"Thetis Moon" © DigitalBlasphemy.com
Recent work by Caleb Scharf, Columbia University's Director of Astrobiology, points to the possibility that habitable moons may not be rare - they may even be as common as habitable planets.
Take a look at our own solar system as an example. Our solar system has several moons that, if orbited the sun instead of a planet, would be large enough to be considered planets themselves. Several of them have atmospheres, and at least one, Europa, is almost certain to have liquid water - though recent articles, which will be discussed in later posts, suggest that life can exist in ice, and that, thus, liquid water may not be necessary for life to exist (though it may be necessary for sentient life to evolve).
If our system is not unusual, then it should be common for extrasolar systems to have many moons as well, some of them large enough to have atmospheres and to retain water. As we've seen in previous posts, water is found to be rather common in planetary discs and systems (1).
However, a heat source is needed. And to the rescue comes tidal forces: tidal forces caused by the moon's parent planet which will create internal heat for the moon. This is due to the fact that gravity decreases with distance and the gravitational pull on the near side of the moon is greater than the gravitational pull on the far side. For a moon in a circular orbit, the moon will adjust its shape to adapt to this gravitational differential, and no tidal heating will occur. But for a moon in an eccentric orbit, the gravitational differential will change rhythmically, and the moon will be kneaded like a lump of bread dough (OK, a bit of an exaggeration). This will heat a moon even if it is outside of the solar system's main habitable zone. This increases the areas in a solar system where life can form.
So when we are looking for extrasolar life, we need to look at large moons as well as planets. Right now our technology allows us to detect only large gas giants. However, rapid advances will (possibly as early as this year) allow the ability to detect terrestrial planets. The ability to detect water planets is on the horizon as well. I can not say if the ability to detect habitable moons will exist in the near future, but I would not rule it out.
1. Many chemicals are found in space, including interstellar gas clouds of sugar and of beer!
Browne, Malcom W. "Alcohol-Laden Cloud Holds the Story of a Star." New York Times. 30 May 1995. 5 February 2008. <http://query.nytimes.com/gst/fullpage.html?res=990CE7D81531F933A05756C0A963958260>.
Scharf, Caleb A. “The potential for tidally heated icy and temperate moons around exoplanets.” Astrophysical Journal. 648 (2006) 1196-1205.