「無家可歸」的行星遍布銀河系（圖）
06-05
「無家可歸」的行星遍布銀河系（圖）
關於是否存在「自由漂行星」，天文學家們現在獲得了最充分的證據。
"Homeless" Planets May Be Common in Our Galaxy
| Our galaxy could be teeming with "homeless" planets, wandering the cosmos far from the solar systems of their birth, astronomers have found. The study could help clear up a long-running debate of whether free-floating planets really exist, and how common they are. | 天文學家們發現：「無家可歸」的行星可能遍布我們的星系，它們從自己的誕生地——遙遠的恆星系統而來，漫遊於宇宙之中。該研究有助於澄清一個長期的爭論：「自由漂行星」是否真的存在？它們是否普遍存在？ |
| "The results are convincing enough that I suspect this paper will be cited for years to come as the best evidence of free-floating planets," says Dimitri Veras, an astrophysicist at the University of Cambridge in the United Kingdom, who was not involved with the study. | 「該研究成果令人信服，我認為這篇論文中的資料作為存在自由漂行星的最充分的證據，將會在未來多年的時間裡被引用，」英國劍橋大學的天體物理學家迪米特里·薇拉斯說。薇拉斯沒有參與這項研究。 |
| Over the past 2 decades, astronomers have identified more than 500 planetlike objects outside of our solar system. Most of these "exoplanets" orbit stars. The few that don"t could be either free-floating planets or stars themselves; astronomers aren"t sure, because their mass is so uncertain. Anything less massive than about 13 Jupiters is generally considered a planet, while anything between about 13 and 80 Jupiters is a small star known as a brown dwarf. | 在過去的20年中，天文學家們在我們太陽系之外發現了500多顆類似行星的天體，這些「系外行星」中的絕大多數行星在圍繞著恆星運轉。少數沒有圍繞恆星運轉的行星要麼是自由漂行星，要麼是恆星本身。天文學家們對此不能確定，因為它們的質量難以確定。在質量上，任何小於木星13倍的天體通常被認為是行星，而質量介於木星13倍至80倍之間的任何天體就是被稱為「褐矮星」的小型恆星。 |
| Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues—who form the Microlensing Observations in Astrophysics (MOA) and the Optical Gravitational Lensing Experiment (OGLE) collaborations—now appear to have figured out what is what. In a paper published online today in Nature, the researchers list 10 objects in our galaxy that are very likely to be free-floating planets. What"s more, they claim that in our galaxy, free-floaters are probably so populous that they outnumber stars. | 目前，日本大阪大學的天體物理學家孝弘須美及同事將天體物理微引力透鏡觀測法（MOA）和光學重力透鏡實驗(OGLE)聯合起來，好像解決了那些行星的問題。今天，在《自然》雜誌在線版上發表的一篇論文中，研究人員列出了我們星系中的10顆天體，它們很可能是自由漂行星。況且，他們聲稱：在我們星系中，自由漂行星數目眾多，很可能超過了恆星的數量。 |
| Most exoplanets have been discovered using one of two techniques: either seeing a star blink as a planet passes in front of it, or seeing a star wobble because of the shifting pull of a planet"s gravity. Searching for planets that have no stars, however, requires a different approach: microlensing. In this technique, astronomers use a planet"s gravity to bend light like a magnifying lens. As the planet passes in front of a distant star—one it isn"t orbiting—it gives itself away by magnifying the star"s light. Generally speaking, the shorter the magnifying time as the planet crosses the star, the smaller the planet. | 多數系外行星是利用兩種方法中的一種發現的：行星在其恆星前面經過時觀察到恆星所發生的閃爍，或者觀察到由於行星的引力變化而造成恆星的搖擺。然而，在尋找沒有恆星的行星時需要利用一種不同的方法：微引力透鏡效應。在這項技術中，天文學家們就像利用放大鏡那樣利用行星的引力來使光線發生折射。一顆不繞軌道運行的行星在一顆遙遠的恆星前面經過時，就會因放大恆星的光而暴露自己。一般說來，行星在經過恆星前面時放大的時間越短，行星就越小。 |
| Over 2 years, Sumi and others in the MOA collaboration monitored 50 million stars in our galaxy using the 1.8 meter MOA-II telescope at New Zealand"s Mount John Observatory and the 1.3 meter Warsaw University Telescope at Chile"s Las Campanas Observatory. They found 474 incidents of microlensing, just 10 of which were brief enough to be planets of around Jupiter"s size. For each of those 10 planets, the researchers couldn"t find any trace of a parent star within 1.5 billion kilometers—about the distance Saturn orbits around our sun. Although planets do orbit at greater distances, it is exceedingly rare for Jupiter-sized ones, and so the 10 planets were very likely to be free-floaters. The data were backed up by the OGLE collaboration. | 在兩年多的時間裡，孝弘須美及MOA協作組中的其他成員利用紐西蘭約翰山天文台的1.8米MOA-II望遠鏡和智利拉斯坎帕納斯天文台的1.3米華沙大學望遠鏡，監視了我們星系中的5,000萬顆恆星。他們發現了474起微引力透鏡事件，其中只有10起時間短暫，表明是大約相當於木星大小的行星。對於這10顆行星中的任何一顆，研究人員在方圓15億公里的範圍內沒有發現母恆星的蹤跡，這個範圍的距離大約相當於土星環繞太陽的軌道長度。儘管有的行星確實在更加遙遠的距離上做環繞運動，但是對於木星大小的行星來說那種情況是極為稀少的。因此，這10顆行星很有可能是自由漂行星，而且這些數據也得到了OGLE協作組的證實。 |
| The other 464 microlensing events were due to bigger objects--live stars, dead stars, and brown dwarfs. But because longer microlensing events like these are easier to spot, they skew the statistics. Taking that bias into account, the researchers estimated that there are nearly two free-floaters for every star in our galaxy. | 其他的464起微引力透鏡事件是由於較大的天體造成的——包括活恆星、死恆星和褐矮星。像這樣微引力透鏡效應持續時間較長的情況比較容易被發現，因為統計資料會產生偏差。把這種偏差考慮在內，研究人員估計我們星系中自由漂行星與恆星的數量之比接近二比一。 |
| "It is not surprising that there are such free-floating planets, as people have been expecting [they] exist," says Sumi. "But it is surprising that they are so common." | 「正如人們一直期待的那樣，存在自由漂行星並不奇怪，」孝弘須美說，「然而，奇怪的是它們竟然如此普遍地存在著。」 |
| Perhaps the biggest question arising from the discovery is where free-floaters come from. One option proposed by astronomers was that they formed in the same way as stars—by using gravity to suck up nearby material. Yet that process is unlikely to have formed so many small objects, says Sumi. Instead, he and his colleagues think it"s more likely that the free-floaters started out in planetary systems but were slingshotted away during a chaotic orbit. The question of formation is an important one, not least because—according to some—life on Earth could have originated from a free-floater that crashed into our solar system billions of years ago. | 或許，該發現所產生的最大問題是這些自由漂行星來自哪裡的問題。天文學家們提出的一個答案是，它們跟恆星的形成方式相同——通過引力吸收附近的物質而形成。然而孝弘須美說，這個過程不可能形成這麼多小型天體。相反，孝弘須美及同事認為情況更有可能是這樣的：自由漂行星源自某個行星系統，但是在軌道無序期間被彈射出去。自由漂行星的形成問題是個重要的問題，尤其是一些理論認為，地球上的生命可能起源於幾十億年前闖入太陽系的一顆自由漂行星。 |
| "These results suggest that violent, dynamical events are quite common in the history of planetary systems," says Sascha Quanz, an astronomer at the Swiss Federal Institute of Technology in Zurich, who was not involved with the research. "So, forming planets is one thing, but keeping them is another." | 「這些研究結果表明，在行星系統的歷史上，劇烈的動態事件相當普遍，」蘇黎世瑞士聯邦理工學院的天文學家薩沙·匡茲說，「因此行星的形成是一回事，把它們保留在系統之內是另一回事。」匡茲沒有參與這項研究。 |
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