走向滅亡的行星（圖）

Astronomers have found a giant planet orbiting so close to its parent star that it"s bound to spiral inward to its doom or else be ripped to shreds by the star"s gravity. Either way, the planet called WASP-18b should provide astronomers with a mother lode of data about the delicate gravitational balancing act that affects all solar systems.

天文學家們發現了一顆巨大的行星在很近的距離上圍繞著其主恆星旋轉，該行星正在盤旋著向內靠攏，註定要走向滅亡，或者要被主恆星的引力撕成碎片。不管是哪一種下場，這顆叫做「WASP-18b」的行星可以為天文學家們提供豐富的資料來源，使他們了解影響所有恆星系統的微妙的引力平衡作用。

Stars and their planets can influence one another in subtle but inevitable ways. Even though Jupiter orbits about 500 million kilometers away from our sun, for example, the planet"s huge mass can shift the sun"s center of gravity by more than 10,000 kilometers, enough to possibly reveal the planet"s presence to alien astronomers. The moon"s pull on Earth raises and lowers sea levels in the form of tides. And conversely, Earth"s gravity has slowed the moon"s rotation so that it always presents the same face to its parent planet.

恆星及其行星以一種微妙而必然的方式產生相互影響。例如，儘管木星在大約5億公里之外圍繞著太陽旋轉，但該行星的巨大質量可以使太陽的引力中心產生1萬多公里的偏差，足以向太陽系以外的天文學家展示該行星的存在。月球對地球的引力使海平面時升時降，表現形式為漲潮和退潮；反過來，地球的引力減慢了月球的自轉速度，結果月球總是將其相同的一面展現給地球。

Now researchers have found an extreme version of tidal forces at work. WASP-18b is roughly 10 times the mass of Jupiter, and it orbits only about 3 million kilometers from its star. WASP-18b, which was discovered with twin telescopes located in the Canary Islands and South Africa, is located about 400 light-years away in the constellation Phoenix.

現在，研究人員發現一種極端形式的潮汐力在起作用。行星WASP-18b大約是木星的10倍，它在距其主恆星僅約300萬公里的軌道上運行。WASP-18b位於大約400光年之外的鳳凰座，是利用座落在加那利群島和南非的一對望遠鏡發現的。

WASP-18b and its star are so close to each other that it takes the planet, which is described in tomorrow"s issue of Nature, less than a day to complete a revolution. As it orbits, WASP-18b drags bulges tens of kilometers high around the star"s equator, making it spin faster. Those same forces also cost the planet its angular momentum, pulling it inward, eventually to the point where it will smash into the star"s surface. Or perhaps it will be shredded to form a Saturn-like ring system. In either case, WASP-18b appears doomed to die within half a million years.

研究人員在明天出版的《自然》雜誌上描述道：行星WASP-18b距其主恆星非常近，僅用不到一天的時間就繞行一周。在做環繞運行時，WASP-18b的引力使主恆星的赤道周圍隆起高達幾十公里，並使主恆星的自轉加速。這種相互作用使得該行星的角動量減小，因而不斷向內靠攏，最終會撞入恆星的表面。或者，該行星也可能會被撕碎，形成象土星那樣的光環系統。不管屬於哪一種情況，WASP-18b似乎在50萬年以內註定要滅亡。

Or maybe not. Astronomer and co-author Andrew Collier Cameron of the University of St. Andrews in Fife, U.K., says there could be a third alternative: Some as-yet-unknown manifestation of tidal forces could prolong the planet"s life by as much as 500 million years. That"s because the close and fierce interaction between two large and essentially gaseous bodies could produce turbulence or other effects that could somehow cancel out WASP-18b"s orbital deterioration. Astronomers may know whether that"s happening within a decade or so, Collier Cameron says. By then, they will have collected enough data to determine whether WASP-18b"s orbit has degraded and by how much. If WASP-18b somehow escapes imminent destruction by tidal forces, then astronomers will know that "our understanding of orbital dynamics, particularly tidal interactions, needs revision," says lead author Coel Hellier of Keele University in the United Kingdom.

或者，也有可能不會滅亡。該研究的發起人之一、英國法夫市聖安德魯斯大學天文學家安德魯·科利爾·卡梅倫說，可能存在第三種情況：到目前為止仍不為人所知的某種潮汐力現象，可能會將該行星的壽命延長達5億年之久。這是由於兩個本質為氣態的大型天體在近距離上發生劇烈的相互作用，這種作用會產生動蕩或其他效應，也許會莫名其妙地修正WASP-18b走形的軌道。卡梅倫說，在十年左右的時間內，天文學家們可能會知道是否正在發生上述情況。到那個時候，他們將收集到足夠的資料來確定WASP-18b的軌道是否發生了退化變形，以及在多大程度上發生了變形。「如果WASP-18b以某種方式靠潮汐力逃脫了即將滅亡的命運，那麼天文學家們將會認識到我們對軌道動力學的理解需要改變了，特別是對潮汐作用的理解更需要改變，」該研究的首要發起人、英國基爾大學的科爾·赫利爾說。

"It"s the find of a lifetime," says astronomer Douglas Hamilton of the University of Maryland, College Park. "The beauty of this discovery is that we will be able to know [how WASP-18b will meet its end] within 5 to 10 years," Hamilton says. "I think that it could go either way--which makes it very exciting."

「通常，這需要用一生的時間去發現，」馬里蘭大學帕克分校的天文學家道格拉斯·漢密爾頓說。「該發現的魅力在於：在5至10年之內，我們將能夠知道WASP-18b是否會滅亡，」漢密爾頓說，「而我認為，兩種可能性都有，這使得該行星非常令人激動。」