It is inferred from the empirical study of natural satellites in the Solar System that they are likely to be common elements of planetary systems. The majority of detected exoplanets are gas giants. In the Solar System, the gas giants have large collections of natural satellites (see Moons of Jupiter, Moons of Saturn, Moons of Uranus and Moons of Neptune), therefore it is reasonable to assume that exomoons are equally common.
Though exomoons are difficult to detect and confirm using current techniques observations from missions such as Kepler have observed a number of candidates including some that may be habitats for extraterrestrial life.
- 1 Definition of satellites around brown dwarfs
- 2 Characteristics
- 3 Proposed detection methods
- 4 Candidates
- 5 Detection projects
- 6 Habitability
- 7 See also
- 8 References
- 9 External links
Definition of satellites around brown dwarfs
Although traditional usage implies moons orbit a planet, the discovery of planet-sized satellites around brown dwarfs blurs the distinction between planets and moons, due to the low mass of such failed stars. To resolve this confusion, the International Astronomical Union declared, "Objects with true masses below the limiting mass for thermonuclear fusion of deuterium, that orbit stars or stellar remnants, are planets."
Characteristics of any extrasolar satellite are likely to vary, as do the Solar System's moons. For extrasolar giant planets orbiting within their stellar habitable zone, there is a prospect a terrestrial planet-sized satellite may be capable of supporting life.
Proposed detection methods
The existence of exomoons around many exoplanets is theorized. Despite the great successes of planet hunters with Doppler spectroscopy of the host star, exomoons cannot be found with this technique. This is because the resultant shifted stellar spectra due to the presence of a planet plus additional satellites would behave identically to a single point-mass moving in orbit of the host star. In recognition of this, there have been several other methods proposed for detecting exomoons, including:
- Transit timing effects
- Direct imaging
- Transit method
- Doppler spectroscopy of host planet
- Pulsar timing
Transit timing effects
In 2009, University College London-based astronomer David Kipping published a paper outlining how by combining multiple observations of variations in the time of mid-transit (TTV, caused by the planet leading or trailing the planet-moon system's barycenter when the pair are oriented roughly perpendicular to the line of sight) with variations of the transit duration (TDV, caused by the planet moving along the direction path of transit relative to the planet-moon system's barycenter when the moon-planet axis lies roughly along the line of sight) a unique exomoon signature is produced. Furthermore, the work demonstrated how both the mass of the exomoon and its orbital distance from the planet could be determined using the two effects.
Even direct imaging of an exoplanet is extremely challenging due to the large difference in brightness between the objects and the small angular size of the planet. These problems are exacerbated for small exomoons.
When an exoplanet passes in front of the host star, a small dip in the light received from the star may be observed. This effect, also known as occultation, is proportional to the square of the planet's radius. If a planet and a moon passed in front of a host star, both objects should produce a dip in the observed light. A planet–moon eclipse may also occur during the transit, but such events are inherently low probability.
In 2002, Cheongho Han & Wonyong Han proposed microlensing be used to detect exomoons. The authors found detecting satellite signals in lensing light curves will be very difficult because the signals are seriously smeared out by the severe finite-source effect even for events involved with source stars with small angular radii.
Doppler spectroscopy of host planet
The spectra of exoplanets have been successfully partially retrieved for several cases, including HD 189733 b and HD 209458 b. The quality of the retrieved spectra is significantly more affected by noise than the stellar spectrum. As a result, the spectral resolution, and number of retrieved spectral features, is much lower than the level required to perform doppler spectroscopy of the exoplanet.
In 2008, Lewis, Sackett, and Mardling of the Monash University, Australia proposed using pulsar timing to detect the moons of pulsar planets. The authors applied their method to the case of PSR B1620-26 b and found that a stable moon orbiting this planet could be detected, if the moon had a separation of about one fiftieth of that of the orbit of the planet around the pulsar, and a mass ratio to the planet of 5% or larger.
It has been surmised that the star 1SWASP J140747.93-394542.6, in the constellation Centaurus, might have a planet with a moon. The confirmed extrasolar planet WASP-12b may also possess a moon.
Habitability of exomoons has been considered in at least two studies published in peer-reviewed journals. René Heller & Rory Barnes considered stellar and planetary illumination on moons as well as the effect of eclipses on their orbit-averaged surface illumination. They also considered tidal heating as a threat for their habitability. In Sect. 4 in their paper, they introduce a new concept to define the habitable orbits of moons. Referring to the concept of the circumstellar habitable zone for planets, they define an inner border for a moon to be habitable around a certain planet and call it the circumplanetary "habitable edge". Moons closer to their planet than the habitable edge are uninhabitable. In a second study, René Heller  then included the effect of eclipses into this concept as well as constraints from a satellite's orbital stability. He found that, depending on a moon's orbital eccentricity, there is a minimum mass for stars to host habitable moons and located it around 0.2 solar masses.
- Kipping D. M. (2009). "Transit timing effects due to an exomoon". Monthly Notices of the Royal Astronomical Society 392 (3): 181–189. arXiv:0810.2243. Bibcode:2009MNRAS.392..181K. doi:10.1111/j.1365-2966.2008.13999.x.
- "Position statement on the definition of a planet by the International Astronomical Union". International Astronomical Union. November 12, 2008. Retrieved 2008-11-11.
- Canup, R. & Ward, W. (2006). "A common mass scaling relation for satellite systems of gaseous planets". Nature 441 (7095): 834–839. Bibcode:2006Natur.441..834C. doi:10.1038/nature04860. PMID 16778883.
- "The Exoplanet Catalogue". Jean Schneider. November 11, 2008. Retrieved 2008-11-11.
- "Hunting for Exoplanet Moons". Centauri Dreams. November 11, 2008. Retrieved 2008-11-11.
- Kipping D. M., Fossey S. J. & Campanella G. (2009). "On the detectability of habitable exomoons with Kepler-class photometry". Monthly Notices of the Royal Astronomical Society 400 (1): 398–405. arXiv:0907.3909. Bibcode:2009MNRAS.400..398K. doi:10.1111/j.1365-2966.2009.15472.x.
- Simon A., Szatmary, K. & Szabo Gy. M. (2007). "Determination of the size, mass, and density of exomoons from photometric transit timing variations". Astronomy and Astrophysics 480 (2): 727–731. arXiv:0705.1046.
- Cabrera J. & Schneider J. (2007). "Detecting companions to extrasolar planets using mutual events". Astronomy and Astrophysics 464 (3): 1133–1138. arXiv:astro-ph/0703609. Bibcode:2007A&A...464.1133C. doi:10.1051/0004-6361:20066111.
- Han C. & Han W. (2002). "On the Feasibility of Detecting Satellites of Extrasolar Planets via Microlensing". The Astrophysical Journal 580 (1): 490–493. arXiv:astro-ph/0207372. Bibcode:2002ApJ...580..490H. doi:10.1086/343082.
- Lewis K. M., Sackett P. S. & Mardling R. A. (2008). "Possibility of Detecting Moons of Pulsar Planets through Time-of-Arrival Analysis". The Astrophysical Journal Letters 685 (2): L153–L156. arXiv:0805.4263. Bibcode:2008ApJ...685L.153L. doi:10.1086/592743.
-  - "Mamajek thinks his team could be either observing the late stages of planet formation if the transiting object is a star or brown dwarf, or possibly moon formation if the transiting object is a giant planet"
- Российские астрономы впервые открыли луну возле экзопланеты (in Russian) - "Studying of a curve of change of shine of WASP-12b has brought to the Russian astronomers unusual result: regular splashes were found out.<...> Though stains on a star surface also can cause similar changes of shine, observable splashes are very similar on duration, a profile and amplitude that testifies for benefit of exomoon existence."
- "The Detection and Characterization of a Nontransiting Planet by Transit Timing Variations". sciencemag.org.
- Heller, René; Rory Barnes (January 2013). "Exomoon habitability constrained by illumination and tidal heating". Astrobiology (Mary Ann Liebert, Inc.) 13 (1): 18–46. arXiv:1209.5323. Bibcode:2012arXiv1209.5323H. doi:10.1089/ast.2012.0859.
- Heller, René (09 2012). "Exomoon habitability constrained by energy flux and orbital stability". Astronomy and Astrophysics 545: L8. arXiv:1209.0050. Bibcode:2012A&A...545L...8H. doi:10.1051/0004-6361/201220003.
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