Rare Moon, Plentiful Moons

NASA astronomers studying other star systems using the Spitzer Space Telescope contend that bodies like Earth's Moon are fairly rare. They base their conclusion on the paucity of dust they observe around other stars.

Current planetary formation theory holds that planets form early in a star system's history, when large amounts of dust are around. That dust eventually forms bodies like planets, but that's not the end of the process. After planets have established themselves, smaller bodies would still be ripping around, crashing into the planets or each other. Those collisions, soon after the era of planet formation, produce bodies like Earth's Moon-- plus huge amounts of dust. So, the astronomers reasoned, a star system of a certain age without lots of dust is also likely a star system without many bodies comparable to our Moon. The majority of systems, it turns out, fit that bill.

On the other hand, there's a huge number of stars, and almost certainly an even bigger number of planets. So, even though the process leading to the development of bodies like the Moon may be fairly rare, there are still likely millions or billions of such worlds orbiting planets. Such is the magic of mathematics.