But scientists, writing in a new research paper, theorize a Mars-sized exomoon of a gas giant planet and ask whether or not liquid water could be found on its surface.
In our own solar system, the closest analog is Jupiter's Ganymede, the biggest moon in the solar system and about five-sixth the size of Mars.
NASA confirmed in 2015 the presence of a liquid ocean on Ganymede after performing Hubble Space Telescope observations of the moon's auroras, which appear to rock back and forth less than expected with Jupiter's magnetic field. The space agency said the attenuation is likely due to a salty ocean under Ganymede's surface.
As for this theoretical Mars-sized exomoon, the picture is murky. The scientists considered energy sources such as stellar radiation (which changes as a function of distance to the star), the stellar reflected light from a Jupiter-sized planet on the moon, the planet's own thermal emission on the moon, and the tidal heating inside the moon that is generated due to the changing gravitational pull of the planet. (This tidal heating would be most pronounced if the moon had an eccentric orbit, like the volcanic moon Io has around Jupiter.)
It's know that tidal heating rates decrease if a moon is molten inside, because lava creates an inherent negative feedback mechanism where the heating sort of switches off, and the moon cools down inside. This is called the "tidal thermostat effect," co-author of the paper Rene Heller said in an email. Heller is an astrophysicist at the Max Planck Institute for Solar System Research in Germany.
"We investigate, for the first time, the interplay of all the possible exomoon heat sources as a function of various distances from the host star," he added. "Actually, we even consider two possible types of host stars: a sun-like star, and a red dwarf star (an M dwarf)."
For a sun-like star, the authors found that any moon around a gas giant beyond three astronomical units, or three Earth-sun distances, would have a high enough energy flux to stop the tidal thermostat effect from happening. But if the moon is volatile enough, it could have global volcanism - just like what we see on Io.
Heller described this situation as "dangerous" for organisms.
"They might have lots of liquid surface water, but their surfaces could at the same time be blotched with devastating volcanoes," he wrote. "Nevertheless, we illustrate that they could be habitable given the right amount of tidal heating, and we show at which distances to their planets these moons would need to be."
M-dwarfs are a common target for exoplanet searches because they are smaller and dimmer, making it easier to see planets passing across their surfaces, or the effect of planets tugging on the star itself. But for exomoons, it's even less clear how habitable they would be in such a system. "Moons cannot be stable in the very inner regions of the stellar habitable zone," Heller said.
The best examples for tidally heated bodies in our own solar system are all moons: Jupiter's Io and Europa, as well as Saturn's Enceladus. While Europa and Enceladus are strongly suspected to have oceans underneath an icy surface, Heller pointed out his research is more focused on habitability on the surface of the moon. A better analog, he said, might be Saturn's moon Titan - but with a much warmer surface. Titan has a thick orange atmosphere, as well as liquid hydrocarbon lakes.
"Due to observational selection effects, which will prefer big moons around low-mass planets, I think the first exomoon will be unlike anything we know from the solar system," Heller said.
Read more at Discovery News
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