ANALYSIS: IAU: No, You Can't Name That Exoplanet
Exomoons are likely to be tidally locked to their planet just as our moon keeps one hemisphere facing Earth. Days would last for half of an orbital period. Seasons would be possible depending on the moon's axial tilt plus that of the host planet, and how elliptical the planet's orbit is about the central star. Eclipses of the parent star would happen frequently and this would alter the weather abruptly on a moon. Life might have a biorhythm in synch with the eclipses, just as some organisms on Earth sense the lunar phase cycle.
Living alongside a gas giant planet is likely rough on moons and could inhibit the emergence of life, say the researchers. If the moon is too close to the planet, tidal forces can make it so hot that the surface is always erupting volcanically. Jupiter's innermost major satellite Io is an example of this.
A water-laden moon with a thick atmosphere might suffer from a runaway greenhouse effect. The water vapor, combined with other greenhouse gasses such as methane and carbon dioxide might make the moon too warm, as is the case of Venus. The closer the moon is to its gas giant planet, the more radiation it will absorb, simply from starlight reflected off the planet.
But if the moons are farther out they are not protected from galactic cosmic rays by the planet's magnetic field, and are too small to generate a substantial field of their own. This radiation might sterilize the surface of life.
By analogy with the circumstellar habitable zone, these constraints define a circum-planetary "habitable edge," say the researchers. They conclude that if either exoplanet hosted a moon at a distance greater than 10 planetary radii, then it could be habitable.
Characterizing such moons is certainly an order of magnitude or more difficult that identifying inhabited planets. So although there might easily be more inhabited moons in out galaxy than planets, characterizing them is a daunting task for far future super-telescopes... or maybe starships.
Image credit: NASA