In May, NASA announced the discovery of 1,284 new planets beyond our solar system. It’s exciting news. The idea of life on alien worlds has always had the power to set the human imagination alight.

But before we get too excited, let’s consider just how unlikely life here on Earth actually is. Most astronomers agree that a very long list of specific preconditions must be in place before life can emerge on a given world. Did you know we might not be here at all if the moon didn’t help stabilize our climate? Or if Jupiter didn’t shield us from asteroid and comet strikes?

The point is, life is fickle. It only happens when the conditions are just right.