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

Astronomers described it this week 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.

But Kepler-78b would not be a pleasant 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 81⁄2 hours. (By contrast, Earth is 93 million miles from the sun and, of course, completes its yearly orbit in a little more than 365 days.)

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 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 sunlike star turned out to be about the size of Jupiter, but orbiting 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 and a planet blacker than coal.

“Exoplanets are just surprising us with their diversity,” said Dimitar Sasselov, a professor of astronomy at Harvard and a member of Pepe’s team, using the name for planets outside our solar system.

Kepler-78b is one of more than 150 planets spotted by NASA’s Kepler spacecraft, which noted the dimming of the starlight when a planet passed in front.

Those findings were published in August. But while Kepler can determine exoplanets’ size and orbit, it cannot measure their mass. For that, two teams of astronomers looked at Kepler-78b star from Earth. Howard’s team used the Keck 1 telescope in Hawaii; Pepe’s team used a telescope in the Canary Islands. They could not directly see the planet, but they could spot undulations in the frequency of light from the star caused by the gravitational pull of the planet. The heavier the planet, the larger the swings in frequency.

The two teams coordinated their work, agreeing to publish their results at the same time, but they did not collaborate. Indeed, they decided that they would not exchange their data and answers until their papers were almost complete so that each would serve as an independent check on the other.

In the end, the two teams came up with nearly identical answers. The density of Kepler-78b is 0.2 pounds per cubic inch. That is also the density of Earth, suggesting that the two planets’ makeup is very similar — an iron core with rocky, if melted, outer layers.

“It’s the first really well measured Earthlike composition for a rocky extrasolar planet,” said L. Drake Deming, a professor of astronomy at the University of Maryland who was not a member of either team but wrote an accompanying commentary for Nature.

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

“You can reasonably conclude from that that it’s not rare, because you’ve found it pretty easily,” he said.

That still leaves a mystery: how Kepler-78b got where it is.

“Right now, we have no clue,” Sasselov said.

It could not have formed there, because the star as a youngster would have extended into its orbit. A near-miss with another planet could have flung it toward the star, but in that case its orbit would have been elliptical, not circular. Or it was nudged inward by the material that formed the planets.

Another possibility is that it was originally a gas giant like Saturn and that as the planet spiraled in toward the star, all of the gases were stripped away, leaving just the rocky core at the center.

“Right now, this scenario doesn’t work either,” Sasselov said. “If you want me to choose out of four bad ones, that’s probably the one which seems least so.”

Kepler-78b appears to be similar to our own world, but much closer to its sun.