Relative sizes of Kepler habitable zone planets discovered as of April 18, 2013 in this artist’s rendition provided by NASA. (L to R) Kepler-22b,
Relative sizes of Kepler habitable zone planets discovered as of April 18, 2013 in this artist's rendition provided by NASA. (L to R) Kepler-22b, Kepler-69c, Kepler-62e, Kepler 62f and Earth. Scientists using NASA's Kepler space telescope have found the best candidates yet for habitable worlds beyond the solar system officials said Thursday. REUTERS/NASA Amers/JPL-Caltech/Handout (HANDOUT)