A new year every four hours? It's possible with exoplanets

Image: Hot Earth-sized planet
An artist's conception shows a hot, Earth-sized planet in a close orbit around its sun.

Astronomers have detected a scorching-hot planet that zooms around its alien sun in just eight and a half hours. And if you think that's something, wait until you hear about the iron planet that spins through a whole year in just a little more than four hours.

Two overlapping teams of researchers made those discoveries as part of an exercise that tested the limits for detecting Earth-sized planets around distant stars.

“We’ve gotten used to planets having orbits of a few days,” Josh Winn, a physicist at the Massachusetts Institute of Technology who was a member of both teams, said in an MIT news release Monday. “But we wondered, what about a few hours? Is that even possible? And sure enough, there are some out there.”

Automated search
To find such planets, Winn and his colleagues ran data from NASA's Kepler space telescope through an automated program that looked for tiny, periodic dips in starlight. The Kepler probe, which is currently out of operation due to a mechanical problem, generally detects planets by tracking how a star dims when a planet passes in front of its disk.

In most cases, the dimming is due to the presence of a planet, but there are some cases when the dimming is caused by two stars in a double-star system that repeatedly eclipse each other. So the researchers had to find a way to confirm that they were really seeing planets rather than some other effect.

They theorized that quick-moving, close-in planets would give off an extra bit of light — either by reflecting starlight or emitting their own heat signature. That would produce another characteristic pattern of brightening as the planet moved beyond the parent star's disk, and then dimming as it passed behind its parent star. Graduate student Roberto Sanchis-Ojeda was given the task of looking for such patterns in a sampling of Kepler's data sets.

"I was just looking by eye, and all of a sudden I see this extra drop of light right when it was expected, and it was really beautiful," he said. "I thought, we're actually seeing the light from the planet. It was a really exciting moment."

Tale of two planets
Winn, Sanchis-Ojeda and their colleagues detected a planet called Kepler 78b, which orbits its star once every 8.5 hours. The planet is thought to be 16 percent wider than Earth, but it's not at all Earthlike. Because it's about 40 times closer to its star than Mercury is to our sun, the surface temperature is estimated at about 5,000 degrees Fahrenheit (3,000 Kelvin). That's so hot that the planet's surface would be a bubbling sea of lava.

The other team, led by MIT physics professor emeritus Saul Rappaport, observed KOI 1843.03, a previously discovered exoplanet with an orbital period of 4.245 hours. The alien world is less than two-thirds as wide as Earth. Rappaport, Winn and their colleagues estimated that planet's density at more than 7 grams per cubic centimeter — which suggests that it's at least 70 percent iron.

"Whether nature actually makes planets that are dense enough to survive even closer in, that's an open question, and would be even more amazing," Winn said.

Kepler 78b and KOI 1843.03 are both on a list of candidate planets where a year lasts 10 hours or less. And they're both too hot to be hospitable to life as we know it. But Winn said it's possible to conceive of life on a planet that whips around a cool brown dwarf once every few days. "It would still be habitable," he said, "at the right temperature."

More about exoplanets:

Sanchis-Ojeda, Rappaport and Winn are among the authors of "Transits and Occultations of an Earth-sized Planet in an 8.5-Hour Orbit," published in the Aug. 16 issue of The Astrophysical Journal. Other authors include Alan Levine, Michael Kotson, David Latham and Lars Buchhave.

Rappaport, Sanchis-Ojeda, Levine, Winn and Leslie Rogers are authors of "The Roche Limit for Close-Orbiting Planets: Minimum Density, Composition Constraints, and Application to the 4.2-Hour Planet KOI 1843.03," published in the Aug. 10 issue of The Astrophysical Journal Letters.

Alan Boyle is's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. To keep up with's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.