It is reasoned that from the empirical study of natural satellites in the Solar System that they are likely to be common elements of other planetary systems. The majority of detected extrasolar planets are gas giants. In our Solar System, the gas giants have a large collection of natural satellites, (Jupiter has 79, Saturn has 62, Uranus has 27, and Neptune has 14). Therefore, it is reasonable to assume that extrasolar moons exist and are common.
Though extrasolar moons 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 have extraterrestrial life, and one that may be a rogue planet.
Definition of satellites around brown dwarfsEdit
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.
For impact-generated moons of terrestrial planets not too far from their star, with a large planet-moon distance, it is expected that the orbital planes of moons will tend to be aligned with the planet's orbit around the star due to tides from the star, but if the planet-moon distance is small it may be inclined. For gas giants, the orbits of moons will tend to be aligned with the giant planet's equator because these formed in circumplanetary disks.
Lack of moons around planets close to their starsEdit
Planets close to their stars on circular orbits will tend to despin and become tidally locked. As the planet's rotation slows down the radius of a synchronous orbit of the planet moves outwards from the planet. For planets tidally locked to their stars, the distance from the planet at which the moon will be in a synchronous orbit around the planet is outside the Hill sphere of the planet. The Hill sphere of the planet is the region where its gravity dominates that of the star so it can hold on to its moons. Moons inside the synchronous orbit radius of a planet will spiral into the planet. Therefore, if the synchronous orbit is outside the Hill sphere, then all moons will spiral into the planet. If the synchronous orbit is not three-body stable then moons outside this radius will escape orbit before they reach the synchronous orbit.
In December 2013, a candidate exomoon of a free-floating planet MOA-2011-BLG-262, was announced, but due to degeneracies in the modelling of the microlensing event, the observations can also be explained as a Neptune-mass planet orbiting a low-mass red dwarf, a scenario the authors consider to be more likely. This candidate also featured in the news a few months later in April 2014.
In October 2018, researchers using the Hubble Space Telescope published observations of a candidate exomoon orbiting Kepler-1625b, named Kepler-1625b I. The discovery of this exomoon suggests that the host planet is likely several Jupiter masses, while the moon might have a mass and radius similar to that of the planet Neptune. The study concluded that the exomoon hypothesis is the simplest and best explanation for the available observations, though warned that it is difficult to assign a precise probability to its existence and nature. Some argue that this may be an example of a double planet rather than an exomoon.
|Planet||Mass||Semimajor axis||Exomoon semimajor axis||Exomoon mass||Notes|
|1SWASP J140747.93-394542.6 b||14-26 Jupiter mass||2.2-5.6 AU||0.40 AU||<0.8 Earth mass||Possible exomoon residing in a large ring gap around the planet|
|WASP-12b||1.35 Jupiter mass||0.0221-0.0237 AU||?||0.57-6.4 Earth mass||Found by studying periodic increases and decreases in light given off from WASP-12b.|
|MOA-2011-BLG-262||<189 Jupiter mass||N/A||?||8-46 Earth mass||Rogue planet. Found by microlensing; however it is unknown if the system is a low Neptune-mass planet orbiting a free floating planet, or a low Jupiter-mass planet orbiting a low-mass red dwarf.|
|Kepler-1625b||10 Jupiter mass||0.85 AU||0.0023||~17-19 Earth mass||Possible Neptune-sized exomoon or double planet, indicated by transit observations.|
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 at around 0.2 solar masses.
Taking as an example the smaller Europa, at less than 1% the mass of the Earth, Lehmer et al. found if it 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