Earth has a key ingredient for life: water. As Earth’s atmosphere was still forming, the first signs of life, in the form of microbes, emerged. This early life broke down carbon dioxide and produced oxygen. Mixed with nitrogen – its main ingredient – and argon, this makes up the mixture of gases we call ‘air’. Gravity plays a key role in keeping the air in place. Other planets and moons that don’t have gravity as strong as Earth can’t keep an atmosphere, just as those with gravity too strong end up compressing theirs much more strongly.