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Astronomers have found the first Earth-sized exoplanet within a star's habitable zone. The planet is the closest thing yet to the coveted 'Goldilocks' orb that scientists have long sought — a world roughly the size of Earth orbiting a star at a distance that is just right for liquid water to exist.

"We definitely think it's one step closer to finding a true Sun–Earth analogue," says study co-author Elisa Quintana, an astronomer at the SETI Institute in Mountain View, California, and at the nearby NASA Ames Research Center in Moffett Field. But because the star that the exoplanet orbits is a cool, dim one unlike the Sun, Quintana and her colleagues consider the planet more of a cousin to Earth than a twin.