多行星的恆星系統大量存在（圖）
06-02
多行星的恆星系統大量存在（圖）
美國國家航空航天管理局的開普勒太空望遠鏡發現了171個包含不止一顆行星的系統，如圖中的二行星系統。
Stars With Multiple Planets Abound
| BOSTON—Planets like company. No less than one in three of all planets around other stars found by NASA"s Kepler space telescope are in multiple-planet systems. What"s more, the sheer number of those systems suggests that they are more tranquil places than our own solar system, say astronomers working with the telescope. | 行星喜歡結伴。美國國家航空航天管理局發射的開普勒太空望遠鏡發現：在圍繞其他恆星旋轉的所有行星中，不少於三分之一的行星處於多行星系統中。而且，利用該太空望遠鏡工作的天文學家們稱，單單根據其中行星的數目就可以說明那些系統比我們的太陽系更為平靜。 |
| Since it was launched just over 2 years ago, Kepler has found 116 systems with two planets, 45 with three, eight with four, one with five, and one with six planets, astronomer David Latham of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, reported here today at the meeting of the American Astronomical Society. That"s a total of 171 multiple-planet systems. "We thought we might see a few multiplanet systems," Latham says. "Instead, we found lots of them." | 今天，馬薩諸塞州劍橋市哈佛-史密森天體物理中心的天文學家大衛·萊瑟姆在波士頓舉行的美國天文協會會議上報道說：自兩年多以前發射升空以來，開普勒太空望遠鏡發現了116個擁有兩顆行星的系統、45個擁有三顆行星的系統、8個擁有四顆行星的系統、1個擁有五顆行星的系統和1個擁有六顆行星的系統，總數為171個多行星系統。「我們本以為可能會發現為數不多的多行星系統，」萊瑟姆說，「相反，我們發現了許多這樣的系統。」 |
| Kepler detects a planet when it passes in front of its parent star, as seen from Earth. Such transits produce minute, periodic dips in the brightness of the star. Kepler continuously monitors 165,000 stars to search for those transits. | 如同在地球上所觀察到的情況，當一顆行星從其主恆星前面經過時，開普勒太空望遠鏡就會發現該行星。這種凌日現象會使主恆星的亮度產生周期性的微弱變暗。開普勒太空望遠鏡連續地監視著165,000顆恆星，目的是為了尋找那些凌日的行星。 |
| The low expectation for multiple-planet systems was based on the fact that the eight planets in our own solar system do not orbit in exactly the same plane. For instance, the plane of Mercury"s orbit is tilted by 7? relative to that of Earth. So if the solar system were viewed from a vast distance, if one planet produced transits, most others would not. | 天文學家們對多行星系統的期望值不大，主要是因為我們太陽系的八顆行星並不是恰好在同一個平面上運行的。比如，相對於地球的軌道平面，水星的軌道平面傾斜度為7?。因此，如果從遠處來觀察太陽系，在一顆行星發生凌日現象時，大多數其他就不會發生凌日現象了。 |
| This means Kepler"s multiplanet systems must be very flat, with orbital tilts of less than a degree. "Most likely, if our solar system didn"t have large planets like Jupiter and Saturn to have stirred things up with their gravitational disturbances, it would be just as flat," Latham says. In the Kepler systems found so far, smaller planets such as Neptune are much more common. Latham says, "Systems with smaller planets probably had a much more sedate history." | 這意味著，開普勒太空望遠鏡發現的多行星系統一定非常扁平，行星軌道平面的傾斜度不超過一度。「如果我們太陽系沒有像木星和土星這樣的大型行星，系內的一切不受它們的引力干擾，那麼太陽系很可能也會同樣扁平，」萊瑟姆說。迄今為止，在開普勒太空望遠鏡所發現的系統中，像海王星那樣的較小行星要普遍得多。萊瑟姆說：「對於擁有較小行星的系統來說，其歷史很可能會平靜得多。」 |
| Still, the planets do slightly disturb each other with their mutual gravity, pulling each other around a little bit and thus causing tiny variations in the exact timing of the transits. In fact, such transit timing variations provide information on the masses of the planets. "This is a completely new [tool in the] field that might become very important," says planetary scientist Sara Seager of the Massachusetts Institute of Technology in Cambridge. | 然而，由於行星之間的相互引力，它們確實會輕微地受到彼此的干擾，把相互之間的距離稍微拉近一點，這樣就會在凌日的確切時間上引起微小的變化。事實上，凌日時間變化可以提供有關行星質量的信息。「這在該領域是個全新的研究方法，可能會成為極其重要的方法，」劍橋市麻省理工學院的行星科學家薩拉·西格說。 |
| Transit timing variations even show "indications" of the existence of nontransiting planets in some of the multiplanet systems, says Kepler principal investigator William Borucki of NASA"s Ames Research Center in Moffet Field, California. "We"re looking into it," Borucki says. Those unseen planets" orbits may have been tilted a bit by the perturbations of a Neptune-like planet in the system. But Ames theorist Jack Lissauer says that if no transits are observable, it will be much harder to pin down the orbits and masses of these planets. | 加州莫菲特場市美國國家航空航天管理局艾姆斯研究中心的工作人員、開普勒太空望遠鏡的首席研究員威廉·博盧基說：在一些多行星系統中，凌日時間變化甚至可以顯示出非凌日行星存在的跡象。「我們正在研究這種情況，」博盧基說。那些未被發現的行星，由於受到系統中類似海王星的行星造成的輕微干擾，它們的軌道可能有些傾斜。但是艾姆斯研究中心的理論家傑克·利索爾稱，如果沒有可以觀察的凌日現象，要想確定這些行星的軌道情況和質量就會難得多。 |
| Meanwhile, a thorough statistical analysis of the sizes of the planets Kepler has seen so far reveals "a very profound discovery," planet hunter Geoff Marcy of the University of California, Berkeley, reported at the meeting. "There are some Jupiters and some Saturns, but there are many more smaller planets out there," Marcy says, probably including a lot of them comparable in size to Earth. | 加州大學伯克利分校的行星搜尋專家傑夫·馬西在這次會議上報道說：同時，研究人員對開普勒太空望遠鏡迄今所發現的行星進行了徹底的大小統計分析，分析顯示出「一個影響極為深遠的發現」。「宇宙中有一些類似木星和土星的行星，但是較小的行星要多得多，」馬西說，「其中很可能包括許多大小相當於地球的行星。」 |
| The ultimate goal of the Kepler mission, which has an operational lifetime of 3? years, is to find the abundance of Earth-like planets orbiting stars like our own sun at distances where liquid water—and possibly life—could exist on their surfaces. Borucki expects an additional release of Kepler data in June 2012 and doesn"t want to speculate about what it might show. "We don"t want to get premature information out," he says. "There"s still a lot of analysis that needs to be done." | 開普勒太空望遠鏡的運行壽命為3?年，該航天任務的最終目的是：找到在合適的距離上圍繞著類日恆星旋轉的大量類地行星，在它們的表面上可能存在水，也可能會存在生命。博盧基預料，2012年6月可望再次發布一批開普勒太空望遠鏡收集的資料，但是他不想推測那批資料將會揭示什麼情況。「我們不想過早地透露信息，」他說，「我們仍然需要進行許多分析。」 |
譯自：美國《科學》雜誌網站原著：Govert Schilling
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