Two new Earth-sized exoplanets discovered

Hunting for habitable worlds, NASA's Kepler space telescope has unveiled two new planets, some 950 light-years away, that are the smallest yet detected, and the closest in size to Earth.

Two new exoplanets, Kepler 20e and 20f, are part of a five-planet system orbiting a sun-like star, similar to the artist's rendering above. Researchers have found the new planets are likely scorching hot, circling their star at a much closer distance than Mercury orbits the Sun. Image: Tim Pyle/NASA

In a paper published this week in Nature, scientists from MIT and elsewhere report that the planets - one just about Earth's size, and the other a bit smaller - likely have rocky compositions, similar to Earth, and orbit a star much like the sun. But that's where the similarities end.

Compared with Earth's leisurely 365-day orbit, the new planets practically whiz around their star in a matter of days or weeks. Their tight circuits, closer even than Mercury's orbit around our sun, make the planets extremely hot - most likely too hot to sustain life. While either planet is far from Earth's twin, scientists say the discovery is a technological milestone.

''For the Kepler space telescope, it's extremely significant, because it proves we can reach down to Earth's size,'' says co-author Sara Seager, the Ellen Swallow Richards Professor of Planetary Science and Professor of Physics at MIT. ''It's a massive accomplishment just to find anything at all like this.''

The new planets, which orbit the star Kepler 20, are part of a five-planet system, and have been named Kepler 20e and Kepler 20f.

''This will hopefully open the floodgates to discovering more of these Earth-sized and sub-Earth-sized planets,'' says co-author Leslie Rogers, a physics graduate student at MIT. ''Then we'll be able to determine some sort of context for how common are habitable Earth-sized planets.''