最小的系外行星（圖）

06-03

最小的系外行星（圖）

雖說根本不象家園，但就系外行星來說，比起天文學家們所遇到的任何其他行星，它在大小上更為接近地球。

The Smallest Extrasolar Planet

When it comes to planets outside our solar system, smaller is better. That"s because smaller planets are more likely to resemble Earth. So the discovery of the smallest known extrasolar planet, announced today at the June meeting of the American Astronomical Society in St. Louis, Missouri, comes as good news. "It gives us hope of finding lots of habitable planets," says astronomer Sara Seager of the Massachusetts Institute of Technology in Cambridge, who was not involved in the discovery.

就太陽系以外的行星而言，越小就越理想，因為較小的行星更有可能跟地球類似。因此，今天在密蘇里州聖路易斯舉行的美國天文學會六月份會議上，科學家們宣布發現了最小的已知系外行星，這是個好消息。「這使我們有望發現許多適合居住的行星，」劍橋麻省理工學院的天文學家薩拉·西格說。西格沒有參與這項發現。

The detection comes courtesy of a technique called microlensing. Employed by consortia that operate global networks of telescopes (the Optical Gravitational Lensing Experiment and Microlensing Observations in Astrophysics), microlensing allows continuous monitoring for subtle brightening of far-distant stars as a relatively nearby star passes in front of them. The nearer star"s gravity can slightly bend--or lens--the background star"s light toward Earth, temporarily brightening it. If a planet circling the nearer star also lines up and bends some starlight, the network picks up a secondary brightening.

該發現藉助一種叫做「微引力透鏡」的技術。天文協會啟動全球的望遠鏡網路，利用微引力透鏡效應，可以連續監視諸多遠距離恆星微妙的亮度增加情況。當一顆較近的恆星在遙遠的恆星前面經過時，較遠恆星的亮度就會有所增加。較近恆星的引力可以使背景恆星射向地球的光線發生折射，暫時地使其更加明亮。如果一顆行星圍繞著較近的恆星旋轉，若與其恆星及背景恆星排列成直線，也會使恆星的一些光線發生折射，望遠鏡網路可以探測到亮度的再度增加。

At the meeting, astronomer David Bennett of the University of Notre Dame in Indiana, speaking for the consortia, announced the microlensing discovery of a planet just three times the mass of Earth. Most of the 300 or so known extrasolar planets are many times the mass of Jupiter, or thousands of times the mass of Earth. The newly discovered planet, which the team is referring to as MOA-2007-BLG-192L, orbits its star at 70% of the sun-Earth distance. That means the planet probably formed with lots of ice and gases, Bennett said, more like Neptune in composition than Earth.

在會議上，印第安納聖母大學的天文學家大衛·貝內特代表天文協會發了言。他宣布說，利用微引力透鏡技術發現的這顆行星僅僅是地球質量的三倍。然而，在已知的約300顆系外行星中，多數行星的質量是木星的許多倍。這顆新發現的行星，被研究小組稱為MOA-2007-BLG-192L，公轉軌道距其恆星的距離為地球到太陽距離的70％。貝內特指出，這意味著該行星很可能是由許多冰和大量氣體構成的，在構成成分上更象海王星，而不象地球。

The planet"s host star is small as well. At roughly 6% the mass of our sun, the star is probably too small to sustain fusion reactions in its core, said Bennett, making it a dimly glowing brown dwarf.

該行星的主恆星也很小，約為太陽質量的6％。因此貝內特稱，這顆恆星太小，其核心部分很可能無法維持聚變反應，使其成為光線暗淡的褐矮星。

Bennett said the microlensing technique holds promise for finding planets even closer in size to Earth. What"s more, he noted, future orbiting telescopes may be able to image a planet distinct from its star and therefore search for signs of life.

貝內特說，微引力透鏡技術有望用來發現跟地球大小更為接近的行星。而且他還指出，將來繞軌道運行的望遠鏡也許能夠拍到清晰的行星圖像，可以明顯地與其恆星區別開來，因此可以用來搜尋生命的跡象。