For the first time, NASA's Kepler spacecraft has found two Earth-size planets outside our solar system – a landmark achievement. But the planets are in a solar system that baffles scientists and could overthrow current models of planet formation.
Harvard-Smithsonian Center for Astrophysics/AP
Scientists have found the first Earth-size planets orbiting a star like the sun, but the pair appear in a solar system so bizarre that it is for now upending current explanations for how solar systems form, the discoverers say.
The two planets, thought to be rocky, form a kind of cosmic triple-decker sandwich, with each interspersed among three Neptune-scale gas planets. All five are closer to their host star than Mercury is to the sun, meaning they are too hot for life.
But the find is proof that NASA's Kepler spacecraft can find Earth-size planets orbiting distant stars. Kepler 20e is slightly smaller than Venus, or about 0.87 times Earth's size. Kepler 20f is 1.03 times Earth's size.
Combined with the discovery, announced Dec. 5, of a "super Earth" in another star's habitable zone, these new planets move the Kepler team closer to its goal: detecting Earth-size planets in their stars' habitable zones – orbital distances where temperatures on the planet are warm enough to allow water to remain stable on the surface.
The newest discovery is "the most important milestone" for the Kepler team, says Francois Fressin, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and the lead author of the team's formal report, which is being published by the journal Nature.