|υ Andromedae d and its potential moons|
The dream of astronomers and astrobiologists in particular is to find a planet in a star's habitable zone with atmospheric properties very similar to Earth's. This planet could potentially harbor some form of life.
Of course, this "planet" could actually be a moon, if the primary planet orbits the sun at a close enough distance.
Image from Wikipedia - υ Andromedae d.