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Washington Post
This diagram compares the planets of the inner solar system to Kepler-186, a five-planet system recently discovered about 500 light-years from Earth.

New Earth-size planet found

Discovery raises hope that water, life forms exist

The hunt for Earth’s alien twin reached a new milestone with the discovery of a faraway exoplanet that’s not much bigger than our own globe and is theoretically capable of retaining liquid water.

The planet is the first Earth-size sphere found outside our solar system that also resides in the “Goldilocks zone” – the habitable range where the temperature is neither too hot nor too cold. In other words, the right conditions for life to potentially thrive.

The planet, called Kepler-186f, meets what researchers believe are two basic requirements for life. One is that its size is similar to Earth’s, which increases the chance it has a rocky, rather than gaseous, surface. The other is that it gets the right amount of stellar radiation to support liquid water, as opposed to ice or vapor.

“We don’t fully understand what makes a planet habitable, so we look for what we know,” theoretical astrophysicist Brad Hansen of UCLA said. “The basic assumption is that you need to have a rocky surface to stand on and liquid water for life to use.”

Using data gathered by NASA’s Kepler space-based telescope, astronomers discovered a group of five planets orbiting a star 500 light-years from Earth. The star, called Kepler-186, is a relatively cool red dwarf about half the size of our sun. Four of the planets venture extremely close to the star, making them too hot for liquid water – and therefore, life as we know it. But the outermost planet soaks in just enough energy for surface water to stay liquid.

Last year, the Kepler spacecraft discovered three exoplanets, all larger than Earth, within the habitable zone of two different stars. One of these three, Kepler-62f, is 40 percent larger than Earth and previously held the record for the habitable exoplanet that is closest to the size of our planet. The newly found Kepler-186f set a record by being only 10 percent larger than Earth.

“We thought it was special when we first saw the little blip in the data,” said study author and astronomer Elisa Quintana said of the SETI Institute. The findings were published Thursday in the journal Science.

To find Kepler-186f, Quintana and her colleagues sifted through the mounds of data gathered by the telescope as it scrutinized one patch of the sky continuously for four years, looking for signs of planets outside our solar system.

So Kepler-186f may be close to the Earth in size, but is it truly Earth’s twin? Most likely, no.

For one thing, the planet is colder than Earth. The amount of stellar energy it receives is only a third of the energy that the Earth gets from the sun.

“This planet actually receives less warmth than Mars does,” said astronomer David Kipping at the Harvard-Smithsonian Center for Astrophysics.

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