Cosmic Perspectives from Amid the Stars:
Astronomy Blog

Earth is the only planet we know in the entire universe to support life. In a universe as big as ours, life likely exists out there somewhere by sheer odds alone. But for now, Earth is all we know and the only example of a habitable planet we can study. So why can Earth support life when so many other planets fail? In order to predict what other planets might support life, we first need to understand life on Earth. What is it that makes Earth perfect for life?