Jump to content

Super-Earth

From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by Henrykus (talk | contribs) at 12:52, 9 May 2011 (→‎References). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Illustration of the inferred size of the super-Earth Kepler-10 b (right) in comparison with Earth.

A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's gas giants.[1] The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability. An alternative term "gas dwarf" may be more accurate for some examples, especially higher mass ones as suggested by MIT professor Sara Seager.

Definition

In general, super-Earths are defined exclusively by their mass, and the term does not imply temperatures, compositions, orbital properties, or environments similar to Earth's. A variety of specific mass values are cited in definitions of super-Earths. While sources generally agree on an upper bound of 10 Earth masses,[1][2][3] (~69% the mass of the smallest Solar System gas giant Uranus), the lower bound varies from 1[1] or 1.9[3] to 5,[2] with various other definitions appearing in the popular media.[4][5][6] Some authors further suggest that the term be limited to planets without a significant atmosphere.[7] Planets above 10 Earth masses are termed giant planets.[8]

Discoveries

The Solar System does not contain examples of this category of planets, as the largest terrestrial planet in the Solar System is the Earth, and all larger planets have at least 14 times Earth's mass.

First super-Earth found

Mass and radius values for transiting super-Earths in context of other detected exoplanets and selected composition models.

The first super-Earths were discovered by Aleksander Wolszczan and Dale Frail around the pulsar PSR B1257+12 in 1992. The two outer planets of the system have masses approximately 4 times Earth, too small to be gas giants.

The first super-Earth around a main sequence star was discovered by a team under Eugenio Rivera in 2005. It orbits Gliese 876 and received the designation Gliese 876 d (two Jupiter sized gas giants had previously been discovered in that system). It has an estimated mass of 7.5 Earth masses and a very short orbital period of just about 2 days. Due to the proximity of Gliese 876 d to its host star (a red dwarf), it may have a surface temperature of 430–650 kelvin[9] and may support liquid water.[10]

Other super-Earths discovered in 2006

Two further super-Earths were discovered in 2006, OGLE-2005-BLG-390Lb with a mass of 5.5 Earth masses, which was found by gravitational microlensing, and HD 69830 b with a mass of 10 Earth masses.[1]

First super-Earth in habitable zone

In April 2007, a team headed by Stephane Udry based in Switzerland announced the discovery of two new super-Earths around Gliese 581,[11] both on the edge of the habitable zone around the star where liquid water may be possible on the surface. With Gliese 581 c having a mass of at least 5 Earth masses and a distance from Gliese 581 of 0.073 astronomical units (AU; 6.8 million mi, 11 million km), it is on the "warm" edge of the habitable zone around Gliese 581 with an estimated mean temperature (without taking into consideration effects from an atmosphere) of −3 degrees Celsius with an albedo comparable to Venus and 40 degrees Celsius with an albedo comparable to Earth. Subsequent research suggests Gliese 581 c has likely suffered a runaway greenhouse effect like Venus, but that its sister planet, Gliese 581 d, does in fact lie within the star's habitable zone, with an orbit at 0.22 AU and a mass of 7.7 Earths.

Super-Earths found in 2008

The smallest super-Earth found to date is MOA-2007-BLG-192Lb was announced by astrophysicist David P. Bennett for the international MOA collaboration on June 2, 2008.[12][13] This planet has approximately 3.3 Earth masses and orbits a brown dwarf. It was detected by gravitational microlensing.

In June 2008, European researchers announced the discovery of three super-Earths around the star HD 40307, a star that is only slightly less massive than our Sun. The planets have at least the following minimum masses: 4.2, 6.7, and 9.4 times Earth's. The planets were detected by the radial velocity method by the HARPS (High Accuracy Radial Velocity Planet Searcher) in Chile.[14]

In addition, the same European research team announced a planet 7.5 times the mass of Earth orbiting the star HD 181433. This star also has a Jupiter-like planet that orbits every three years.[15]

Super-Earths found in 2009

Illustration of the inferred size of the super-Earth COROT-7b (center) in comparison with Earth and Neptune.

Planet COROT-7b, with a mass estimated at 4.8 Earth masses and an orbital period of only 0.853 days, was announced on 3 February 2009. The density estimate obtained for COROT-7b points to a composition including rocky silicate minerals, similar to the four inner planets of Earth's solar system, a new and significant discovery.[16] COROT-7b, discovered right after HD 7924 b, is the first super-Earth discovered that orbits a main sequence star that is G class or larger.[17]

The discovery of Gliese 581 e with a minimum mass of 1.9 Earth masses was announced on April 21, 2009. It is the smallest extrasolar planet discovered around a normal star and the closest in mass to Earth. Being at an orbital distance of just 0.03 AU and orbiting its star in just 3.15 days, it is not in the habitable zone,[18] and may have 100 times more tidal heating than Jupiter’s volcanic satellite Io.[19]

Additionally, Gliese 581 d, at 0.2 AU with a 67 day orbital period, has been confirmed to be within the habitable zone of the red dwarf star, making it the first exoplanet where the existence of liquid water is a real possibility.[18]

A planet found in December 2009, GJ 1214 b, is 2.7 times as large as Earth and orbits a star much smaller and less luminous than our sun. "This planet probably does have liquid water," said David Charbonneau, a Harvard professor of astronomy and lead author of an article on the discovery.[20] However, interior models of this planet suggest that under most conditions it will not have liquid water.[21]

By November 2009, a total of 30 super-Earths have been discovered, 24 of which were first observed by HARPS.[22]

Super-Earths found in 2010

Discovered on January 5, 2010, a planet HD 156668 b with a minimum mass of 4.15 Earth masses, is the second least massive planet detected by the radial velocity method.[23] The only confirmed radial velocity planet smaller than this planet is Gliese 581 e at 1.9 Earth masses (see above). On August 24 astronomers using ESO’s HARPS instrument announced the discovery of a planetary system with up to seven planets orbiting a Sun-like star, HD 10180, one of which, although not yet confirmed, has an estimated minimum mass of 1.35 ± 0.23 times that of Earth, which would be the lowest mass of any exoplanet found to date orbiting a main-sequence star.[24] Although unconfirmed, there is 98.6% probability that this planet does exist.[25]

The National Science Foundation announced on September 29 the discovery of a fourth super-Earth (Gliese 581 g) orbiting the M dwarf star Gliese 581. The planet has a minimum mass 3.1 times that of Earth and a nearly circular orbit at 0.146 AU with a period of 36.6 days, placing it in the middle of the habitable zone where liquid water could exist and midway between the planets c and d. It was discovered using the radial velocity method by scientists at the University of California at Santa Cruz and the Carnegie Institution of Washington.[26][27][28] However, the existence of Gliese 581 g has been questioned by another team of astronomers, and it is currently listed as unconfirmed at The Extrasolar Planets Encyclopaedia.[29]

Super-Earths found in 2011

On February 2, 2011, the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including 68 candidates of approximately "Earth-size" (Rp < 1.25 Re) and 288 candidates of "super-Earth-size" (1.25 Re < Rp < 2 Re).[30][31] In addition, 54 planet candidates were detected in the "habitable zone." Six candidates in this zone were less than twice the size of the Earth [namely: KOI 326.01 (Rp=0.85), KOI 701.03 (Rp=1.73), KOI 268.01 (Rp=1.75), KOI 1026.01 (Rp=1.77), KOI 854.01 (Rp=1.91), KOI 70.03 (Rp=1.96) - Table 6][30] A more recent study found that one of these candidates (KOI 326.01) is in fact much larger and hotter than first reported.[32] Based on the latest Kepler findings, astronomer Seth Shostak estimates "within a thousand light-years of Earth" there are "at least 30,000 of these habitable worlds."[33] Also based on the findings, the Kepler Team has estimated "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone.[34]

Characteristics

Comparison of sizes of planets with different compositions

Due to the larger mass of super-Earths, their physical characteristics differ from Earth's. A study on Gliese 876 d by a team around Diana Valencia[1] revealed it would be possible to infer from a radius measured by the transit method of detecting planets and the mass of the relevant planet what the structural composition of a relevant super-Earth is. For Gliese 876 calculations range from 9,200 km (1.4 Earth radii) for a rocky planet and very large iron core to 12,500 km (2.0 Earth radii) for a watery and icy planet. Within this range of radii the super-Earth Gliese 876 d would have a surface gravity between 1.9g and 3.3g (19 and 32 m/s²). High surface gravity (generally higher than Neptune-, Uranus-, and Saturn-class planets, and in certain circumstances higher than Jupiter-class planets) is one of the predominant known characteristics of super-Earths.[dubious – discuss]

Further theoretical work by Valencia and others suggests that super-Earths would be more geologically active than Earth, with more vigorous plate tectonics due to thinner plates under more stress. In fact, their models suggested that Earth was itself a "borderline" case, just barely large enough to sustain plate tectonics.[35] However, other studies determine that strong convection currents in the mantle acting on strong gravity would make the crust stronger and thus inhibits plate tectonics. The planet's surface would be too strong for driving forces of magma to break the crust into plates.[36]

Temperatures

Since the atmospheres and greenhouse effects of super-Earths are unknown, the surface temperatures are unknown and generally only an equilibrium temperature is given. For example, the black body temperature of the Earth is 254.3 K (−19 °C or −2 °F ).[37] It is the greenhouse gases that keep the Earth warmer. Venus has a black-body temperature of only 231.7 K (−41 °C or −43 °F ) even though Venus has a true temperature of 737 K (464 °C or 867 °F ).[38] Though the atmosphere of Venus traps more heat than Earth, NASA lists the black body temperature of Venus based on the fact Venus has an extremely high albedo, giving it a lower black body temperature than the more absorbent (lower albedo) Earth.

See also

References

  1. ^ a b c d e Valencia et al., Radius and structure models of the first super-Earth planet, September 2006, published in The Astrophysical Journal, February 2007 Cite error: The named reference "Valencia" was defined multiple times with different content (see the help page).
  2. ^ a b Fortney; Marley, M. S.; Barnes, J. W.; et al. (2007). "Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation: Application to Transits". The Astrophysical Journal. 659 (2): 1661–1672. arXiv:astro-ph/0612671. Bibcode:2007ApJ...659.1661F. doi:10.1086/512120. {{cite journal}}: Explicit use of et al. in: |author= (help)
  3. ^ a b Charbonneau, David (2009). "A super-Earth transiting a nearby low-mass star". Nature. 462 (17 December 2009): 891–894. Bibcode:2009Natur.462..891C. doi:10.1038/nature08679. PMID 20016595. Retrieved 2009-12-15. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  4. ^ Peter N. Spotts. Canada's orbiting telescope tracks mystery 'super Earth', Hamilton Spectator, 2007-04-28
  5. ^ Life could survive longer on a super-Earth - space - 11 November 2007 - New Scientist Space
  6. ^ ICE - News Detail
  7. ^ Seager, S. (2007). "Mass–radius relationships for solid exoplanets". The Astrophysical Journal. 669 (2): 1279–1297. Bibcode:2007ApJ...669.1279S. doi:10.1086/521346. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  8. ^ Page 20 of The quest for very low-mass planets, M Mayor, S Udry - Physica Scripta, 2008
  9. ^ Rivera, E.; et al. (2005). "A ~7.5 M⊕ Planet Orbiting the Nearby Star, GJ 876". The Astrophysical Journal. 634 (1): 625–640. arXiv:astro-ph/0510508. Bibcode:2005ApJ...634..625R. doi:10.1086/491669. {{cite journal}}: Explicit use of et al. in: |author= (help)
  10. ^ Zhou, J.-L.; et al. (2005). "Origin and Ubiquity of Short-Period Earth-like Planets: Evidence for the Sequential Accretion Theory of Planet Formation". The Astrophysical Journal. 631 (1): L85–L88. arXiv:astro-ph/0508305. Bibcode:2005ApJ...631L..85Z. doi:10.1086/497094. {{cite journal}}: Explicit use of et al. in: |author= (help)
  11. ^ Udry; Bonfils, X.; Delfosse, X.; Forveille, T.; Mayor, M.; Perrier, C.; Bouchy, F.; Lovis, C.; Pepe, F.; et al. (2007). "The HARPS search for southern extra-solar planets XI. Super-Earths (5 and 8 M) in a 3-planet system". Astronomy and Astrophysics. 469 (3): L43–L47. Bibcode:2007A&A...469L..43U. doi:10.1051/0004-6361:20077612. {{cite journal}}: Explicit use of et al. in: |author= (help)
  12. ^ Oasis, Online Abstract Submission and Invitation System - Program Planner
  13. ^ [0806.0025] A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192
  14. ^ "Trio of 'super-Earths' discovered". BBC News. 2008-06-16. Retrieved 2010-05-24.
  15. ^ AFP: Astronomers discover clutch of 'super-Earths'
  16. ^ Queloz, D., Bouchy, F., Moutou, C., Hatzes, A., Hebrard, G., Alonso, R., Auvergne, M., Baglin, A., Barbieri, M., Barge, P., Benz, W., Bordé, P., Deeg, H., Deleuil, M., Dvorak, R., Erikson, A., Ferraz Mello, S., Fridlund, M., Gandolfi, D., Gillon, M., Guenther, E., Guillot, T., Jorda, L., Hartmann, M., Lammer, H., Léger, A., Llebaria, A., Lovis, C., Magain, P., Mayor, M., Mazeh, T., Ollivier, M., Pätzold, M., Pepe, F., Rauer, H., Rouan, D., Schneider, J., Segransan, D., Udry, S., and Wuchterl, G. (2009). "The CoRoT-7 planetary system: two orbiting Super-Earths" (PDF). Astronomy and Astrophysics. 506 (1): 303. Bibcode:2009A&A...506..303Q. doi:10.1051/0004-6361/200913096.{{cite journal}}: CS1 maint: multiple names: authors list (link) Also available from exoplanet.eu
  17. ^ Howard; Johnson; Marcy; Fischer; Wright; Henry; Giguere; Valenti; et al. (28 January 2009). "The NASA-UC Eta-Earth Program: I. A Super-Earth Orbiting HD 7924". The Astrophysical Journal. arXiv:0901.4394. Bibcode:2009ApJ...696...75H. doi:10.1088/0004-637X/696/1/75. {{cite journal}}: |format= requires |url= (help); Explicit use of et al. in: |author= (help); Missing |author8= (help)
  18. ^ a b "Lightest exoplanet yet discovered". ESO (ESO 15/09 - Science Release). 2009-04-21. Retrieved 2009-07-15.
  19. ^ Barnes, Rory; Jackson, Brian; Greenberg, Richard; Raymond, Sean N. (2009-06-09). "Tidal Limits to Planetary Habitability". arXiv:0906.1785v1 [astro-ph]. {{cite arXiv}}: Unknown parameter |accessdate= ignored (help)
  20. ^ "Scientists spot nearby 'super-Earth' - CNN.com". CNN. Retrieved 2010-05-24.
  21. ^ Rogers, L; Seager, S (2010-06-01). "Three Possible Origins for the Gas Layer on GJ 1214b". arXiv:0912.3243 [astro-ph]. {{cite arXiv}}: Unknown parameter |accessdate= ignored (help)
  22. ^ "32 planets discovered outside solar system - CNN.com". CNN. Retrieved 2010-05-24.
  23. ^ "Second Smallest Exoplanet Found To Date At Keck". W.M. Keck Observatory. 2010-01-07. Retrieved 2010-01-07.
  24. ^ "Richest Planetary System Discovered". 2010-08-24. Retrieved 24 August 2010.
  25. ^ Lovis, C (2010-08-12). "The HARPS search for southern extra-solar planets XXVII. Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems" (PDF). Submitted to Astronomy & Astrophysics. Retrieved 2010-08-26. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); line feed character in |coauthors= at position 93 (help)
  26. ^ Overbye, Dennis (September 29, 2010). "New Planet May Be Able to Nurture Organisms". The New York Times. Retrieved October 2, 2010. {{cite web}}: Italic or bold markup not allowed in: |publisher= (help)
  27. ^ "Press release 10-172: Newly Discovered Planet May Be First Truly Habitable Exoplanet". National Science Foundation. September 29, 2010.
  28. ^ Vogt, Steven S.; R. Paul Butler; E. J. Rivera; N. Haghighipour; Gregory W. Henry; and Michael H. Williamson (posted September 29, 2010). "The Lick-Carnegie Exoplanet Survey: A 3.1 M Planet in the Habitable Zone of the Nearby M3V Star Gliese 581⊕" (PDF). Draft to be submitted to Astrophysical Journal. {{cite journal}}: Check date values in: |date= (help); Italic or bold markup not allowed in: |journal= (help)CS1 maint: multiple names: authors list (link)
  29. ^ "Notes for star Gl 581". The Extrasolar Planets Encyclopaedia.
  30. ^ a b Borucki, William J. (1 February 2011). "Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data" (PDF). arXiv. Retrieved 2011-02-16. {{cite web}}: Check date values in: |date= (help); More than one of |author= and |last= specified (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); line feed character in |date= at position 12 (help)
  31. ^ Characteristics of Kepler Planetary Candidates Based on the First Data Set: The Majority are Found to be Neptune-Size and Smaller, William J. Borucki, for the Kepler Team (Submitted on 14 Jun 2010)
  32. ^ Grant, Andrew (8 March 2011`). "Exclusive: "Most Earth-Like" Exoplanet Gets Major Demotion—It Isn't Habitable". 80beats. Discover Magazine. Retrieved 2011-03-09. {{cite web}}: Check date values in: |date= (help); External link in |work= (help)
  33. ^ Shostak, Seth (3 February 2011). "A Bucketful of Worlds". Huffington Post. Retrieved 2011-02-03.
  34. ^ Borenstein, Seth (19 February 2011). "Cosmic census finds crowd of planets in our galaxy". AP News. Retrieved 2011-02-19.
  35. ^ CfA Press Release Release No.: 2008-02 January 9, 2008 Earth: A Borderline Planet for Life?
  36. ^ Barry, Carolyn (2007). "The plate tectonics of alien worlds". Cosmos.
  37. ^ Williams, David R. (2009-05-20). "Earth Fact Sheet". NASA. Retrieved December 23, 2009.
  38. ^ Williams, David R. (2005-04-15). "Venus Fact Sheet". NASA. Retrieved 2009-12-23.

Template:Link GA

Retrieved from "https://en.wikipedia.org/w/index.php?title=Super-Earth&oldid=428240070"