Science in Mind

Earth-sized planet discovered surprisingly close to its sun

An artist’s rendering of the hot, rocky world, Kepler 78b.
An artist’s rendering of the hot, rocky world, Kepler 78b.Credit: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics
An artist’s rendering of the hot, rocky world, Kepler 78b.David A. Aguilar, Harvard-Smithsonian Center for Astrophysics

A few years ago, a team of scientists began to wonder whether there might be other solar systems that look wildly different than ours.

The search for Earth-like planets orbiting other suns has been incredibly fruitful, bringing scientists closer and closer to the goal of finding a habitable world—a planet somewhere in the universe where life could plausibly emerge. But the discovery of these exoplanets provided evidence of a kind of solar system zoo—other configurations of stars and planets that don’t look much like ours. And they raised questions about theories that described how solar systems and planets form in the first place. What were the outer limits of what was possible, the scientists began to wonder?

“How close can a planet really be to a star?” said Joshua Winn, an associate professor of physics at MIT. “We’ve kind of gotten used to the idea that giant planets at least can be very close to their stars, because the very first exoplanet discovered back in 1995 went around every three days or so. But we were interested in three hours.”

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Two teams of scientists described a small, rocky planet that whips around its sun in 8.5 hours in the journal Nature on Wednesday. To detect the planet, scientists depended on the NASA space-based telescope, Kepler, which can detect the dip in the light that occurs as a planet passes between a star and the telescope, blocking starlight. Then, they used ground-based telescopes to observe the star’s light in greater detail and use that to estimate the planet’s mass—about 1.7 times as heavy as the Earth, and about as dense.

The rocky world, called Kepler 78b, is far from habitable—it’s likely to be a few thousand degrees hotter than Earth. But it raises plenty of questions about how it got there that could fuel further study, in part because it isn’t an anomaly.

MIT graduate student Roberto Sanchis-Ojeda made the search for extremely close planets the subject of his thesis, and he has now discovered about 100 small planets that circle their stars in less than a day. Winn said that there are probably about three to five such close-in planets for every thousand stars. One of the planets they’ve identified takes only four and a quarter hours to make an orbit. It’s even possible, Winn conceded, that there could have once been more of them—if there had been one in our own solar system that was pulled into the sun by tidal gravity, it could have vanished without a trace.

What may be most perplexing about Kepler-78b is how it got there.

The traditional story about how planets form is one of aggregation: dust grains come together, joined by larger rocks, becoming an asteroid, and eventually accruing enough mass to form a planet. But that story can’t possibly describe how Kepler-78b formed; congealing out of tiny bits of rock and dust would be impossible so close to the sun, because all those particles would be vaporized.

“It must have formed somewhere else, and spiraled in and gotten knocked into this very tight orbit around a star. Nobody knows how that happens. It’s one of the big preoccupations” in the field, Winn said. “We already had this problem with the [close] giant planets. Now we have this question renewed, because of these rocky planets that are even closer. That’s going to be fun to try and find out what it means.”

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