天文學家們發現了首顆「小型地球」（圖）

 MOFFET FIELD, CALIFORNIA—NASA"s Kepler space telescope appears to have confirmed the existence of an alien world smaller than our own Earth—the first time such a planet has been discovered around a star like our sun. However, scientists are reluctant to provide any details about the find—announced on Tuesday at the First Kepler Science Conference here at NASA"s Ames Research Center—before their work is published, probably early next year.

 加州莫非特場消息——美國國家航空航天管理局（NASA）的開普勒太空望遠鏡證實了一顆系外星球的存在，該星球比我們的地球還要小，這是首次發現類地行星圍繞著類日恆星旋轉的情況。星期四，在莫非特場NASA埃姆斯研究中心舉行的 「首屆開普勒科學大會」上，宣布了這一發現。然而，科學家們的研究成果很可能將要在明年年初發表，在這之前他們不願意透露該發現的詳細情況。

 Despite its size, the new world—provisionally dubbed KOI-70.04—is very unlike our home planet. It orbits its star, a sun similar to our own known as Kepler-20, at a distance of just 9.6 million kilometers, completing one circuit every 6.1 days. Surface temperatures probably soar to almost 600°C, so there"s no hope of finding liquid water there, let alone life.

 儘管這顆被臨時稱為「KOI-70.04」的新行星比較小，但是它跟作為我們家園的地球相比仍有很多不同之處。這顆行星在960萬公里的近距離上圍繞著其主恆星旋轉，環繞一周僅用6.1天。這顆主恆星被稱為「開普勒-20」，類似於我們的太陽。KOI-70.04的表面溫度很可能高達近600°C，因而在那裡是沒有希望找到液態水的，更不用說尋找生命了。

 Just like Earth, KOI-70.04 is part of a multiplanet system. Four other, larger planets orbit the same star—one even closer in, and three farther away. All five planets are closer to their star than Mercury is to the sun. In that respect, the planetary system resembles Kepler-11, an extremely compact system of six planets announced in February.

 恰恰跟地球一樣，KOI-70.04也位於一個多行星系統中：其他四顆較大的行星也環繞著同一顆恆星，其中一顆在距離恆星更近的內圍，另外三顆距離恆星更遠，但是所有的五顆行星距離它們的恆星都比水星距離太陽還要近。從這一點來看，這個行星系統很像2月份宣布的一個星球極為密集的系統——開普勒-11（包含著六顆行星）。

 The Kepler telescope, launched in March 2009, scrutinizes over 150,000 stars for tiny periodic dips in brightness that might mean orbiting planets are passing in front of the stars, blocking part of their light from reaching the telescope. But other observations need to confirm the true planetary nature of these candidate events, because the observed dips may also be due to an eclipsing binary star in the background whose light blends with the foreground star studied by Kepler. An eclipse within the background binary would mimic a small brightness dip in Kepler-20.

 開普勒望遠鏡於2009年3月份發射升空，它仔細審查了15萬顆恆星，尋找在亮度上微弱的周期性變暗情況。這種變暗可能意味著軌道上的行星正在恆星前面經過，阻礙了恆星的部分光線到達望遠鏡。但是，還需要利用其他的觀察來驗證這些候選行星事件中真正的行星特性，因為所觀察到的變暗也可能是這樣形成的：在雙星系統中，背景中一顆子星的光線跟前景中另一顆子星的光線混合，致使背景子星受到遮擋而變暗，被開普勒望遠鏡觀察到。背景子星遭受遮擋變暗跟開普勒-20中的微弱變暗極為相似。

 The best way to confirm a candidate planet is by measuring the small, periodic wobbles it induces in its parent star. If that doesn"t work, astronomers have a couple of other tricks up their sleeves to validate the planet.

 驗證候選行星的最佳方法是，測量主恆星中微弱的周期性擺動，這種擺動是由行星引起的。如果該方法仍不奏效，天文學家們還可以利用其他幾個驗證行星的保留方法。

 Kepler project scientist Nick Gautier of NASA"s Jet Propulsion Laboratory presented new data at the meeting on three of the larger planets in the Kepler-20 system, now officially known as Kepler-20b, -20c and -20d. From the star"s wobbles, observed with ground-based telescopes, Gautier and his colleagues were able to deduce the masses of Kepler-20b and -20c: 8.7 and 16.1 Earth masses, respectively. Kepler-20d, the outermost planet in the system, was found to weigh in at less than 20.1 Earth masses. The three planets are between 1.9 and 3 times the size of Earth.

 開普勒望遠鏡項目的科學家、NASA噴氣推進實驗室的尼克·戈蒂埃在會議上提交了開普勒-20系統中三個較大行星的新數據，這三顆行星現在正式被稱為開普勒-20b、開普勒-20c和開普勒-20d。通過地面望遠鏡觀察到的恆星擺動，戈蒂埃及同事推測出開普勒-20b和開普勒-20c的質量分別為地球質量的8.7倍和16.1倍。開普勒-20d是該系統中最外圍的行星，質量不超過地球質量的20.1倍。三顆行星的大小均介於地球的1.9倍至3倍之間。

 So what about the other two? "I can"t disclose any details yet," says Fran?ois Fressin of the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who has a paper about the two smaller planets in press with Nature, "but if they are also validated, and if their size estimates are right, KOI-70.04 is significantly smaller than the Earth." KOI-70.04 was already in Kepler"s list of candidate planets back in February, together with the three Kepler-20 planets described by Gautier; KOI-70.05 was found in June.

 那麼，另外兩顆行星的情況又是如何呢？「我還不能透露任何詳細情況，」馬薩諸塞州劍橋市哈佛-史密森天體物理中心的弗朗素瓦·弗雷桑說，「但是，如果這兩顆行星也得到確認，如果對它們大小的估計也是準確的，行星KOI-70.04應該比地球小得多。」弗雷桑擁有一篇關於兩顆較小行星的論文，準備發表在《自然》雜誌上。早在2月份的時候，KOI-70.04和戈蒂埃描述的三顆開普勒-20行星就已經登上開普勒候選行星的列單了；行星KOI-70.05是六月份被發現的。

 Fressin won"t confirm whether the two planets have indeed been validated, but if they weren"t, they probably wouldn"t warrant a Nature publication. Moreover, according to theoretical astrophysicist Jack Lissauer of NASA"s Ames Research Center, "most Kepler multicandidates must be real planets." The reason is simple statistics: It"s extremely unlikely to encounter two background eclipsing binaries (or one background binary and one genuine planet) at the same location.

 弗雷桑還不能肯定兩顆小型行星是否真正地得到確認，如果沒有得到確認，他們就不能保證論文能夠在《自然》雜誌上發表。此外， NASA埃姆斯研究中心的理論天體物理學家傑克·利索爾說：「開普勒望遠鏡發現的絕大多數系統中的多個候選行星一定是真正的行星。」原因在於一項簡單的統計數據：在同一個位置，極不可能遇到兩顆被遮擋的背景子星；或者，一顆為背景子星而另一顆為真正行星的情況也是不可能的。

 Indeed, asked about Kepler-20, Daniel Fabrycky of the University of California, Santa Cruz, confirmed that it is the first genuine five-planet system found by the space telescope. But while Fressin agrees that Lissauer"s statistical argument is strong, he says validation of a candidate planet, especially one as small as KOI-70.04 appears to be, is of paramount importance. "I can"t tell you anything more at this moment," he says.

 的確，當有人問及有關開普勒-20系列行星的情況時，加州大學聖克魯茲分校的丹尼爾·法布里基肯定地說，這是開普勒太空望遠鏡所發現的首個真正的五行星系統。然而，儘管弗雷桑承認利索爾的統計證據是強有力的，但他表示一顆候選行星的確認，尤其是確認一顆看起來像KOI-70.04這麼小的行星，是至關重要的。「目前，我不能透露其他情況了，」他說。

 From the data published in February and the new estimates for the properties of the parent star presented by Gautier, it would follow that KOI-70.04 is between 80% and 90% of the size of Earth. For KOI-70.05, no data have been released yet, except its orbital period, which, according to Gautier, is 19.6 days. In Fressin"s upcoming Nature paper, the two planets will almost certainly be renamed Kepler-20e and Kepler-20f.

 根據2月份公布的數據以及戈蒂埃提交的有關主恆星特性的最新估計資料，情況應該是這樣的：行星KOI-70.04的大小介於地球大小的80%至90%之間。至於行星KOI-70.05，除了其公轉周期之外，一切數據均未公布。根據戈蒂埃的描述，KOI-70.05的公轉周期為19.6天。幾乎可以肯定的一點是，在弗雷桑即將發表於《自然》雜誌上的論文中，這兩顆行星將被重新命名為開普勒-20e和開普勒-20f。

 Earlier this week, Kepler scientists announced the discovery of Kepler-22b, a relatively large planet (2.4 Earth diameters) in the habitable zone of its parent star, where temperatures allow the existence of liquid water. With Kepler-20e, they have bagged a real Earth-sized planet, albeit much too close to its star to harbor life. It probably won"t be too long before the planet hunters hit upon their Holy Grail: a planet just like Earth in the habitable zone of a sunlike star. Stay tuned.

 本周早些時候，開普勒望遠鏡項目的科學家們宣布，他們發現了一顆較大的行星——開普勒-22b。該行星相當於地球直徑的2.4倍，對於其主恆星來說，它位於適合居住的區域上，上面的溫度允許液態水的存在。儘管開普勒-20e距其主恆星太近，不可能擁有生命，但是有了該發現，科學家們算是真正地獲得了一顆地球大小的行星。也許過不了多長時間，行星的搜尋者們就會找到最為理想的目標：恰似地球的行星位於類日恆星的可居住帶上……讓望遠鏡保持調諧狀態，讓我們拭目以待！