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. Dr. Howard’s team used the Keck 1 telescope in Hawaii; Dr. 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 teams coordinated their work, agreeing to publish their results at the same time, but they did not collaborate. 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, the same as Earth’s, 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,” Dr. 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,” Dr. Sasselov said. “If you want me to choose out of four bad ones, that’s probably the one which seems least so.”