649. Bibcode:2013ApJ...767L...8K. doi:10.1088/2041-8205/767/1/L8.
- Emspak, Jesse. "Kepler Finds Bizarre Systems". International Business Times. International Business Times Inc. Retrieved 2 March 2011.
- Fabrycky, Daniel C. (2010). "Non-Keplerian Dynamics". arXiv:1006.3834 [astro-ph.EP].
- Equilibria in the secular, non-coplanar two-planet problem, Cezary Migaszewski, Krzysztof Gozdziewski, 2 Feb 2009
- Staff (May 3, 2013). "Special Issue: Exoplanets". Science. Retrieved May 18, 2013.
|Wikimedia Commons has media related to Exoplanets.|
|Wikiversity has learning materials about Observational astronomy/Extrasolar planet|
- The Extrasolar Planets Encyclopaedia (Paris Observatory)
- NASA Exoplanet Archive
- Open Exoplanet Catalogue
- The Habitable Exoplanets Catalog (PHL/UPR Arecibo)
- The Habitable Zone Gallery
- Exoplanet Orbit Data Explorer interactive table and plotter for exploring data from Exoplanet Orbit Database
- Exoplanets: Interactive Visual of XKCD 1071
- Exoplanet database for iPhone/iPod/iPad with visualizations
- NASA's PlanetQuest
- A Zoo of Extra-Solar Planets (audio and transcript) — Astronomy Cast on 9 February 2009 with Pamela Gay and Chris Lintott
- Transiting Exoplanet Light Curves Using Differential Photometry
- Extrasolar Planets – D. Montes, UCM
- Exoplanets at Paris Observatory
- "Exoplanets in relation to host star's current habitable zone". planetarybiology.com.
- Doyle, Laurence R. (19 March 2009). "Naming New Extrasolar Planets". SETI institute. SPACE.com. Retrieved 2010-06-02.
- ETD – Exoplanet Transit Database (Exoplanet Transit Database)
- Exomol Project Spectroscopic database of molecules of importance for the characterization of exoplanets.
- Characterizing bulk composition of Solid Planets
- Graphical Comparison of Extrasolar Planets
- Video (86:49) - "Search for Life in the Universe" - NASA (July 14, 2014).
- Arxiv: Earth and Planetary Astrophysics
- Extrasolar News and Discoveries
- astrobites the astro-ph reader's digest
- Virtual Planetary Laboratory