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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. |
天文学家们发现了一颗来自其他星系的行星，他们相信这颗行星的大小至少相当于木星。如果该发现得到证实，这颗星球就是在银河系中所发现的第一颗外来的行星，同时该发现也将违背有关行星起源的最主要的假说。 |
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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. |
这颗行星位于2200光年之外的赫米恒星流之中，而赫米恒星流是横截银河系平面的一个环形的古老恒星带。天文学家们认为，赫米恒星流形成于60亿至90亿年以前，当时银河系将另外一个星系撕成碎片，在这个过程中吞没了其中的一些恒星。德国马普学会天文学分会的天文学家约翰尼·塞蒂亚万喜欢观察这些恒星，因为它们有着不同寻常的特性。然而，即使采用不同的标准进行观测，一颗特别的恒星仍然吸引了他的眼球，该恒星是HIP 13044。 |
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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. |
塞蒂亚万和马普学会的同事们以及欧洲航天局的同事们注意到：相对于我们的太阳来说，恒星HIP 13044并没有以恒定的速度移动。他们的仪器捕捉到了一种规则的、以16天为周期的移动模式，在这种模式下该恒星时近时远。这种情况可能是由恒星上的耀斑引起的，耀斑可能会给速度测量造成混乱；还有可能是恒星的膨胀和收缩（被称为恒星脉动）引起的。但是，这两种情况研究人员都没有发现。那么，只剩下一种可能性：一颗行星的引力拉动了这颗恒星。 |
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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，研究小组在《科学》杂志在线版上报道：如果情况属实，该星球就是在银河系所发现的第一颗外来的行星。但是，这不是该行星唯一的不同寻常之处。首先，它本不该在那里形成——因为HIP 13044是一个非常古老的、特别缺乏金属的恒星。该恒星的年龄比我们的太阳大大约30亿岁，但是所含金属只相当于太阳的1%。 |
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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. |
目前流行的假说是这样的：随着恒星的演化，金属在恒星周围盘旋的圆盘中形成“种源”，种源吸引其他物质而逐渐地发展成为行星。“金属”是天文学家们的术语，用来指代任何重于氢和氦的元素。到目前为止，绝大多数观测符合这个理论：富含金属的恒星，例如我们的太阳，比那些不含金属的恒星更有可能拥有行星。“在金属如此之少的恒星周围可以形成行星，这是一个非常令人惊讶的异乎寻常之事，”德克萨斯州休斯顿市莱斯大学的天文学家克里斯托弗·约翰斯-科拉尔说，他并没有参与这项新研究。 |
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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，有关行星的形成，还有一个更为激进的理论。约翰斯-科拉尔称：如果旋转的气体圆盘质量足够大，达到其恒星质量的十分之一或更大，圆盘的引力可以致使其自身产生不稳定，在这样的情况下，恒星的大小不足以控制圆盘，因此圆盘开始在自己的引力作用下发生塌缩，从而形成行星。 |
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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. |
美国国家航空航天管理局的天文学家史蒂文·普拉夫多说：在这颗恒星的速度变化之后是否隐藏着一颗行星，这一点尚不确定；但是到目前为止所有的发现都表明，这样一颗行星是存在的。为了确定下来，天文学家们仍需要进行其他的测试。普拉夫多称：从理想的角度讲，天文学家们可以寻找这颗行星从其恒星前面经过的迹象，但是只有行星的轨道处于合适的角度时，这一招才能奏效。“这颗行星来自外星系的说法只是一种猜测，”普拉夫多说。几十亿年前，银河系与其他星系碰撞时，那颗恒星有可能本来就是银河系的一部分。 |
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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." |
普拉夫多说，尽管行星来自外星系只是一种极具吸引力的可能性，但是更加令人兴奋的发现将是，这颗行星在行星缓慢渐进的形成过程方面能够造成哪些观念上的影响。约翰斯-科拉尔同意普拉夫多的说法，他说：“或许这颗行星可以表明，那个观点不正确，或许它的形成是另外一个更加引人注目的过程。” |
