... Kepler discovers smallest "habitable zone" planets to date . ... The artist's concept depicts NASA's Kepler misssion's smallest habitable zone planet. Seen in the foreground is Kepler-62f, a super-Earth-sized planet in the habitable zone of a star smaller and cooler than the Sun. ... So far, the Kepler spacecraft has discovered four habitable-zone planets, as illustrated here with their relative sizes compared to Earth: (from left) Kepler-22b, Kepler-69c, Kepler-62e, Kepler-62f, and Earth. ...