類似太陽系的恆星系統很稀少（圖）

06-07

類似太陽系的恆星系統很稀少（圖）

圖為哈勃太空望遠鏡拍下的可見光圖像，可見光是由獵戶座星雲中一個被稱為proplyd 170-337的原行星盤發出的。圖像顯示：高溫電離氣體（紅色）環繞著一個盤（黃色），並不斷地從該盤流出。這些輪廓線揭示了這個隱藏在高溫氣體中的塵埃盤。該原行星盤的質量大於太陽質量的百分之一，也就是說，大於形成木星大小行星所需的最低質量限度。

Solar Systems Like Ours May Be Rare

As humans look farther into the universe and discover more and more planets beyond the sun, many wonder how typical our own solar system is. Often astronomers in the planet-hunting business say discoveries of Earth-like worlds are just around the corner.	隨著人類對宇宙的觀測越來越深遠，並發現了越來越多太陽系以外的行星，很多人想知道我們的太陽系在多大程度上具有代表性。參與行星搜尋活動的天文學家們通常說，發現類似地球的行星不過是輕而易舉的事。
But a new study indicates our setup may be rare indeed.	然而一項新研究顯示，我們太陽系的結構是極其罕見的。
A group of astronomers surveyed sun-like stars in the Orion nebula open cluster and found that fewer than 10 percent have enough surrounding dust to make Jupiter-sized planets.	一組天文學家觀測了獵戶座星雲中疏散星團內的類日恆星，發現僅有不到10%的類日恆星周圍的塵埃足以形成木星大小的行星。
"We think that most stars in the galaxy are formed in dense, Orion-like regions, so this implies that systems like ours may be the exception rather than the rule," said researcher Joshua Eisner, an astrophysicist at the University of California, Berkeley.	「我們認為，星系中多數恆星形成於類似獵戶座的高密度區域，因此這意味著象我們這樣的系統可能是一個特例，而並非尋常，」研究人員、加州大學伯克利分校的天體物理學家約書亞·艾斯納說。
That"s important because giant planets like Jupiter may be instrumental in fostering life on rocky worlds like Earth.	這一點很重要，因為象木星那樣的巨大行星有助於在類地岩石星球上產生生命。
Eisner and his team observed about 250 stars in the million-year-old Orion Nebula, looking for dense disks of dust surrounding the stars that could be forming planets. They found that only about 10 percent of the stars emitted radiation in the frequency that would indicate they have these proto-planetary disks of warm dust. And only 8 percent of the stars surveyed had dust disks with masses greater than one-hundredth the mass of the sun, a mass thought to be the lower limit for formation of Jupiter-sized planets.	獵戶座星雲已有上百萬歲，艾斯納及其研究小組觀察了其中大約250顆恆星，在它們周圍尋找可能正在形成行星的高密度塵埃盤。他們發現，僅有約10%的恆星發出的射線頻率表明其周圍擁有溫暖塵埃構成的原行星盤；被觀測恆星中，僅有8%的塵埃盤質量大於太陽質量的百分之一。太陽質量的百分之一被認為是形成木星大小的行星所需的最低質量限度。
These findings seem to agree with what planet hunters are finding so far when they use radial velocity studies to detect extrasolar planets around other stars. (The radial velocity approach involves looking for a wobble in a star"s motion caused by the slight gravitational pull of an orbiting planet.)	這些發現好象跟一些行星搜尋專家到目前為止所發現的情況一致，他們利用徑向速度研究法來探測其他恆星周圍的系外行星。（徑向速度法涉及尋找恆星運動中的擺動，擺動是由於繞其旋轉的行星發出的輕微引力所致。）
"The current numbers are suggesting 6 to 10 percent of stars have Jupiter-sized planets, which is exactly consistent with our findings," Eisner told SPACE.com.	「目前的數據表明，有6%至10%的恆星擁有木星大小的行星，這跟我們的發現恰好一致，」艾斯納告訴《太空網》的記者說。
The researchers will detail their findings in the Aug. 10 issue of the Astrophysical Journal.	研究人員將把他們的發現詳盡地記錄在8月10日出版的《天體物理學雜誌》上。
Snapshot in time	時期不定的快照
Still, it"s too soon to completely despair of finding the universe filled with Jupiters around other suns.	然而，現在為時尚早，對於在宇宙其他恆星周圍找到類木行星還不能完全絕望。
Since the survey only looked at dust around the stars, and would not have detected any already-formed planets, it could be that some of those sun-like stars already had planets.	由於該觀測只是關注了恆星周圍的塵埃，不會發現任何已經形成的行星。情況有可能是這樣：其中某些類日恆星早已擁有了行星。
"Perhaps we"re only detecting the stars that have not formed planets yet," said John M. Carpenter, an astronomer at Caltech who worked with Eisner on the Orion research. "Perhaps some other stars already formed planets. It"s only a snapshot in time and as you look at other clusters at different ages you can build up a better picture."	「也許我們只是在觀測還沒有行星形成的恆星，」加州理工學院的天文學家約翰·M·卡彭特說，「也許一些其他的恆星已經有行星形成。這只不過是時期不定的快照，隨著對其他不同時期星系團的觀測，可以建立起一個更為完善的圖片庫。」卡彭特跟艾斯納一起參與了獵戶座研究。
Other scientists agree there are many unanswered questions about solar systems beyond our own.	其他科學家們承認：關於我們太陽系以外的恆星系統，有很多問題有待解答。
"As the precision with which we can measure improves, we find more planets," said Harvard planet hunter David Charbonneau, who was not involved in the Orion study. "The rate of occurrence has gone up since we started looking."	「隨著我們的觀測精度不斷提高，我們會發現更多行星，」哈佛大學行星搜尋專家大衛·夏伯諾說，「自從我們開始搜尋以來，發現行星的頻度一直在提高。」夏伯諾並沒有參與這項獵戶座研究。
He said it"s too soon to tell for sure whether Earth"s system is atypical, but studies that look at whether other stars have the raw materials necessary to form solar systems like our own can help.	夏伯諾指出：如果現在就確切地說明地球系統是否具有代表性，恐怕為時尚早；然而，研究其他恆星是否具備形成類似我們太陽系的系統所必需的原材料，也是有所幫助的。
"Certainly knowing that there is enough stuff around stars to make planets is a crucial step," he said.	「當然，了解恆星周圍的材料是否足以形成行星是至關重要的一步，」夏伯諾說。
Rare life?	罕見的生命
If it turns out to be true that sun-like stars with Jupiter-sized planets are rare, it may mean that extraterrestrial life is rare as well.	如果證明類日恆星擁有類木行星的情況實屬罕見，那麼就意味著地外生命也是罕見的。
Some scientists have suggested that having our own Jupiter has been instrumental in forming life on Earth. For one thing, large planets can protect smaller inner planets from being bombarded too heavily by space rocks, which could crush any budding bits of life.	一些科學家提出，我們太陽系擁有木星對於地球生命的形成有所幫助。原因之一在於，大型行星可以保護較小的內部行星，使它們不至於受到太空岩石特別猛烈的撞擊。如若不然，任何處於發育狀態的、哪怕是星星點點的生命都會被撞得稀巴爛。
Plus, large planets could kick comets and asteroids out of their orbits and onto paths toward the smaller terrestrial planets. These space rocks could be delivery systems for organic materials and water	此外，大型行星會將彗星和小行星踢出運行軌道，並使它們踏上通往較小的類地行星之路。這些太空岩石可能是有機材料和水的輸送系統。
"If you don"t have a Jupiter it"s kind of hard to build a wet planet," Eisner said.	艾斯納說：「從某種程度上說，如果沒有類木行星，就難以形成濕潤的行星。」