 It's hard to imagine life without Earth. We take the vast variety of life on this planet and even our own existence for granted. But numerous times in our planet's history, life as we know it has come close to disappearing entirely. We call these events mass extinctions. And we even teach schoolchildren about those times of great death on our planet. For example, we know that long ago on a much more unstable planet, the dinosaurs were killed by an asteroid striking the Earth.