As some scientists search for habitable planets outside our solar system, other researchers are tackling a similar question for the moons of these planets. So-called exomoons have yet to be found outside our solar system, and a detection could be a decade away - or more.
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.)