Free Astronomy Magazine November-December 2018

ASTROBIOLOGY 39 oon awaits NOVEMBER-DECEMBER 2018 T he planetary system de- picted in the back- ground is an art- work, but it effec- tively illustrates how an Earth- sized moon could orbit around a giant planet. A bout 200 moons are orbiting the planets of our Solar System. Among them, Ganymede, Titan and Callisto have dimensions comparable to those of Mercury, and if they or their associated planets had formed within the habitable zone of our Sun, these moons might have formed environments hospitable to life. It is likely that the other planetary systems of our galaxy (and perhaps of the whole uni- verse) also contain a large number of moons, some of which are of planetary in size. But as we are limited to the moons of our Solar system for knowledge, we can only assume that the oldest possible extrasolar moons around a given star were born from to the accretion of material left behind by the for- mation of the planet around which they orbit or, similarly, material resulting from planetary impacts between protoplanets occurring in primordial epochs (as hap- pened for the Moon and Charon).