天文學家首次觀察到系外行星的相位（圖）

Since prehistoric times, people have watched the moon waxing and waning through its phases. Four hundred years ago, Galileo was the first to observe similar phases of the planet Venus. Now, for the first time, astronomers have seen the changing phases of a planet outside of our solar system. They expect similar observations in the future to reveal important details about the atmosphere, cloud cover, and climate of planets orbiting other stars.

自從史前的年代，人們就開始通過月相觀察月球的盈虧。四百年前，伽利略第一次觀察到金星具有類似的相位。現在，天文學家們第一次觀察到我們太陽系以外一顆行星的不斷變化的相位，他們期望將來可以利用同樣的觀察來揭示其他恆星周圍的行星上有關大氣、雲量和氣候的重要詳細情況。

Most extrasolar planets (or exoplanets for short) can"t be seen directly, especially if they are very close to their parent stars; astronomers see only the combined light of the star and the planet. Sometimes, they happen to see the orbits of exoplanets edge-on. In those cases, the planet alternately shows more of its dayside and then more of its nightside as it circles its star. These phases should show up as tiny variations in the intensity of the observed light, researchers have predicted.

多數太陽系以外的行星（簡稱「系外行星」）不能被直接觀察到，特別是當距離其主恆星太近的時候，情況更是如此，天文學家們只能觀察到來自恆星和行星的混合光線。有時，天文學家們碰巧從側面觀察到系外行星的軌道，在這種情況下，行星在圍繞恆星旋轉的時候就會交替地呈現出有時光面較多、有時暗面較多。研究人員預計，這些相位應該表現為所觀測到的光線在亮度上的微弱變化。

Astronomers Ignas Snellen of Leiden Observatory in the Netherlands and colleagues have now demonstrated that such optical observations are feasible. The researchers scanned publicly available data on a star known as CoRoT-1, which is 1600 light-years away. In the numbers, they found minute periodic variations in brightness that they believe are caused by the changing phases of its gaseous giant planet, known as CoRoT-1b.

現在，荷蘭萊頓天文台的天文學家伊格納斯·斯奈倫及同事證明了這樣的光學觀測是可行的。研究人員仔細查看了有關一顆恆星的可以公開使用的資料，該恆星叫做CoRoT-1，位於1600光年之外。在諸多數據中，研究人員發現了在亮度上存在周期性的細微變化，他們認為這些變化是由於其巨大的氣態行星不斷變化的相位造成的，該行星叫做CoRoT-1b。

Because it is less than 5 million kilometers from its parent star, the planet"s dayside temperature is about 2000°C, and part of the planet"s light is probably thermal glow instead of reflected star light. But, says Snellen, "even if we assume that the observed brightness variation is fully due to reflected star light, we find that the planet"s reflectivity is probably lower than 10%." That"s much less than the reflectivity of Jupiter, indicating that CoRoT-1b probably lacks extensive clouds. The team reports its results tomorrow in Nature.

由於距離其主恆星不到500萬公里，該行星的光面溫度大約為2000攝氏度，而且部分光線很可能是熱釋光，而不是反射自恆星的光線。然而，斯奈倫稱：「即使假定所觀察到的光線完全源於反射自恆星的光，我們仍然發現該行星的反射率很可能不到10%。」這比木星的反射率要低得多，表明行星CoRoT-1b很可能缺少大範圍的雲層。研究小組在明天的《自然》雜誌上報道了他們的研究成果。

"It"s a trailblazing result," says astrophysicist Sara Seager of the Massachusetts Institute of Technology in Cambridge. In this pioneering case, the measurements aren"t precise enough yet for a detailed analysis of CoRoT-1b, she says, but knowing a planet"s reflectivity is key to learning about its cloud cover and climate. "It heralds an interesting new way to characterize planets."

「這是一項開創性的成果，」麻省理工學院的天體物理學家薩拉·西格指出。她說：在這項具有先導性的研究中，所利用的測量數據還不夠準確，還無法對行星CoRoT-1b進行詳細的分析，但搞清一顆行星的反射率對於了解其雲量和氣候是很關鍵的。「這預示著找到了一個可以用來分析行星特徵的有趣的新方法。」

NASA"s Kepler satellite, launched in March, will probably reveal many more phase-changing exoplanets, and so might the newly refurbished Hubble Space Telescope and even large, ground-based telescopes. "If it can be done for this planet, you can do it for other systems too," says astronomer Heather Knutson of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "Someday, this kind of observation will become commonplace."

美國國家航空航天管理局的開普勒人造衛星於3月份發射升空，很可能將會揭示更多具有相位變化的系外行星。整修一新的哈勃太空望遠鏡，甚至還有大型的地面望遠鏡都可能會發現這樣的系外行星。「如果能夠對這顆行星進行這樣的觀測，那麼這種觀測對其他的行星系統也能夠行得通，」馬薩諸塞州劍橋市哈佛-史密森天體物理中心的天文學家希瑟·克努森說，「有朝一日，這種觀測將會得到普遍運用。