Kepler-78b: This scorched planet is more like Earth than you think

This artist's rendition released by NASA shows Kepler-78b orbiting its parent star.
(David A. Aguilar / NASA )

Astronomers have discovered a world that is incredibly close to Earth-sized and may also be made of rock and iron, like our own planet. The only problem? It’s a scorched hellscape of molten lava that sits less than a million miles from its star’s surface, and it shouldn’t exist in the first place.

This planet defies astronomers’ expectations even as it lends hope that increasingly Earth-like planets may be found (though hopefully farther away from their parent stars, in more bearable climes).

“For the first time we have something pretty close to the size of the Earth that has the same density as the Earth,” said astronomer David Latham of the Harvard-Smithsonian Center for Astrophysics, one of the coauthors of one of two studies published in Nature.

The planet, named Kepler-78b, “foreshadows leaps forward in the search for life beyond the solar system,” Drake Deming, an astronomer at the University of Maryland at College Park who was not involved in the paper, wrote in a commentary.


Kepler-78b has a mass that’s 80% greater than Earth’s and a radius that’s only 20% longer, which comes out to a density that nearly matches Earth’s density of 5.5 and makes it “a virtual twin of Earth by astronomical standards,” Deming wrote.

But the planet defies theoretical expectations for how planets formed. Scientists really aren’t sure how it got there, given that its parent star was bigger when it was younger. If the planet had formed where it is today, it would have been inside the star.

“That doesn’t work – so it would have to form further away from the star and then gradually migrate its way in to where it is now,” Latham said. “But that’s got a bit of a problem too because it’s so close.”

The planet is doomed, he added. In a few hundred million years, “it’ll get swallowed by the star.”

The rocky planet was discovered using the Kepler Space Telescope, which until its recent crippling would stare at a single patch of the heavens, waiting for dips in starlight that meant a planet was crossing in front of its star.

Finding such a small planet is a challenge. But because the planet is so close to its star, it completes a full orbit in just 8.5 hours, and its many transits in a short amount of time made it easier to pick out.

Figuring out its mass (and density) was another challenge. That requires a whole other technique called radial velocity, which uses the Doppler effect to measure the little wobbles in the star caused by the planet’s gravitational tug. This wobble causes the star’s light to stretch to redder wavelengths and squeeze to bluer ones, and scientists can pick out this red shift and blue shift and determine how big the troublesome planet must be.

Using data from the HARPS-N instrument on the Galileo National Telescope in Italy, the researchers were able to determine this planet’s mass (and thus the density), but it’s too difficult to use the radial velocity technique to find mass for every planet they’ve found using Kepler, Latham said.

“The Doppler measurements will be the bottleneck for the foreseeable future,” Latham said.

But they do hope it’s the start of something.

“The existence of Kepler-78b shows that, at the very least, extrasolar planets of Earth-like composition are not rare,” Deming said.


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