太陽系外行星新發現 地球大小 可能有水

For the first time, astronomers have discovered a world nearly the size of Earth orbiting a far star where water might exist as a life-giving liquid, the scientists announced Thursday.

科學家週四宣佈，在圍繞一顆遙遠恆星運行的軌道上，天文學家首次發現了一顆大小與地球相近、並且可能存在水這種生命液體的行星。

Using the Kepler space telescope, Elisa Quintana of The SETI Institute at NASA's Ames Research Center in California and her colleagues detected the planet around a relatively cool red dwarf star in the constellation Cygnus, located about 459 light years from Earth.

通過使用開普勒(Kepler)太空望遠鏡，美國國家航空及太空總署(NASA)艾姆斯研究中心(Ames Research Center)地外文明搜尋研究所(The SETI Institute)的昆塔納(Elisa Quintana)和同事發現了這顆行星。這顆行星環繞天鵝座(Cygnus)中一顆溫度相對較低的紅矮星運行，距離地球大約459光年。

Known officially as Kepler-186f, the planet is the outermost of five Earth-sized worlds orbiting in that star's solar system, the scientists said at a news conference held Thursday by the space agency. They also reported their findings in the journal Science.

科學家週四在NASA舉辦的新聞發佈會上說，這顆行星的正式名稱爲開普勒-186f (Kepler-186f)，其所處的太陽系有五顆地球大小的行星，它是最外面的一顆。《科學》(Science)雜誌刊登了這些科學家的發現。

In the search for worlds where life might take hold, scientists so far have detected 20 potentially habitable planets around other stars. But this one is the first so close in size to Earth that is located within its star's so-called habitable zone, where it receives the right amount of solar radiation so that water there wouldn't boil or freeze, the researchers said.

到目前爲止，在搜尋可能存在生命的星球過程中，科學家已經發現了20個有孕育生命潛力的行星圍繞其他恆星運轉。但這次還是科學家們首次發現尺寸與地球如此接近，並位於所謂恆星“宜居帶”的行星。研究人員們說，恆星“宜居帶”是指行星所處位置能接收到不多不少的恆星輻射量，從而使行星上的水維持在液態。

'This is the first validated Earth-sized planet in the habitable zone of another star,' said Dr. Quintana. 'We can now say other potentially habitable worlds the size of Earth can exist.'

昆塔納博士說，這是首次在另一個恆星“宜居帶”確認發現地球大小的行星。她說，現在可以這樣說，還有與地球大小類似、可能孕育生命的星球存在。

Other experts, though, cautioned that the planet orbits at the outer edge of that habitable zone, where it likely receives about the same amount of light as Mars in our own solar system. Kepler-186f could easily be so cold that any water there is frozen, unless it has a greenhouse-like atmosphere that can trap heat, they said.

不過，其他專家提醒說，這顆行星的運行軌道位於“宜居帶”的外緣，那裏接收到的光線可能與太陽系裏的火星接收到的光線一樣多。他們說，開普勒-186f很可能非常寒冷，水是冰凍狀態的，除非那裏有一個類似溫室的環境，能夠使熱量滯留。

'It is Earth-sized, but not Earthlike,' said Abel Mendez, director of the Planetary Habitability Laboratory at the University of Puerto Rico, who wasn't involved in the research effort. 'The main point of this discovery is the size of the planet. So far, all potentially habitable planets were big ones.'

波多黎各大學(University of Puerto Rico)星球宜居性實驗室主任門德斯(Abel Mendez)說，這顆行星的大小與地球類似，但環境與地球不同。門德斯沒有參與這項研究。門德斯說，這項發現的主要一點是行星的大小，目前爲止，全部可能宜居的行星體積都較大。

Given its small size, the researchers believe that Kepler-186f is most likely a rocky planet, but they don't have enough information to calculate its actual composition or mass. The researchers cannot see the planet directly, but detected it by measuring the periodic dip in light as it passes in front of its star.

鑑於開普勒-186f的體積較小，研究人員認爲它最有可能是一顆巖態行星，但他們沒有足夠多的信息來計算出其實際成分或質量。研究人員無法直接看到這顆行星，而是通過測量它在其恆星前面經過時光線的週期性減弱而探測到它的。

Once the stuff of science fiction, such habitable planets may be common in the cosmos--as many as 40 billion Earthlike planets in the Milky Way, researchers estimate.

這類宜居行星過去都只是在科幻小說中才會出現。它們在宇宙中可能有很多，據研究人員估計，在銀河系有多達400億顆環境與地球類似的行星。