The Moon is ridiculously big relative to the size of Earth, and this was thought to be a cosmic rarity. Now it looks like rocky planets with huge moons are actually extremely common, which might help us find alien life.
Earth's Moon was formed billions of years ago, when an object the size of Mars smashed into our young planet. The resulting cataclysm threw up a massive molten disc of debris around our planet, which over time merged together to become our satellite. While the gas giants have moons that are larger than ours in absolute terms, relatively speaking no satellite comes close to matching the size of our moon when compared to its planet.
In fact, the Moon is so big compared to Earth that some astronomers have argued we really live in a double planet system, as both objects are large enough to meet the criteria to be considered planets. That's very much a minority view, but it speaks to to just how unique our moon really is when compared to the others in our solar system.
But it appears the Moon might be surprisingly ordinary compared to those in other solar systems. Researchers at the University of Zurich and the University of Colorado ran a series of computer simulations to see how planets form from gas and smaller chunks of rock. They then used that data to see how often huge impacts like that between Earth and the Mars-sized object might occur.
They discovered that forming something similar to an Earth-Moon system is far from a one in a million chance - if anything, it's closer to one in twelve. Specifically, they found that the chances of creating a system with a planet at least half Earth's mass and a satellite at least half the Moon's mass is between 1 in 4 and 1 in 45...but the much simpler way of saying that is just that the odds are about 1 in 12, once everything's averaged together.
What's exciting about all this is that the Moon is generally credited with a huge role in the development of life on Earth. The presence of a large satellite helped stabilize Earth's orbit, which means our climate has remained far more gentle and less volatile than it would have without the Moon around. That's not to say we needed the Moon for life to evolve, but it sure made things easier, and the fact that 1 in 12 rocky exoplanets might also have Moons is good news in the search for alien life.
We've still not seen any moons in other solar systems (or exomoons, to match exoplanets), but if any exomoons are going to be relatively easy to detect, then it will probably be large moons orbiting relatively small planets. Now that we know these big moons aren't a cosmic rarity, we can start looking for them...and that might just be the first step to finding planets with life on them.