一個極為怪異的系外行星系統（圖）

 Today"s news that the Kepler spacecraft has turned up an alien solar system with five of its six known planets bunched tightly around their star had a familiar ring. In the past 15 years, astronomers have identified hundreds of solar systems, each bizarre in its own way. Yet this new system offers a unique way to learn more about how planets form and evolve.

 今天的一條新聞報道說：開普勒太空望遠鏡發現了一個系外恆星系統，六顆已知行星中的五顆緊緊地環繞在恆星周圍。這類新聞聽起來有些耳熟，在過去的15年當中天文學家們發現了幾百個恆星系統，每個系統都有其本身的奇異之處。然而這個新發現的系統為天文學家們提供了獨一無二的途徑，能夠使他們更加清晰地了解行星是如何形成和演化的。

 "We keep hearing it over and over again: Other planetary systems aren"t like our own," says exoplanet astronomer Debra Fischer of Yale University. But "this six-planet system is just incredible."

 「我們不斷地聽到這樣的說法：其他的行星系統跟我們太陽系有所不同，」耶魯大學研究系外行星的天文學家黛布拉·費希爾說，「但是，這個擁有六顆行星的系統簡直不可思議！」

 The find comes from NASA"s Kepler telescope, launched in March 2010 to hunt for planets orbiting other stars. For months on end, it has been staring at 156,000 stars in the constellation Cygnus, searching for the tiniest fading of a star"s light that signals a planet passing in front of it. Reporting online today in Nature, planetary dynamicist Jack Lissauer of NASA"s Ames Research Center in Mountain View, California, and 39 Kepler co-authors announced that the effort paid off with the discovery of six planets orbiting a single star designated Kepler-11 2000 light-years away.

 這個系統是由美國國家航空航天管理局的開普勒太空望遠鏡發現的。開普勒太空望遠鏡於2010年3月份發射升空，用以搜尋環繞其他恆星的行星。連續幾個月以來，該望遠鏡一直對準天鵝座的156,000顆恆星，尋找恆星最微弱的變暗跡象，這種變暗表示有一顆行星正在恆星的前面經過。加州山景城市美國國家航空航天管理局埃姆斯研究中心的行星動力學家傑克·利索爾，以及39位研究開普勒望遠鏡的同事，今天在《自然》雜誌在線版上報道該發現時宣布：他們的努力得到了回報，他們發現了六顆行星圍繞著一顆叫做開普勒-11的恆星旋轉，該恆星位於2000光年之外。

 None of the planets is Earth-like or even as small as Earth. All fall between Earth and Uranus in mass, and three are clearly gas giants with thick atmospheres of hydrogen and helium like Uranus"s. Most odd is that five of the six orbit much closer to their star than Mercury orbits the sun, whereas the sixth orbits closer to where Venus would be. "It is by far the most densely packed planetary system," Lissauer said during a press teleconference this week. "Why there are so many so close is a mystery."

 所有的行星都不同於地球，甚至都不像地球那麼小。所有的行星在質量上都大於地球而小於天王星，很明顯其中三顆是氣態的巨型行星，跟天王星一樣擁有氫和氦構成的厚厚的大氣層。最為奇怪的是，六顆行星中有五顆在距其恆星很近的軌道上運行，比水星軌道跟太陽距離要近得多，而第六顆行星跟恆星的距離也比金星跟太陽的距離近。「這是一個行星密度最高的行星系統，」本周利索爾在一次電話新聞發布會上說，「為什麼在這麼近的距離上有這麼多行星呢？這是一個謎。」

 But that mystery may eventually help astrophysicists understand how and where planets arise. Researchers have long debated how close in gas giant planets form from a flat disk of primordial gas and dust. One theory posits that they form far from their parent stars—about where Jupiter and Saturn are now—and move inward over time. Another says that they form in place. Researchers try to figure out which way real planets form by simulating planet formation in computer models.

 但是，這個謎最終可能會幫助天體物理學家們理解行星產生的方式和位置。有關氣態巨型行星形成於原始氣體和塵埃平盤內的什麼位置問題，長期以來研究人員一直在爭論。一種理論提出，他們形成於遠離主恆星的位置，大約相當於木星和土星目前的位置，隨著時間的推移它們向內部移動。另外的一種理論指出，氣態行星形成於適當的距離上。研究人員通過計算機模型來模擬行星形成的情況，以期搞明白真正的行星是按照哪一種方式形成的。

 The Kepler find dramatically shows how poorly the models are doing. "The usual planet formation model is severely challenged to explain the Kepler-11 system," says planetary dynamicist Peter Bodenheimer of the Lick Observatory at the University of California, Santa Cruz. Making all five Kepler-11 planets far out and moving them in "is so complicated it"s unlikely," he says. But forming them about where they are now would require an unrealistic amount of solid particles to form a rocky core, he says. For now, Bodenheimer leans toward formation in place, if only because it is "a little bit less problematic."

 引人矚目的是，開普勒望遠鏡的這項發現表明了那些模擬的效果有多麼差。「在解釋開普勒-11系統時，對普通行星形成情況的模擬受到了嚴峻的挑戰，」加州大學聖克魯茲分校利克天文台的行星動力學家彼得·博登海默說。他指出，使開普勒-11中的五顆行星在遠處的外圍形成，然後再將它們移進來，這太複雜了，不可能是這樣的。但是他又說道，如果就在它們目前位置形成，那麼將需要大量的固體粒子才能形成固體核心，這是不現實的。目前，博登海默傾向於認為行星在適當的距離上形成，只有這樣問題才會較少一些。

 Kepler-11"s tightly bunched planets may have deepened the mystery, but researchers see a ray of hope in them as well. The combination of Kepler"s extreme sensitivity and closely spaced planets gravitationally tugging on one another allows team members to learn more about the Kepler-11 planets than astronomers using other techniques have found out about other, less-populated systems. Those additional properties include a planet"s mass, density, and proportion of rock and gas—just the sort of information modelers need to test their theories of planet formation convincingly.

 開普勒-11系統中緊密環繞的行星使行星形成之謎更加深奧了，但是研究人員通過這些行星也看到了一線希望。天文學家們利用其他技術發現過行星比較稀疏的其他系統，但是相比之下，開普勒望遠鏡的極端敏感性結合在近距離上互相吸引的行星，使研究小組的成員們對開普勒-11系統中的行星情況了解得更多一些。其他的特徵包括行星的質量、密度以及岩石和氣體的比例等，這些正是進行模擬的人員需要驗證的信息，驗證之後才能使他們的行星形成理論令人信服。