Newly discovered planet is just 4.4 light-years distant

The Alien Next Door

By Nadia Drake

Astronomers searching for Earthlike worlds need look no further than Alpha Centauri, the stellar system next door.

An Earth-sized planet has been discovered circling a star in the system, just 4.4 light-years away. The planet's mass is similar to Earth's, but its orbit is not. Tucked in close to its star — 25 times closer than the Earth is to the sun — the planet is likely a roasted world incapable of hosting life.

Still, the discovery, reported October 18 in Nature, ignites dreams of sending a spacecraft to Alpha Centauri, a perennially favored interstellar target because of its location in Earth's celestial backyard.

"A rocky planet around Alpha Centauri, our nearest neighbor — this is incredible," says astronomer Debra Fischer of Yale University. "If you were going to send a spacecraft anywhere, or a probe anywhere, that's where you'd go first. And if you have evidence that there are rocky planets there, you'd be insane to skip that target."

The rocky planet circles Alpha Centauri B, a star just a bit smaller and cooler than the sun. But with a year there lasting just 3.236 days, the planet is not Earth's twin. Being snuggled so close to the star means the planet has a surface temperature of around 1,200 degrees Celsius, notes astronomer Greg Laughlin of the University of California, Santa Cruz. "It's utterly uninhabitable, utterly scorched, utterly un-Earthlike in every respect," he says.

But a rocky planet so close to Alpha Centauri B suggests there could be more planets in the same system — perhaps rocky and a bit farther out, in the area where life could comfortably thrive. "I think that the odds that there's an interesting planet, a truly interesting planet in the system, are very high, given that this one is there," Laughlin says. Data returned from NASA's Kepler spacecraft — which looks for planets around a population of stars that mostly lie more than 600 light-years away — suggest that multiple-planet systems are common, especially when small, rocky planets are found in close orbits.

There's also a good chance that Alpha Centauri A — the bigger, binary partner of Alpha Centauri B — also hosts planets. But because Alpha Centauri A is bigger, brighter and more rambunctious, any small planets orbiting it would be harder to find.

Previous observations indicate that there are no planets more massive than Neptune hovering around any of Alpha Centauri's three stars. But those observations don't rule out smaller, harder-to-detect planets.

"The way we describe it is, the easy planets were all found a long time ago," says Sara Seager, an astrophysicist at MIT.

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