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Astronomers have discovered an exoplanet that orbits its star every 704 days, the longest known year for an exoplanet, according to the Harvard-Smithsonian Center for Astrophysics.

Kepler-421b orbits a star about 1,000 light years from Earth at the "frost line" — the crucial distance that divides rocky planets from gaseous planets.

The discovery shows that scientists' understanding of how planets form is "not too far off," said Steve Howell, project scientist for NASA's Kepler Mission, in an e-mail to USA TODAY Network.

The frost line is "a distance at which it is believed gas and ice giant planets form. Kepler-421b provides support for this idea," Howell wrote.

At this distance of 110 million miles from its star, Kepler-421b is cold enough for little ice grains to form and stick together to form gaseous planets, said David Kipping, an astronomer with the center and lead author on a paper about Kepler-421b's discovery accepted for publication in The Astrophysical Journal.

Within the frost line, the ice grains "boil off, and you get something like Earth," Kipping told USA TODAY Network.

In our solar system, the frost line would be between Mars and Jupiter, he said.

KEPLER FIND: Another Earth-size planet

A planet at this "special" point was expected to exist, "but we didn't really have this evidence" before Kepler-421b's discovery, Kipping said.

Finding a planet like Kepler 421-b is rare. NASA's Kepler telescope typically spots planets that are "really, really close to their star," Kipping said. To see a planet so far away from its star requires the planet and its star to be in the telescope's field of vision, he said.

The Uranus-sized planet — about four times the size of Earth — will be one of the exoplanets astronomers will continue to study.

Although it's not a candidate for humans to live on, Kepler-421b may have an "Earth-like" moon that is habitable, Kipping said.

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