來自其他星系的行星（圖）

Astronomers have detected what they believe to be a planet at least the size of Jupiter that came from another galaxy. If true, the world is the first planetary immigrant ever detected in the Milky Way. The find would also violate the leading hypothesis of how and where planets form.

天文學家們發現了一顆來自其他星系的行星，他們相信這顆行星的大小至少相當於木星。如果該發現得到證實，這顆星球就是在銀河系中所發現的第一顆外來的行星，同時該發現也將違背有關行星起源的最主要的假說。

The planet lives 2200 light-years away inside the Helmi stream, a ring of ancient stars that cuts through the plane of the Milky Way. Astronomers believe the stream formed 6 billion to 9 billion years ago, when the Milky Way ripped another galaxy to shreds, swallowing some of its stars in the process. Astronomer Johny Setiawan of the Max Planck Institute for Astronomy in Heidelberg, Germany, likes looking at these stars because they tend to have unusual properties. But even by these standards, one star in particular caught his eye: HIP 13044.

這顆行星位於2200光年之外的赫米恆星流之中，而赫米恆星流是橫截銀河系平面的一個環形的古老恆星帶。天文學家們認為，赫米恆星流形成於60億至90億年以前，當時銀河系將另外一個星系撕成碎片，在這個過程中吞沒了其中的一些恆星。德國馬普學會天文學分會的天文學家約翰尼·塞蒂亞萬喜歡觀察這些恆星，因為它們有著不同尋常的特性。然而，即使採用不同的標準進行觀測，一顆特別的恆星仍然吸引了他的眼球，該恆星是HIP 13044。

Setiawan and colleagues at Max Planck and the European Space Agency noticed that the star didn"t move at a constant velocity relative to our sun. Their instruments picked up a regular 16-day pattern, where sometimes the star was closer and sometimes farther away. That could be caused by spots on the star, which could confuse velocity measurements, or by the star expanding and contracting (known as stellar pulsations). But the researchers didn"t detect either. That left only one possibility: a planet gravitationally pulling on the star.

塞蒂亞萬和馬普學會的同事們以及歐洲航天局的同事們注意到：相對於我們的太陽來說，恆星HIP 13044並沒有以恆定的速度移動。他們的儀器捕捉到了一種規則的、以16天為周期的移動模式，在這種模式下該恆星時近時遠。這種情況可能是由恆星上的耀斑引起的，耀斑可能會給速度測量造成混亂；還有可能是恆星的膨脹和收縮（被稱為恆星脈動）引起的。但是，這兩種情況研究人員都沒有發現。那麼，只剩下一種可能性：一顆行星的引力拉動了這顆恆星。

If that"s the case, the world is the first foreign planet detected in the Milky Way, the team reports online today in Science. But that"s not the only unusual thing about it. It shouldn"t have formed in the first place. That"s because HIP 13044 is a very ancient, very metal-poor star. It"s about 3 billion years older than our own sun and has only 1% as much metal.

今天，研究小組在《科學》雜誌在線版上報道：如果情況屬實，該星球就是在銀河系所發現的第一顆外來的行星。但是，這不是該行星唯一的不同尋常之處。首先，它本不該在那裡形成——因為HIP 13044是一個非常古老的、特別缺乏金屬的恆星。該恆星的年齡比我們的太陽大大約30億歲，但是所含金屬只相當於太陽的1%。

Until now, the prevailing hypothesis has said that as stars evolve, metals (astronomers" term for any chemical elements heavier than hydrogen and helium) in the swirling disk around them form tiny "seeds" that attract other matter and slowly grow into planets. So far most surveys have backed up the theory: Stars rich in metals, such as our Sun, are much more likely to have planets than stars that don"t. "The fact that you can form planets around a star that has so little of this material is a very surprising and unusual thing," says Christopher Johns-Krull, an astronomer at Rice University in Houston, Texas, who was not involved in the new work.

目前流行的假說是這樣的：隨著恆星的演化，金屬在恆星周圍盤旋的圓盤中形成「種源」，種源吸引其他物質而逐漸地發展成為行星。「金屬」是天文學家們的術語，用來指代任何重於氫和氦的元素。到目前為止，絕大多數觀測符合這個理論：富含金屬的恆星，例如我們的太陽，比那些不含金屬的恆星更有可能擁有行星。「在金屬如此之少的恆星周圍可以形成行星，這是一個非常令人驚訝的異乎尋常之事，」德克薩斯州休斯頓市萊斯大學的天文學家克里斯托弗·約翰斯-科拉爾說，他並沒有參與這項新研究。

But there is a second, more violent, theory about how planets form. If the swirling disk of gas is massive enough—one-tenth the mass of its star or more—Johns-Krull says the gravitational power of the disk can make the disk unstable. In those cases, the star isn"t big enough to keep the disk in check, so the disk starts to collapse under its own gravity and forms planets.

然而，有關行星的形成，還有一個更為激進的理論。約翰斯-科拉爾稱：如果旋轉的氣體圓盤質量足夠大，達到其恆星質量的十分之一或更大，圓盤的引力可以致使其自身產生不穩定，在這樣的情況下，恆星的大小不足以控制圓盤，因此圓盤開始在自己的引力作用下發生塌縮，從而形成行星。

Although it"s still not certain that a planet is behind the variations in the star"s speed, all the findings so far point that way, says NASA astronomer Steven Pravdo. To be sure, astronomers will need to run other tests. Ideally, they would look for the planet to pass in front of the star, but that would work only if the planet orbits at just the right angle, he says. "The claim that it"s extragalactic is kind of a guess," says Pravdo. It"s possible the star was originally part of the Milky Way when it collided with the other galaxy billions of years ago.

美國國家航空航天管理局的天文學家史蒂文·普拉夫多說：在這顆恆星的速度變化之後是否隱藏著一顆行星，這一點尚不確定；但是到目前為止所有的發現都表明，這樣一顆行星是存在的。為了確定下來，天文學家們仍需要進行其他的測試。普拉夫多稱：從理想的角度講，天文學家們可以尋找這顆行星從其恆星前面經過的跡象，但是只有行星的軌道處於合適的角度時，這一招才能奏效。「這顆行星來自外星系的說法只是一種猜測，」普拉夫多說。幾十億年前，銀河系與其他星系碰撞時，那顆恆星有可能本來就是銀河系的一部分。

Although an extragalactic planet is "a nice possibility," says Pravdo, the more exciting find is what this planet could do to the idea of tame, gradual planet formation. Johns-Krull agrees. "This planet says, maybe that"s not right," he says. "Maybe it"s this other, more dramatic process."

普拉夫多說，儘管行星來自外星系只是一種極具吸引力的可能性，但是更加令人興奮的發現將是，這顆行星在行星緩慢漸進的形成過程方面能夠造成哪些觀念上的影響。約翰斯-科拉爾同意普拉夫多的說法，他說：「或許這顆行星可以表明，那個觀點不正確，或許它的形成是另外一個更加引人注目的過程。」