本帖最后由 positron 于 2009-1-21 09:36 编辑
一个老掉牙的话题，但是不错的文章，原帖由nngs发表：
http://www.astronomy.com.cn/bbs/viewthread.php?tid=57511&highlight=Is%2BSedna%2Ba%2Bplanet
这几天整理电脑文件时，发现自己不知何时将该文译了出来，没在论坛上搜到译文，就把自己的发上来，水平所限，不足之处请高手指正。
Is Sedna a planet?
赛德娜是行星吗？
不是，至少在我们的定义下它不是。虽然天文学家至今未能在行星的精确定义上达成一致，但是我们可以使用如下的定义，它即符合历史又在科学上具有合理性。根据我们的定义，赛德娜不是行星，冥王星也不是，但是其他八颗是。
首先，行星的定义是什么？
令人吃惊的是，现在“行星”这个词并没有准确的科学定义。让科学家定义一个已经普遍使用并且每个人从小学就开始理解的词是很少见的，如何让人给一个已经是事实的名词构想一个科学的定义？
在这种情况下，我们相信对一个词进行科学地、精确地、合理地定义时，对这个词的意思不论是忠于历史的还是流行的理解都是重要的。我们将用这个观点——忠于历史的和科学正确的——作为标准给“行星”一个可能的定义。我们分析了关于“行星”定义的四种主流观点（虽然在我们所知的范围内还没有最流行的观点）。
1.纯粹历史的：水星、金星、地球、火星、木星、土星、天王星、海王星和冥王星是行星。太阳系内没有其他天体是行星。这一定义是忠于历史的，但是在科学合理上却不幸地失败了。如果找到一个比冥王星大的天体呢？它是什么？为什么冥王星是行星，而一个有它四分之三大的天体，像赛德娜，就不是行星呢？这种定义完全缺乏科学依据，使“行星”这个词在科学上完全没有意义。
2.超越历史的：从水星到冥王星是行星，任何一个被发现比冥王星大的天体也是。我们相信这个定义是世界范围内最通俗流行的用法，即使人们并没有意识到他们已经使用了这个定义。事实上，如果赛德娜比冥王星大，大部分人都会为它成为第十大行星而欢呼。这个定义和上一个定义一样是符合历史理解的，但是，也如上一个定义一样，仍然缺乏科学性。为什么把冥王星作为截至大小呢？在冥王星、赛德娜和Quaoar之间真的就存在足够大的不同，以至于其中的一个可以被叫做行星而其他的就不能吗？科学的回答是大大的不！
3.重力球形的。行星是任何在自身的重力作用下呈球形并且绕太阳公转的天体。这一个定义很有特点！它是严格科学的，也是符合历史习惯的，因为所有的在历史定义下被我们叫做行星的天体都确实在它们自身的重力作用下呈球形。更为重要的是，天体是否呈球形的分界线仅仅比冥王星稍小一点。那为什么不利用这一个偶然简单地定义行星是呈球形的天体呢？这样做意味着我们必须同意把其他几个天体称为行星。赛德娜、Quaoar、谷神星，以及可能会有很多柯伊柏带天体也会是球形的，这样，根据这个定义，它们都是行星。而增加这些行星仅仅会对建立在严格科学原则上的定义带来很小的不便。
不幸的是，这一定义在历史检验下却完全失败。历史上，除了行星的历史定义和行星是否是球形的转变尺寸巧合地近似外，球形的标准从何而来？以前，在讨论一个天体是否算是一个行星的时候，并没有考虑这个天体是否是球形的。谷神星最初被认为是行星并不是因为它是球形的（实际上当时还没有球形这个概念），而是因为它是被发现存在于火星和木星之间的唯一天体。当在差不多同一区域发现其他大小近似的小行星后，才决定将它们统称为小行星带，而不是行星。
球形是一个很重要的物理性质，重力是太阳系的支配力量，所以也许有必要用一个特定的词来描述太阳系内呈球形的天体。但是不能单单的因为历史上所有的行星是球形的而认为定义所有呈球形的天体是行星是一个好的科学定义。更好的建议是用另外一个词来描述这类天体。球状体？或是引力层？实际上，我们更喜欢用“行星体”作为描述绕太阳公转的球形天体的新词。行星都是行星体，但行星体不一定是行星。
4.种类分类法。这一定义需要更多的解释和对太阳系的更多了解，但是最终会得到行星最令人满意的定义。就像太阳系内的天体可以自然地分为球形天体和非球形天体，它也可以自然地分为独立天体和某一大天体类群的成员。最好的例子是小行星带的巨大天体数。我们称它们为一个族群是因为在同一个空间区域存在大量的天体，这些天体具有连续范围的大小，从比较大的天体（如谷神星）到一些稍小的天体（如灶神星、智神星、Hermione），再到大量非常小的天体（岩石、尘埃）。但独立天体完全不同。在它们的空间区域只有它们（比如说地球），其次是更小的天体的集合（近地小行星），之间没有连续的天体。简单的例子戏剧性地帮助我们表现了连续天体类群和独立天体的区别。作为最大的小行星，谷神星直径900km。第二大的小行星是智神星，直径520km。然后是灶神星直径500km，Hygiea直径430km，直径如此一直减小。小行星带中大小临近小行星之间直径的变化从不会超过两倍。相比之下，地球直径大约12000km，但是地球附近最大的天体小行星Ganymed，直径大约41km，两者相差300倍！
水星、金星、地球、火星、木星、土星、天王星以及海王星在这种定义下都将算作独立天体。冥王星和Quaoar不是。冥王星显然是一个柯伊柏带天体，这一点可以从在同一区域存在比冥王星稍小的天体上看出（Quaoar, 2004 DW, Varuna），然后恰好有一批更大数目的天体比那个天体稍小，然后一直如此。
那赛德娜呢？赛德娜是它轨道附近现在所知的唯一天体，但是我们怀疑在那一区域发现更多的天体只是时间的问题。因此我们感觉将赛德娜算作一个大天体类群的成员比将它作为一个独立天天更为合理。这一分类确保我们不会在几十年后在同一区域发现更多天体而不得不再回来重新定义赛德娜。
当我们用科学的方法准确区分了独立天体和天体类群，它将会对我们描述这些天体产生指导意义。每一个大天体类群都可以描述为一个特殊的独立类群（如小行星带、柯伊柏带、内奥尔特云、奥尔特云等）。那么独立天体呢？它是描述行星的最佳方法吗？
让我们更深入地分析这一定义。首先，它是严格符合科学的和有历史基础的。但是和前面定义的“引力层”一样，确定一个行星是一个独立天体而不是一个大天体类群的成员，这种做法有历史基础吗？是的！如先前所说，历史上，谷神星和其他几个最早发现的小行星在开始的时候被认为是行星。而当了解到在同一空间区域有大量的小行星后，才决定它们不应再被定义为行星。历史上，行星和天体类群一直有明确的区别。任何忽略此区别的定义在历史面前都是严重站不住脚的。这一对历史的简单看法说明冥王星和谷神星是完全类同的。冥王星最初被认为是独立天体，随着时间的推移，我们在它附近找到了更多的天体，并意识到它是一个天体类群的成员。从历史角度，冥王星当然也不应该再被认为是行星。
这样我们对行星这个词有了最终的概念。太阳系内的每一个天体可以很自然地分为独立天体或者是一个天体类群的成员。独立天体是行星，而天体类群的成员不是。这一定义符合区分小行星和行星的历史要求，也符合所有科学上的要求。
但这个定义仍然不完美。一些人会以一种病态的心理想象（甚至可能发现）上述分类方案会失败。相比之下，前三个定义更为严格并且永远不需要重新定义。我们找不到前三个定义在下述方面的优势。当我们对我们的太阳系有了更多的了解以后，我们的语言——日常的和科学的——应该改变从而符合我们的认识。我们认为我们提出的分类法能够满足我们所发现的太阳系的一切现象，但是我们会更愿意发现一些不服从我们现在认为我们知道的定义的天体，并且迫使我们对基本问题进行全面的重新思考，如“什么是行星”。
原英文如下：
is Sedna a planet?
NO, at least not by our definition. Astronomers have been unable to agree on a precise definition of "planet", but we have a suggestion for a definition below which is both historically and scientifically motivated. By our definition, Sedna is not a planet. Nor is Pluto. But the other 8 are.
What is the definition of a planet?
Astoundingly, no precise scientific definition of the word "planet" currently exists. It is rare for scientists to have to define a word that is already in common usage and that everybody from school children on up already understand. How does one then go about constructing a scientific definition of such a word after the fact?
In such cases, we believe that it is important to be both true to the historical and popular perception of the meaning of the word while being scientifically descriptive, accurate, and meaningful. We will use these points -- historically valid and scientifically meaningful -- as the criteria on which to judge potential definitions of the word "planet." We have identified 4 major ideas for the definition of the word "planet" (though the most common have never been written down to our knowledge):
* Purely historical. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto are planets. Nothing else in the solar system is a planet. This definition is definitely historically valid, but fails miserably under scientific meaning. What if a new object larger than Pluto is found? What is it? Why is Pluto a planet but an object 3/4 its size, like Sedna, is not? This definition, completely lacking in scientific motivation, makes the word "planet" meaningless as a scientific description.
* Historical plus. Mercury through Pluto are planets, as is any newly discovered object larger than Pluto. This definition is, we believe, the one in most common colloquial use throughout the world, even if people don't realize that this is the definition they are using. Indeed, if Sedna had been larger than Pluto, most would have hailed it as a 10th planet. This definition -- like the previous -- is historically consistent, but -- like the previous -- still fails the scientific test. Why is Pluto the cutoff size? Is there really a big enough difference in size between Pluto and Sedna and Quaoar that one should be called a planet while the others are not? The scientific answer remains a resounding no.
* Gravitational rounding. Any object which is round due to its own gravitational pull and which directly orbits the sun is called a planet. This definition is very different! It is strictly scientific, yet historically valid, as all objects that we call planets by the historical definitions are indeed round due to their own gravitational pull. More importantly (and by a complete coincidence) the dividing line between objects which are round and those which are not round is just a few times smaller than the size of Pluto. So why not take advantage of this coincidence and simply define planets to be objects which are round? To do so means that we must admit several other bodies to the class of "planet." Sedna, Quaoar, the asteroid Ceres, and perhaps a dozen Kuiper belt objects are also likely to be round and thus, by this definition, planets. But these additions are perhaps a small price to pay for a definition which rests on solid scientific principles.
Unfortunately, this definition completely fails the historical sanity check. Historically, where does the criterion to be round come from, except for the near coincidence between the historical definition of planet and the transition size from round to not round? At no time in previous history has any discussion of whether or not an object is round been part of the discussion of whether or not it should be called a planet. Ceres was initially considered to be a planet, but not because it is round (which was unknown at the time), but because it was the only object known to exist between Mars and Jupiter. When other asteroids of similar sizes were found at nearly the same location it was decided to call them all members of the asteroid belt, rather than planets.
Roundness is an important physical property, and gravity is the dominant force in the solar system, so perhaps it is important to have a special word which describes the class of objects in the solar system which are round. But simply because all historical planets are round does not at all mean that it is good science to define all round objects to be planets. A much better idea is to use a different word to descibe these objects. Spheroids? Gravispheres? Actually, we prefer the word "planetoid" as a new word to descibe round objects orbiting the sun. All planets are planetoids. Not all planetoids are planets.
* Population classification. This definition requires a little more explanation and a little more understanding of the solar system, but, in the end, leads to the most satisfactory definition of "planet". Just like the solar system very naturally divides itself between round objects and non-round objects, it also very naturally divides itself between solitary individuals and members of large populations. The best known example of a large population is the asteroid belt. We call it a population because one region of space contains objects with a continuous range of sizes from one moderately large object (Ceres) to a handful of slightly smaller objects (Vesta, Pallas, Hermione) to a huge number of extremely small objects (rocks, dust particles). The solitary individuals are much different. In their region of space there is only them (Earth, say) and then a collection of much much smaller objects (the near-earth asteroids), with no continuous population in between. A single example helps to dramatize the difference between a continuous population and a solitary individual. Ceres, the largest asteroid, has a diameter of 900 km. The next largest asteroid, Pallas, has a diameter of 520 km. After that is Vesta at 500 km, and Hygiea at 430 km, and the list continues on down. The jump in size between asteroids is never more than a factor of two. In contrast, the earth has a diameter of about 12,000 km, while the largest other object in the earth's vicinity, the asteroid Ganymed, has a diameter of about 41 km, a factor of 300!
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune all count as solitary individuals by this definition. Pluto and Quaoar do not. Pluto is clearly a member of the Kuiper belt population, as can be seen from the fact that there are objects in the same vicinity slightly smaller than Pluto (Quaoar, 2004 DW, Varuna), and then even a larger number slightly smaller than that, and then on down.
What about Sedna? Sedna is currently the only object known in its orbital vicinity, but we strongly suspect that there will be many others found out there with time. We thus feel it is more reasonable to classify Sedna as a member of a large population (the inner Oort cloud of objects) rather than a solitary object. This classification saves us from having to go back and reclassify Sedna in a decade when we find more objects!
Since there is a clear scientific distinction between solitary individuals and members of large populations it is instructive to come up with words to describe these objects. The large populations can each be described by the particular population (asteroid belt, Kuiper belt, inner Oort cloud, Oort cloud). What about the solitary individuals? Isn't the best word to describe them "planet"?
Let's examine this definition in more details. First, it is certainly scientifically motivated and well-founded. But so was the "gravisphere" definition above. Is there any historical basis for saying that a planet is a solitary individual that is not a member of a large population? Yes! As mentioned earlier, historically Ceres and the first few asteroids were initially classified as planets. Only when it became known that there were many many asteroids in similar orbits was it decided that they should no longer be classified as planets. Historically, there is a clear distinction between planets and populations. Any definition which fails to make this distrinction is in strong trouble on historical grounds. This simple look at history shows that Pluto is completely analogous to Ceres. Pluto was initially thought to be a solitary individual. Over time we found more objects in the vicinity and realized instead that it is a member of a large population. Historically, then, Pluto, too, should no longer be considered a planet.
We are thus left with a final concept of the word planet. Every object in the solar system quite naturally can be classified as either a solitary individual or a member of a large population. The individuals are planets. The populations are not. This definition fits the historical desire to distinguish between asteroids and planets, and this definition fits all of the requirements of scientific motivation.
Even this definition is not perfect. People will always be able to imagine (and perhaps even find) pathological scenarios in which the above classification scheme fails. In contrast, the first three definitions are much more rigorous and will never need refining. We don't find this aspect of the first definitions an advantage, however. As we learn more about our solar system our language -- both popular and scientific -- should change to fit our knowledge. We think that our proposed classification scheme will suffice for everything that is found in our solar system, but we would like nothing better than to find some object which defies everything that we currently think we know and forces us to completely rethink fundamental questions like "what is a planet."
[ 本帖最后由 positron 于 2008-5-30 12:25 编辑 ] |
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