【宇宙新知】地球誕生之謎：行星大搬家

■ 美國自然歷史博物館天體物理館館長麥克洛（Mordecai-Mark Mac Low）在第215屆的美國天文協會會議上，提出修正古典行星遷徙理論的見解。他認為，依照舊有的理論，地球是不可能形成的，它只會落入太陽之中。若將熱能的變化也列入考量，行星就在新的模型中就可能向外或向內「遷徙」，從而安穩地被留在軌道上。

撰文 ∣ Dr. Jonathan Shock??? 翻譯?∣ 盧愛羅

At the American Astronomical Society meeting in Washington on January 6 the exciting new results from a recent paper were presented by Mark Low, a coauthor on one of the latest papers to study planetary formation and migration These results shed new light on how planets like our own may come into being from the lifeless dust and gas which circles stars like the sun.

　　美國天文協會今年一月六日在華盛頓召開的會議中，有個令人興奮的發現。這是由麥克洛提出的，他是這份最新的行星形成與遷徙研究的共同作者。這些結果啟發新靈感，揭示了行星（好比我們居住的地球）從環繞恆星（像是太陽）的一團無生命塵埃與氣體中誕生的奧秘。

It is a scientist's job to look at the world around us and to discover its inner workings. A scientist builds a model of a system which may describe anything from the inner workings of an atom, to the firing of neurons in the human brain, to complex weather patterns all the way up to the large scale structure of the universe.

　　科學家的工作之一，就是觀察我們周遭的世界，進而發掘內在的機制。科學家建立系統性的「模型」（model），期待能夠描述任何事情的內在機制，小自一顆原子的內部結構，到腦中億萬神經元電光石火的一瞬間訊息傳導，甚至是複雜的氣象系統，乃至於廣袤的宇宙構成。

By writing down a theory in the language of mathematics and comparing its predictions to the observations we make in the laboratory or the world around us we are able to get ever closer to a fundamental understanding of nature.

　　透過數學語言寫下理論，再比較預測成果和實際觀測而得的結果，我們期待能對自然的基本構成更瞭解一些——實際觀測的結果或從實驗室的模擬得來，或從我們身處的世界。

It is often the case that a system is too complicated to model exactly, so we must make approximations.

　　但是也常遇到諸如此類的問題：系統實在是太複雜了，無法用模型描述，所以必須取近約數（approximations）。

We don't model the fall of an apple from a tree by calculating what every particle in the apple and the earth experience. Often this is not a problem and we can capture the most important details through approximations (the apple as a single object and not one made up of countless electrons and protons), but sometimes our models fail to capture the very essence of the problem we are attempting to study.

　　一顆蘋果從樹上落下時，我們並不會計算蘋果內每個分子跟地球間的相互運動來建立模型。因為這通常不算是個問題，人們可以仰賴近約法捕捉最重要的細節（我們視蘋果為一個單一物體，而非由無數的電子和中子形成）。但有時這樣的模型也可能失敗，因為它沒辦法捕捉我們想研究的問題中最關鍵的構成細節。

This was the case until very recently when it came to our understanding of the formation of the Earth in the early era of the solar system, some 4.5 billion years ago. Clearly this is an important problem to answer from many perspectives. According to previous models, the world we stand on should simply not be here!

　　直到人們最近開始關注地球在太陽系早期形成的過程為止（那可是45億年前的事了），這都是研究宇宙理論的惱人問題之一。在各方面而言，這很明顯的都是一個亟待回答的問題。根據以往的模型，我們腳下的這個世界，根本不應該存在在這裡！

The formation of planets around a star is an exceedingly complicated problem, with an uncountable number of particles (dust and gas) flowing in a disk around the parent star, interacting in many complex ways.

　　恆星週圍的行星形成是一個超級複雜的問題，無數的分子（灰塵與氣體）環繞恆星碟狀流動，產生許多複雜互動。

In this problem as in many others that scientists face, it is impossible to model the system exactly, so it is important to decide what processes are important and what are extraneous to the question you are asking, so that the mathematical description can be simulated on powerful computers or studied with other analytical techniques.

　　在這個問題中，就跟許多其他科學家面臨的問題一樣，幾乎不可能完全模擬出系統的模型。因此選擇模型的過程以及決定哪些因子對於這個問題而言是可以除外或省略的，變得格外重要。如此一來我們便能藉助現代強大的電腦或其他先進的分析技術，模擬出我們用數學語言定義出的模型。

In the case of planetary formation, the gravitational interactions between the dust in the disk, the mass in the larger aggregates of material, and the sun itself plus the friction of the material, its thermal properties, and the interaction between the magnetic fields from the star and those of the disk material itself plus countless other factors may be vital in order to understand how planets like our own form from the primordial disk of stardust.

　　在行星形成的研究中，碟狀星塵之間重力的互動、較大聚集物的質量、太陽本身與這些物質間的摩擦、物質間本身的摩擦、物質本身的熱力性質、太陽磁力場中這些物質的互動及磁力的影響，加上其他的無數因子都可能是瞭解地球如何從星塵中誕生的關鍵。

The models that have been used until now have in general not taken into account the differences in temperature within regions of the disk and therefore have not correctly accounted for the flow of heat. In these models an Earth like planet quickly becomes unstable and falls into its star. Clearly there was something wrong with such a theory although it provided good answers when modeling the formation of larger planets, like the gas giants in our own solar system.

　　截至目前為止我們所用的各種模型，通常都沒有將碟形區域內的溫度變化視為重要因子，因此也未能將熱能的流動正確計算在內。在這些模型中，像地球大小的行星，將很快變得不穩定並往恆星的方向跌入。這些模型雖能正確預測較大行星的形成，比如太陽系內的氣體巨星（gas giant），但顯然仍有些失誤。

The new results from The American Museum of Natural History and Cambridge University however, have, for the first time taken the subtle effects from the temperature gradients into account and their results show stable Earth-like planetary formation. The heat flow, it was shown, is an absolutely vital part of the process in the stabilisation of planetary orbits. This advance in our understanding takes us closer to being able to estimate, for example, how many planets like the Earth are likely to be out there in our own galaxy and others, circling stars like the sun.

　　然而，美國自然博物館與劍橋大學提出的新研究，首度將溫度梯度的細微變化納入計算，而他們的結果顯示了一個類似地球的穩定行星的形成過程。研究顯示，熱能的流動對於穩定行星軌道是決定性的關鍵。此一認識上的進展讓我們得以估計，在銀河系內外，究竟可能存在著多少像地球一樣的行星，環繞著類似太陽的恆星運轉？

It is results like these which provide moments of enlightenment as to what are the important factors and the unnecessary details of a given complex system, which allow us to get closer to truly understanding the universe we live in. As computers get more powerful and our models improve we will get ever nearer to the truth, but these exciting new results mark an important leap in our comprehension of how we came to be here on this pale blue dot circling an average star in the spiral arm of one galaxy amongst billions.

　　電腦計算能力越來越強大的今日，隨著不斷改進的理論模型，我們將越來越接近真相，但這些令人興奮的新研究，標誌了在理解宇宙的漫漫長路中的一個躍進點——在億萬個銀河系中的一個銀河系的螺旋臂上，繞著一顆尋常星星運轉的蒼藍色小點上，我們，是如何來到這兒的。?

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（本文作者Dr. Jonathan Shock 現為西班牙 Santiago de Compostela 大學高能量物理研究員。在此特別感謝他為CASE撰寫此文，讓我們的讀者得以瞭解這則新發現在物理學上的貢獻。）

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延伸閱讀：地球搬過家、How Earth Survived Its Birth

責任編輯：MissZoe