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Astrobiology is the study of life in the universe from its origins to its evolution into intelligent sentient beings. All life as we know it is carbon-based, reliant on sources of liquid water and energy for its survival, and as far as we are aware, exists only on Earth. Our planet occupies a unique spot in the solar system. It is just the right distance from the Sun--within the so-called Goldilocks Zone--to be not too hot or too cold for liquid water to be stable on its surface, which, together with a protective shielding atmosphere, allowed the four-billion-year journey from a single-celled organism to an upright humanoid species. Most of primordial life, if seen today, would be classified as "alien," as it bears little resemblance to anything that currently exists.