This artist's rendering provided by NASA on Thursday, April 17, 2014 shows an Earth-sized planet dubbed Kepler-186f orbiting a star 500 light-years from Earth. Astronomers say the planet may hold water on its surface and is the best candidate yet of a habitable planet in the ongoing search for an Earth twin. (Photo by T. Pyle/AP Photo/NASA Ames/SETI Institute/JPL-Caltech)
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