The discovery of planets around distant stars has become like space-shuttle launches--newsworthy but just barely. With some 50 extrasolar planets under their belt, astronomers² have to announce something really strange to get anyone's attention.

对围绕着遥远的恒星运行的行星的发现就好像发射航天飞机一样---有新闻价值但仅此而已。现在天文学家已经在太阳系之外发现了50多颗行星，他们只有发现一些真正奇怪的东西才能吸引人们的注意力。

Last week they did just that. Standing³ in front of colleagues and reporters at the American Astronomical⁴ Society's semiannual meeting in San Diego, the world's premier⁵ planet-hunting team--astronomer¹ Geoffrey Marcy of the University of California, Berkeley, and his colleagues--presented not one but two remarkable⁶ finds. The first is a pair of planets, each about the mass of Jupiter, that whirl around their home star 15 light-years from Earth in perfect lockstep. One takes 30 days to complete an orbit, the other exactly twice as long. Nobody has ever seen such a configuration⁷. But the second discovery is far stranger--a solar system 123 light-years away, in the constellation⁸ Serpens, that harbors one "ordinary" planet and another so huge--17 times as massive as Jupiter--that nobody can quite figure out what it can be. It is, says Marcy, "a bit frightening."

上周的发现就属于此类。在圣迭戈举行的美国天文学会半年一度的会议上，来自加州大学伯克利分校的世界顶级行星搜寻小组天文学家杰弗里·马西和他的同事们向同行和新闻记者展示了两项惊人的发现。第一项是一对行星，每个大小和木星相仿，它们一前一后绕着距离地球15光年的一颗恒星运行。其中一颗三十天沿轨道运行一周，而另一颗所需时间恰好是它的运行周期的两倍。这种相对位置以前从未有人见过。但第二项发现更令人称奇---在巨蛇座内距离地球123光年的地方有一个太阳系，它拥有一颗"普通"行星和一颗体积足有木星的17倍那么大的行星---这么巨大的体积叫人不知道它会是什么。马西说它"有点惊人。"

What's frightening is that these discoveries make it clear how little astronomers know about planets, and they add to the dawning realization⁹ that our solar system--and by implication Planet Earth--may be a cosmic oddball. For years theorists figured that other stars would have planets more or less like the ones going around the sun. But starting with the 1995 discovery of the first extrasolar planet--a gassy monster like Jupiter but orbiting seven times as close to its star as Mercury orbits around our sun--each new find has seemed stranger than the last. Searchers have found more "hot Jupiters" like that first discovery. These include huge planets that career around their stars not in circular orbits but in elongated¹⁰ ones; their gravity would send any Earthlike neighbors flying off into space. Says Princeton astronomer Scott Tremaine: "Not a single prediction for what we'd find in other systems has turned out to be correct."

真正令人震惊的是，这些发现说明天文学家对于行星的了解十分有限，并且让人们开始认识到我们的太阳---不言而喻，还有地球这颗行星---也许是宇宙中的异数。很多年来理论学家们都一直认为其它恒星的行星和围绕太阳运行的行星多少有些相似。但1995年发现了第一颗太阳系之外的行星---一颗像木星一样的巨大气态星球，它围绕自己的母星运行的距离比水星绕着太阳运行时的距离还近7倍。从这一发现开始，每一个新发现似乎都比上一次发现更令人惊奇。搜寻者们已经发现了更多第一次发现中的那种"热木星"。这些发现中有一些巨大的行星，它们围绕母星运行的轨道不是圆形的，而是延长的；它们的重力会让周围像地球这样的星体逃逸到太空中去。普林斯顿的天文学家斯科特·特里梅因说："我们对于会在其它星系里的发现所作的预测没有一次是准确的。"

Last week's giant was the most unexpected discovery yet. Conventional theory suggests that it must have formed like a star, from a collapsing¹¹ cloud of interstellar gas. Its smaller companion, only seven times Jupiter's mass, is almost certainly a planet, formed by the buildup of gas and dust left over from a star's formation. Yet the fact that these two orbs¹² are so close together suggests to some theorists that they must have formed together--so maybe the bigger one is a planet after all.

上周发现的这颗巨星是目前为止最出人意料的发现。传统的理论认为它一定像一颗恒星那样是由一团星际气体形成的压缩星云组成的。它那体积较小的伴侣只有木星体积的7倍，几乎可以确定是一颗行星，是由恒星形成时留下的气体和灰尘累积形成的。不过这两个天体如此接近，一些理论家认为它们一定是一起形成的---那样的话，较大的那一颗也是一颗行星。

Or maybe astronomers will have to rethink their definition of "planet." Just because we put heavenly objects into categories doesn't mean the distinctions are necessarily valid¹³. And as Tremaine puts it, "When your classification schemes start breaking down, you know you're learning something exciting. This is wonderful stuff."

也许天文学家们必须重新思考他们对于"行星"的定义。我们把天体进行分类并不意味着我们的这种区分必然有效。正如特里梅因所说："当你的分类模式开始不起作用的时候，你就知道你正在了解某些令人激动的事情。这是很有趣的。"