It doesn't mean all are potentially habitable, but the sheer number of Earth-size planets is a welcome starting point in the search for worlds like our own.
Scientists have yet to find a twin Earth -- one that's not only the right size but also located in the so-called Goldilocks zone, a place that's not too hot and not too cold where water might exist in liquid form.
Two independent groups came up with the new estimate after a fresh analysis of data gathered by NASA's Kepler spacecraft, which was launched in 2009. One team was from the Harvard-Smithsonian Center for Astrophysics; the other from the University of California, Berkeley and University of Hawaii.
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