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First Earth-size planets found around distant star – in a bizarre solar system

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. 

This illustration shows artist's renderings of planets Kepler 20e and Kepler 20f compared with Venus and the Earth.

Harvard-Smithsonian Center for Astrophysics/AP

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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.

Kepler uncovered the two Earth-size planets 1,000 light-years away by tracking the changes the brightness of light coming from their host star, Kepler 20, as the planets pass in front of it. Kepler 20e orbits its sun once every 6.1 days at an average distance of 4.7 million miles. Kepler 20f orbits once in 19.6 days at a distance of 10.3 million miles. 

Earth, by contrast, is 93 million miles from the sun.

The team doesn't yet have an independent confirmation of the planets' masses, but given their sizes and orbits, the planets likely are rocky – probably composed of silicates and iron, as is Earth – according to current models of how solar systems form.

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