Earth is the only planet in the universe known to possess life. There are several million known species of life, ranging from the bottom of the deepest ocean to a few miles into the atmosphere, and scientists think far more remain to be discovered. Scientists figure there are between 5 million and 100 million species on Earth, but science has only identified about 2 million of them.
Earth is the only body in the solar system known to host life, although scientists suspect that other candidates — such as Saturn’s moon Titan or Jupiter’s moon Europa — have the potential to house primitive living creatures. Scientists have yet to precisely nail down exactly how complex life rapidly evolved on Earth from more primitive ancestors. One solution suggests that life first evolved on the nearby planet Mars, once a habitable planet, then traveled to Earth on meteorites hurled from the Red Planet.
Additional reporting by Nola Taylor Redd, Space.com Contributor
Earth has a diameter of roughly 8,000 miles (13,000 kilometers), and is round because gravity pulls matter into a ball, although it is not perfectly round, instead being more of an "oblate spheroid" whose spin causes it to be squashed at its poles and swollen at the equator
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