Tuesday, 8 February 2011

What are the odds of a planet having a large moon?

As Dr. Maggie Alderin-Pocock pointed out in Do We Really Need The Moon?, our Moon is unusually large in comparison to its parent planet, and the large tidal forces it generates are thought to have been very important to the origin of life on Earth. She therefore suggested that when looking for life on other planets, we should concentrate out efforts on other planets that have large moons.

Our Moon, according to the best theories we have, was formed when another planet, the size of Mars, crashed into the early Earth. This, at first, sounds like a spectacularly unlikely event. It may sound quite discouraging if you were hoping to get in touch with aliens. However, models of the early solar system show that it was quite a violent, chaotic place, and space rocks crashing into each other is pretty much how the inner planets were formed. So, is there any way we can calculate the probability of a planet having a large moon like ours?

The best way is to make an empirical estimate, and it turns out to be a lot easier to do than you might have expected, and to have quite encouraging results. There are four rocky planets in the inner solar system. One has a large moon. So the odds of a rocky planet having a large moon, based on the available evidence, is 1/4. (± √3/8)