Two of the new planets are part of a five-planet system orbiting a star some 1,200 light-years away in the constellation Lyra. The star, Kepler 62, is about two-thirds the size of the sun and has 70 percent of the sun's mass. It's also about 3 billion years older than the sun.
The system's three inner planets, one comparable in size to Mars, are too close to their sun to be livable. Kepler 62-e, the fourth planet out, however, falls within the habitable zone. Orbiting once every 122 days, the planet is about 60 percent larger than Earth.
The team speculates that the planet is covered with water, although the system is too far away to take the measurements needed to estimate the planet's mass. Researchers need that measurement to determine the planet's density, a major clue as to its bulk composition.
Instead, modeling studies have indicated that planets ranging from 1.5 to two times Earth's size tend to be far more watery than planets closer to Earth's size. Thus, while the nature of the planet remains speculative for now, "the fascinating idea is that we've actually found the first ocean planet, the first water world out there," said Lisa Kaltenegger, an astronomer with the Max Planck Institute for Astronomy in Germany and the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., during a mission briefing Thursday afternoon.