新发现后的争论：冥王星能称之为行星吗？

心灵

国家地理杂志新闻

Published August 24, 2011

发表日期：2011年8月24日

Officially, Pluto is still not a planet.

按照官方说法，冥王星仍然算不上一颗行星。

But five years after the ruling that demoted the icy object to dwarf planet, people continue struggling with the definition, and the debate over what exactly should be called a planet remains as contentious as any political divide.

把冥王星由行星降级为一颗矮行星已经过去了5年时间，人们还是就其定义争论不休，对于到底什么星体才能称之为行星的讨论如同政治分歧一般充满争议。

"Maybe it's just an argument over semantics, but we ought to be worried about semantics. We learned that lesson when the definition was changed," said Marc Kuchner, a planetary scientist at NASA's Goddard Space Flight Center in Maryland.

作为位于马里兰州的美国航空航天局下属 Goddard 空间飞行中心的一名行星学家， Marc Kuchner说；“也许这只是一场语言意义上的争论，但是我们还是应该有足够的重视，当初行星的定义改变时已让我们意识到了这一点。”

"After the ruling, astronomers everywhere were besieged by complaints from everyone big and small. A planet is a very personal thing—we think of the Earth, the moon, and the other planets as part of our home, and maybe that's why we got so upset about Pluto."

“在那次裁定之后天文学家无论何地都被大大小小的人物所困扰。因为一颗行星是及其个人的事务，我们可能把地球、月球或者其他行星都当作自己生命中的一部分，这也许就是我们被冥王星弄的心烦的原因”

Since the 2006 ruling, astronomers have also made a number of scientific advances that further cloud the issue, from discoveries of planets that don't orbit stars to new models of how our own solar system may have rearranged itself since birth.

在2006年（天文大会）的裁定之后，天文学家得出的一些最新研究成果更是让这个问题扑朔迷离，从对那些没有绕恒心运转的行星研究，到对太阳系可能从诞生之初可能重构了自己的模型的研究。

This time next year the International Astronomical Union (IAU)—the organization that originally called for a vote on a planet definition—will hold its general assembly in Beijing, China, and many in the field wonder whether the time is right to revisit the decision.

明年的这个时候，当初对行星进行定义的国际天文学联合会，将会在中国的北京召开大会，许多相关人士猜想可能此次会议是一个重新审视06年的决议的时机。

Birth of the Dwarf Planet

矮行星定义的诞生

The issue of whether Pluto should be a planet first arose in the 1970s, when scientists were able to refine their estimates for the mass and size of the distant body. With each new measurement, Pluto got lighter and tinier, until astronomers realized that the object is in fact smaller than Earth's moon and has a very low density.

就冥王星能否算得上一颗行星的争论起于上世纪70年代，那时的科学家方能精确测量冥王星的质量和体积。用各种测量方法重新测量后，冥王星变轻了、变小了，这时候天文学家才意识到冥王星实际上还不如月球大，并且密度也很小。

Adding to the oddities, in 1978 scientists announced they'd found a moon of Pluto—but one that's almost half its size, making it the largest moon in relation to its parent body.

更奇怪的是，在1978年科学家宣传他们发现了一颗冥王星的卫星，它的体积居然达到冥王星体积的一半，这颗卫星也就成为了已知卫星中与其母星体积相比最大的一颗。

Over the decades scientists continued to find similarly large objects in Pluto's neighborhood, a region of the solar system beyond the orbit of Neptune called the Kuiper belt.

海王星轨道以外有一个被称之为太阳系的柯伊伯带的区域，接下来的几十年内，在这个区域中靠近冥王星的位置科学家接连发现了许多和冥王星相似的星体

The biggest challenge for Pluto came in 2005, when Caltech astronomer Mike Brown announced that he'd found a Kuiper belt object more massive than Pluto—a potential tenth planet provisionally called 2003 UB313.

2005年出现了对冥王星来说最大的挑战，当时加利福利亚理工学院的天文学家Mike Brown 宣称他在柯伊伯带中发现了一颗比冥王星更大的星体—这颗潜在的第十大行星暂时被称作2003 UB313。

The discovery prompted the IAU to convene a committee to decide on an official definition of a planet.

正是这次发现促使了国际天文学联合会召开了一次委员会，以决定如何给行星做一个官方定义。

It was a bureaucratic problem, as it had to do with naming rights for these kinds of things," said Owen Gingerich, the Harvard-Smithsonian astronomer who chaired the committee. After all, if 2003 UB313 really was a new planet, it would need a proper name on which everyone could agree.

“这是一个官僚性质的问题，因为它涉及到对这些星体的命名权等诸如此类的事务”，作为委员会的主持者，哈佛大学斯密森学会的Owen Gingerich这样说到。毕竟，如果真要把2003 UB313 算作一颗行星的话，它确实需要一个大家都认可的正式名称。

In drafting a definition, "there were two possible routes to take—one that would deal with the physical nature of these objects, and another that would address dynamically where they fit into the structure of the solar system, with respect to their orbits and so on."

在制定一个定义时，“有两个途径可以选择—一种是就这些星体的物理性质来定义，另一种则显得更为的动态化，如这些星体自身的轨道是否与太阳系的结构相适应等等”

The committee initially proposed that there be two categories of planets: the classical planets and the group of planet-like bodies beyond Neptune, to be called plutons, "as a way of tipping our hat to Pluto," Gingerich said.

委员会最初提议将行星分为两类：即经典行星和在海王星轨道以外的类行星体，他们被称为类冥王星。“就当我们对冥王星的一种敬意”Gingerich说到。

The planetlike object Ceres would have to be in a separate class, because it resides in the main asteroid belt, between the orbits of Mars and Jupiter. So the committee suggested it be called a dwarf planet.

如果真这样的话，类行星体谷神星就会被归到另一个体系了，因为它位于火星轨道和木心轨道之间的小行星带中。所以委员会建议把它称为矮行星。

The draft definition was put to a vote in 2006 at the IAU general assembly in Prague, the Czech Republic. What emerged from the session is that, to be a planet, an object must:

该定义的草案于2006年给交给了在捷克举行的国际天文学联合会大会进行投票。最后会议达成决议，能够被称之为行星的星体必须具备以下特征：

a) be in orbit around the sun,

1、位于绕恒心运转轨道中

b) have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,

2、

质量必须足够大，它自身的吸引力必须和自转速度平衡使其呈圆球状

c) have cleared the neighborhood around its orbit.

3、

必须清除轨道附近区域。

Instead of plutons, the IAU members present voted that Pluto, Ceres, and 2003 UB313—now known as Eris—would all be called dwarf planets, and that this term is not for a subclass of planets but is for a unique category of solar system objects.

替换了以前的类冥王星方案，国际天文学大会成员投票决定冥王星、谷神星以及现在被称为阋神星的2003 UB313都被归为了矮行星，这个分类并不是行星的子分类，而是太阳系中星体的一种独立分类。

Exoplanets Complicate Matters

太阳系以外的行星让行星定义更加复杂化

At the time of the ruling, the IAU noted that the new definition does not apply to anything outside the solar system, leaving it unclear how the organization defines the planetary objects found orbiting other stars.

就在做裁定的时候，国际天文学联合会注意到新的定义并不能应用于太阳系以外，使得在定义那些围绕其他恒心运转的类行星体时，变得不明确。

Since 2006 there's been an explosion in the number of these extrasolar planets, or exoplanets, known to exist, with the current count at more than 400 and rising. Many are bigger than the gas giant Jupiter, but astronomers have found an increasing number of worlds close to Earth's mass, some of which may be habitable.

从2006年开始，太阳系外行星被发现的数目开始了激增。就目前数据已经发现了400多颗太阳系以外的行星，并且这个数目还在增长。他们当中的许多都比气体巨行星木星还要大，不过天文学家还是发现了相当数目的和地球差不多大小的星体，其中的一些可能还适宜居住。

And in the past few years astronomers have even found rocky planets akin to Earth's mass that don't orbit stars at all.

在最近的几年，科学家甚至发现了一些不环绕任何恒心运转的和地球相当大小的的岩石行星。

By the current IAU definition, none of these objects are official planets, because they violate the first rule about orbiting the sun.

就目前国际天文学联合会的定义，这些星体都不能算作是官方定义中的行星，因为他们违反了行星必须围绕恒心运转的这第一必备条件。

"I was disappointed when I learned that exoplanets were not included in the definition," said NASA's Kuchner.

美国国家航空航天局的 Kuchner说“在知道这些太阳系以外的行星没有被定义所包括的时候，我感到很失望。”

But for now, he said, the issue is a moot point, because the definition of a planet was necessary mostly to sort out objects that are much smaller than anything we've seen outside our solar system.

但现在，他说这种争论其实是没有实际意义的。因为最需要用行星的定义来进行甄别的，是那些我们根本没还没发现的，小得多的，太阳系外的星体。

The second part of the definition, that planets must be massive enough to be nearly round, helped draw a line between bodies such as Pluto and large asteroids such as 433 Eros, a 21-mile-long (34-kilometer-long) space rock shaped somewh like a peanut.

行星的第二条定义，行星的体积必须足够大到成为一个近似的圆球体，这有助于在冥王星和诸如爱神星（一颗34公里长呈花生米状的小行星）这类的小行星之间划一条界限。

"For now we're good at finding exoplanets that are several times bigger than Earth. The smallest planets we've seen around other stars are much bigger than the bodies for which element two of the definition matters," Kuchner said. "And besides, we usually can't see their shapes to tell if they are round or not..

“现在我们在发现那些比地球大上好几倍的太阳系外行星方面已经做得很不错了，目前我们发现的最小的太阳系外行星比那些能够引起行星分类争议的星体要大上许多”， Kuchner 说到：“更何况，我们通常也不能看到他们的形状来区分他们是不是球体”

"We're also good at seeing big clouds of small particles of dust. ... But in between dust grains up to a millimeter in size and the smallest planets we can see, we're blind."

“我们也同样擅长于发现那些一大堆的尘埃...但是如果要去发现那些毫米级的尘埃，和那些我们能看到的最小的行星，我们几乎就成了盲人。”

According to Kuchner, "the revolution that happened in 2006 was about how dwarf planets are not planets. Instead this is another kind of object in the solar system that we have to be aware of."

Kuchner认为：“2006年变革主要目的是说明为什么矮行星算不上行星，而不是说我们在太阳系中发现了一种新的星体”

He likens the situation to a child adjusting to a new sibling: "You don't know how you're supposed to feel about it at first. I'd like us all to think about the dwarf planets out there as new siblings that we have to get to know and learn to love."

他把这比作一个小孩去适应一个新的兄弟姐妹，“最初你不知到如何去适应这种情形，我希望我们大家都把矮行星当作一个我们的新伙伴，我们必须尝试去了解它，去爱它”

And "it's okay with me if we have another definition for exoplanets. I imagine that's something that will be revised with time."

“对于我而言，对太阳系以外的行星进行一次定义，是可以接受的。我想只是随着发展可能会出现的事情。”

Orbital Neighborhood Boundary Unclear?

轨道周围清除的不干净？

Perhaps the most controversial part of the IAU ruling, though, is whether an object has cleared its orbital neighborhood.

也许对IAU决议最具争议的部分在于，如何确认一颗星体清除了自己轨道附近的物质。

Kuchner, who was a graduate student under Caltech's Brown, thinks this part of the definition is the most subjective.

加利福利亚理工学院布朗的一名研究生Kuchner，认为这是定义中最具主观成分的一条。

In baseball, he said, "if you have a foul ball, it's because the ball landed on one side of the line—that's pretty clear. But it's harder to say if something's a strike. ... That relies on someone calling it."

他拿棒球做了一个比喻：“如果你打出了一个界外球，很明显的这是因为球到了边界的另一边。但是去定义怎样的击打算一次击打则恨难说了....这全看人们怎么去说了”

In the case of defining a planet, IAU made the call, and "now we have to use this definition and try to play the game."

在定义行星这点上，IAU给了行星一个说法，“现在我们就要使用这个定义来完这场游戏了”

Overall, he added, the ruling was crucial for limiting the number of things in the solar system that deserve to be called planets.

他认为总的来说，这个决议在限制太阳系中那些真正配得上为行星的星体的数目上，起到了关键的作用。

"We really didn't have a choice," he said. "It was either going to be eight planets or a whole lotta planets. Nature sort of forced our hand."

“我们确实没有什么更好的办法了”，他说：“要么就只有8颗行星，要么就是一大堆行星。是造物主迫使我们这样做的”

But other astronomers aren't in favor of placing those kinds of limits.

但是还是有另外一些天文学家并不想设置这些对行星的限制。

"There are more than 200 bones in the human body. Does that mean we should redefine bones to make life easier for medical students?" argued Timothy Spahr, head of the IAU's Minor Planet Center based in Cambridge, Massachusetts.

麻省剑桥市IAU小行星中心的主任Timothy Spahr争辩说:"人体内有200多块骨头，是否我们就该为了方便医学院的学生也来重新定义一下骨头"

Instead the IAU definition makes life more complicated for astronomers, he said, because the notion of whether an orbital neighborhood has been cleared remains hazy.

实际上IAU的定义对于天文学家来说，反而上事情更复杂化了，他认为，如何判断一条轨道

附近区域是否得到了清除，还是很让人迷惑。

As an example, Spahr points to the increased number of known Earth-crossing asteroids, including roughly 8,000 that are considered near-Earth objects. While these space rocks don't exactly share our planet's orbit, they do cross it, in the sense that when they are closest to the sun, they are inside Earth's orbital path.

Spahr以不断增加的穿越地球轨道的小行星作为例子，这大约包括8000多颗靠近地球的小行星。虽然这些小行星并未和地球共享轨道，他们却实际上穿过了地球轨道，当他们靠近近日点的时候，他们则在地球绕日轨道内侧。

"There's certainly no big donut where Earth is, just a big mass of objects" that could be said to share our neighborhood, he said.

“显然在地球轨道上没有什么大东西，只有这些大量的小行星”但这还是可以说他们和我们共享了轨道。

This mass of objects will probably always exist, as asteroids in the main belt collide, break apart, and send new material on orbits closer to Earth's.

这众多的小行星可能会一直存在，因为小行星带中的星体会发生碰撞，解体然后输送新的物质到我们地球附近

"In 50 million years our orbital neighborhood will look pretty much the same." By some counts, that means Earth will not ever clear its orbit of debris.

“在5000万年内我们的轨道附近看上去会几乎一样”通过一些计算他得出了这一结论，这意味着，地球始终都不能清除自己的轨道。

Spahr also notes that recent models of the solar system suggest the planets didn't form where they currently reside.

Spahr同时指出，最新的模型认为行星并非是在其现在的位置上形成的。

"There's been a lot of work done that says comets and other objects [including many dwarf planets] were formed between Neptune and Jupiter, and as the giant planets migrated, they threw stuff out," he said.

他说“通过很多的研究发现彗星和一些其他的星体（包括很多的矮行星）是在海王星轨道和木心轨道之间形成的，当这些巨行星移动的时候，他们会抛射出一些物质出来”

"You can see signatures of Jupiter's migration in the main asteroid belt.

“你能在小行星带中看到明显是属于木星的物质”

"Probably as Neptune migrated outward, it swept up objects like Pluto and locked them in orbital resonance" out in the Kuiper belt, as they exist today.

“或许在海王星外移的过程中，它抛射出了像冥王星这样的东西，然后将他们固定在了与自身轨道相交的地方。”就如同他们现在的位置，在柯依伯带以外。

And "certainly in the early solar system, not very much had cleared its zone out," he added. "So are you a planet yesterday, today, and tomorrow? I can't tell you that" based on the IAU definition.

“显然在早期的太阳系，行星们并没有完全清除他们的轨道”，他说到：“昨天是颗行星，今天，明天还能是颗行星吗?依据IAU的定义并不能给你一个答案。”

Planetary Science Constantly Evolving

行星科学在不断的发展

For his part, Spahr favors a simpler definition than the current version.

就Spahr自身来说，他希望一个比目前的定义更为简单的定义。

"Orbiting a star and round is a good way to start," he said. Planetary scientist Alan Stern at the Southwest Research Institute calls it the Star Trek criteria, he added: "If you can look out the viewfinder of the Enterprise and see it's round, it's a planet."

“围绕一颗恒心运转，并且自身是个球形，是个不错的开始”，西南研究院的行星科学家Alan Stern 把这当作是星际迷航的定义标准，他说道：“如果你从企业号航天飞机的取景器上看出去，发现它是球形的，那么它就是颗行星”

From there, Spahr said, planets could be grouped into subclasses: terrestrial planets like Earth, gas giants like Jupiter, and icy outer planets like Pluto. "We could even have a category for rogue planets, to account for the worlds that don't orbit stars."

对于这点， Spahr 说到，可以把行星分类为若干的子类：如地球这样的类地行星，像木星那样的气体巨行星，和像冥王星一样外面覆盖者一层冰的行星。“我们甚至可以给那些孤星划一个类别，就是那些不绕恒心运转的世界”

While NASA's Kuchner thinks the current definition should stand for now, he says he's "happy that we are constantly updating our definitions and revising our vision of universe—that's what science is all about."

同时NASA的Kuchner认为目前的定义还是应该保留，他说：“很高兴能能不断的更新我们定义，重新认识我们的宇宙，—这便是科学的意义所在”

And of course, no matter what you call it, many astronomers will continue to see Pluto as one of the most fascinating objects in solar system, the Minor Planet Center's Spahr said.

小行星中心的Spahr说到，无论你怎样的称呼它，还是会有大量的科学家始终把冥王星视为太阳系中最神奇的星体。

The IAU definition, "doesn't change the fact that we're going to visit Pluto with [the New Horizons] spacecraft, and scientists are still going to go hog wild over all the data we collect."

“IAU的定义不会改变我们将要用探测器（新地平线号）去探测冥王星的事实，科学家仍旧会因我们探测到的数据而兴奋不已。行星系統。

海卓與尼克斯的大小都非常的小，海卓較明亮，也可能較大直徑約為52-160公里，尼克斯的直徑約為16-72公里，大概跟台北縣差不多大。
海卓的命名為希臘神話中的九頭蛇，而尼克斯是神話中的水妖。