It's not quite a twin Earth: a rocky planet orbiting a sunlike star at a distance where the temperature would be mild enough for liquid water to exist. But a new exoplanet that astronomers have detected in the Alpha Centauri triple star system, a mere 4.3 light-years away, comes close to being our planetary twin. Is it going around a sunlike star? Check. It's orbiting Alpha Centauri B, one of the three stars in that stellar system, which is a little smaller and less bright than the sun. Does it have a mass similar to Earth? Check. It's a bit more massive than Earth, actually. Is it in the star's habitable zone? Alas, no. It's much closer to the star than Mercury is to the sun, making it scorching hot. Nonetheless, planet hunters are excited about the find, not least because the discovery shows just how sophisticated planet-searching techniques have become. The European team that made the discovery—reported online today in Nature—did so by measuring the wobbles caused in Alpha Centauri B's motion by the gravitational tug of the planet. Although astronomers have used that so-called radial velocity technique since the mid-1990s, what's remarkable in this instance is that the wobbles they detected are tinier than any they've ever detected before: motion at a speed no more than 1.8 kilometers per hour. The Alpha Centauri system could have other planets, perhaps even one in the habitable zone.
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