The search for life beyond Earth fascinates many and inspires big questions: Are we truly alone in the universe? Is our Earth unique? Is it possible that life beyond Earth may actually be far from little green aliens and much closer to the microbial life with which we share our planet?
Single-celled organisms were the first life forms that evolved on Earth billions of years ago and have been around much longer than humans and other multicellular organisms. They are also metabolically diverse and can thrive in environments that we humans deem extreme—such as at the bottom of the ocean in piping-hot hydrothermal vents, in extremely salty lakes, and even within rocks.
The first place to look for life outside Earth is within our solar system, where distances between us and potentially habitable worlds are still manageable for spacecraft flybys and even sampling missions. Venus, Mars, and many moons of Jupiter and Saturn are all of interest to astrobiologists, though Europa, one of Jupiter's 95 moons, is a particularly promising candidate world. Europa is an icy ocean world where plumes of water spurt from an ocean below a thick ice crust.
Though the surface temperature perpetually lurks below a cool -220°F, Europa excites many astrobiologists as a possible site for life in our solar system because of its subglacial ocean. Water is important for the habitability of a planet by life as we know it; a polar solvent like water is essential for the biochemical reactions that drive all life on Earth and can also provide a thermally stable habitat for organisms to live and evolve.
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