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Around a Star in 8.5 Hours

MIT scientists discover an exoplanet with an extremely short
orbital period.

In the time it takes you to complete a single workday or get a full night’s sleep, a small fireball of a planet 700 light-years away has already completed an entire year.

“We’ve gotten used to planets having orbits of a few days,” says Josh Winn, associate professor of physics. “But we wondered, what about a few hours? Is that even possible? And sure enough, there are some out there.”

Researchers at MIT have discovered an Earth-size exoplanet named Kepler 78b that whips around its host star in a mere 8.5 hours, one of the shortest orbital periods ever detected. The planet is extremely close to its star—the distance is only about three times the star’s radius—and the scientists have estimated that its surface may be as hot as 3,000 K, or more than 2,760 °C. In such a scorching environment, the top layer of the planet is probably completely melted, creating a massive roiling ocean of lava. The researchers reported their discovery in the Astrophysical Journal.

What’s most exciting to scientists is that they were able to detect not only reflected light, but also light emitted by the planet—the first time researchers have been able to do so for such a small exoplanet. Analyzing both kinds of light with larger telescopes may give scientists detailed information about the planet’s surface composition and reflective properties.

Scientists hope they will be able to measure the planet’s gravitational influence on its star, which could help them calculate the planet’s mass. That could make Kepler 78b the first Earth-size planet outside our own solar system whose mass is known.

A previously discovered exoplanet, known as KOI 1843.03, has an even shorter orbital period: just four hours, 15 minutes. In a paper published in Astrophysical Journal Letters, members of the group that discovered Kepler 78b, along with others at MIT and elsewhere, described observations of that planet and determined that in order for it to maintain its extremely tight orbit around its star, it would have to be incredibly dense to keep tidal forces from ripping it to pieces. They think the planet is more than 70 percent iron.

“Just the fact that it’s able to survive there implies that it’s very dense,” says Josh Winn, an associate professor of physics at MIT and a coauthor on both papers. “Whether nature actually makes planets that are dense enough to survive even closer in—that’s an open question.” Finding any, he says, “would be even more amazing.”

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