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The Boston Globe

Science

Astronomers find Earth-like planet, but it’s infernally hot

NEW YORK — Kepler-78b, a planet 400 light-years away, is like hell on Earth.

Astronomers described it Wednesday as the first Earth-size planet that seems to be made of the same mixture of rock and iron as Earth and that orbits a star similar to our sun.

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But Kepler-78b would not be a place to visit. It whirls around its parent star, Kepler-78, at a distance of less than 1 million miles, and its year — the time it takes to complete one orbit — is just 8½ hours. (By contrast, Earth is 93 million miles from the sun.)

At that close proximity, the surface of Kepler-78b is infernally hot: 3,500 to 5,000 degrees Fahrenheit, or “well above the temperature where rock melts,” said Andrew W. Howard, an astronomer at the University of Hawaii and the lead author of one of two papers being published in the journal Nature. “This is probably one of the most hellish planets that have been discovered yet.”

Viewed from the surface of Kepler-78b, its star would cover 80 times more of the sky than the sun does in Earth’s sky.

“It’s certainly not a habitable planet,” said Francesco Pepe, a professor of astronomy at the University of Geneva and the lead author of the other Nature paper.

Kepler-78b is the newest addition to the pantheon of oddball planets in the Milky Way. The first planet discovered around another sun-like star turned out to be about the size of Jupiter but circling its star at what seemed to be an impossibly close orbit. Other discoveries over the years include a fluffy planet with a density less than that of cork.

That astronomers have already found an Earth-like planet suggests that there should be others in cooler, more life-friendly orbits.

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