So far, Earth has the only life that we know of. In studying how life, especially microbial life, has transformed our world, we learn what other living worlds may be like. And in studying the amazing variety of life here, we learn about possible evidence to look for on alien worlds elsewhere in the universe.

Where do we find life on Earth? The answer guides our search for life elsewhere in the cosmos. One surprising discovery is that life exists on our planet in places where we never expected it: in rocks nearly a mile deep, underneath and inside glaciers, near superheated steam vents on the ocean floor, in hot springs, and in other extreme environments.

Most “extremophiles” (lovers of extreme conditions) are single-celled microbes. Similar life forms may exist in extreme conditions on other worlds.

Who knows what kind of life we may find in space? This artist’s view from a passing asteroid shows an Earth-like planet in the habitable zone around a double star system. The habitable zone is the region in a solar system where water can exist in its liquid state on a planet’s surface.