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Earth-size planet found just next door, in Alpha Centauri system

The planet is not habitable, it is too close to Alpha Centauri B. But rocky planets tend to have siblings, researchers note, raising hopes others could be found in the system just 4.4 light-years distant. 

By / October 17, 2012

This artist's impression shows the planet orbiting the star Alpha Centauri B, a member of the triple star system that is the closest to Earth in this image released on October 17, 2012.

L. Calcada/N. Risinger/ESO/Reuters

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Astronomers have discovered a rocky, Earth-mass planet right in our own sun’s galactic neighborhood. The planet is orbiting a star a mere 4.4 light-years away in the multiple-star Alpha Centauri system, which may well turn out to be a cosmic condo, hosting additional planets.

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A habitable planet could well be among them, some astronomers speculate.

The planet, with an estimated mass 1.13 times higher than Earth's, is orbiting Alpha Centauri B, which forms a binary pair with Alpha Centauri A. Both essentially share the sun's mass and are only slightly older than the sun. The system includes a smaller, dimmer star called Proxima Centauri.

With a "year" that corresponds to 3.2 Earth days, the newfound planet is too close to the star to be habitable, notes Xavier Dumusque, a PhD student in astrophysics at the University of Porto in Portugal. He led the team reporting the discovery in Thursday's issue of the journal Nature.

The planet is gravitationally bound to the star in such a way that it presents the same face to Alpha Centauri B all the time. This sets up temperatures on the day side that could top 2,200 degrees Fahrenheit, melting rock – a sharp contrast with the night side, perpetually facing the cold of space.

Still, researchers point out that rocky planets tend to have siblings.

Evidence from 17 years of hunting planets, especially evidence that NASA's Kepler spacecraft is amassing, "shows us where there's one rocky planet, there's more," says Debra Fischer, a Yale University astronomer who has been deeply involved in hunting for extrasolar planets for most of this period.

Snagging a rocky planet with an Earth-like mass one solar system over from ours suggests that "it's a good bet there are other planets there as well," she adds, including planets orbiting Alpha Centauri A.

Detecting an Earth-mass, rocky planet in the habitable zone of either star will be extremely difficult. The notion that one of these lies a cosmically scant 4 light-years away remains speculation, but "it is not a crazy speculation anymore," says Dr. Fischer, who was not part of the research team reporting the results.

The team, which included 11 researchers from Switzerland, Portugal, and France, used the European Southern Observatory's 3.6-meter telescope high in the Chilean desert.

The researchers observed the system on and off for four years using an approach that measures the slight to-and-fro tug a planet imparts to its host star as the planet orbits. Using a highly stable spectrograph bolted to the back of the telescope, the team detected this movement as subtle changes in the star's spectrum – which shifted toward red when the planet tugged the star away from Earth, and to blue when it pulled the star toward observers.

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