Habitable exomoon

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A habitable exomoon is a moon orbiting an extrasolar planet that has the ideal conditions to host life as we know it. A total of 21 exomoon candidates have been detected, but none of them have been confirmed.[1]

Habitability of extrasolar moons will depend on stellar and planetary illumination on moons as well as the effect of eclipses on their orbit-averaged surface illumination. Beyond that, tidal heating might play a role for a moon's habitability. In 2012, scientists introduced a concept to define the habitable orbits of moons; they define an inner border of an habitable moon around a certain planet and call it the circumplanetary "habitable edge". Moons closer to their planet than the habitable edge are uninhabitable.[2]

Characteristics[]

Lehmer et al. found if the moon of Europa were to end up near to Earth orbit it would only be able to hold onto its atmosphere for a few million years. However, for any larger, Ganymede-sized moons venturing into its solar system's habitable zone, an atmosphere and surface water could be retained pretty much indefinitely. Models for moon formation suggest the formation of even more massive moons than Ganymede is common around many of the super-Jovian exoplanets.[3]

Martínez-Rodríguez et al. found exomoons with the mass of Mars around IL Aquarii b and c could be stable on timescales above the Hubble time. Like an exoplanet, an exomoon can potentially become tidally locked to its primary. However, since the exomoon's primary is an exoplanet, it would continue to rotate relative to its star after becoming tidally locked, and thus would still experience a day/night cycle indefinitely. The CHEOPS mission could detect exomoons around the brightest M-dwarfs or ESPRESSO could detect the Rossiter–McLaughlin effect caused by the exomoons.[4]

Host planet[]

Given the general planet-to-satellite mass ratio of 10000,[5] large Saturn or Jupiter-sized gas planets in the habitable zone are believed to be the best candidates to harbour Earth-like moons, with more than 120 such planets by 2018.[6]

Massive exoplanets known to be located within a habitable zone (such as Gliese 876 b, 55 Cancri f, Upsilon Andromedae d, 47 Ursae Majoris b, HD 28185 b and HD 37124 c) are of particular interest as they may potentially possess natural satellites with liquid water on the surface. It was also found that moons at distances between about 5 and 20 planetary radii from a giant planet could be habitable from an illumination and tidal heating point of view.

Host star[]

There is a minimum mass of roughly 0.20 solar masses for stars to host habitable moons within the stellar habitable zone. René Heller & Rory Barnes found that, depending on a moon's orbital eccentricity, there is a minimum mass for stars to host habitable moons at around 0.2 solar masses.[7]

Earth-sized exoplanets in the habitable zone around M-dwarfs are often tidally locked to the host star. This has the effect that one hemisphere always faces the star, while the other remains in darkness. An exomoon in an M-dwarf system does not face this challenge, as it is tidally locked to the planet and it would receive light for both hemispheres.

See also[]

References[]

  1. ^ "Habitable exomoon". Exoplanet Exploration: Planets Beyond our Solar System. Retrieved 2020-07-08.
  2. ^ Kohler, Susanna (29 August 2018). "Habitable Moons Instead of Habitable Planets?". AAS Nova. Retrieved 2020-07-08.
  3. ^ Hadhazy, Adam (March 4, 2013). "The 'Habitable Edge' of Exomoons". Astrobiology Magazine. Retrieved 2020-07-08.
  4. ^ Martínez-Rodríguez, Héctor; Caballero, José Antonio; Cifuentes, Carlos; Piro, Anthony L.; Barnes, Rory (2019-10-26). "Exomoons in the habitable zones of M dwarfs". The Astrophysical Journal. 887 (2): 261. arXiv:1910.12054. doi:10.3847/1538-4357/ab5640. S2CID 204904780.
  5. ^ Canup, Robin M.; Ward, William R. (June 2006). "A common mass scaling for satellite systems of gaseous planets". Nature. 441 (7095): 834–839. doi:10.1038/nature04860. ISSN 1476-4687.
  6. ^ Jorgenson, Amber (5 June 2018). "Kepler data reveals 121 gas giants that could harbor habitable moons". Astronomy.com. Retrieved 2020-07-08.
  7. ^ Heller, René; Barnes, Rory (January 2013). "Exomoon Habitability Constrained by Illumination and Tidal Heating". Astrobiology. 13 (1): 18–46. doi:10.1089/ast.2012.0859. ISSN 1531-1074. PMC 3549631. PMID 23305357.
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