冥王星之外 太陽系可能真的存在第九顆行星

There might be a ninth planet in the solar system after all, and it is not Pluto.

太陽系或許還是擁有第九顆行星，不是冥王星。

Two astronomers reported on Wednesday that they had compelling signs of something bigger and farther away — something that would satisfy the current definition of a planet, where Pluto falls short.

兩名天文學家於週三通報，他們發現了令人信服的跡象，表明存在一顆更大、更遙遠的天體——該星體將滿足目前對行星的定義，而冥王星則不符合。

“We are pretty sure there’s one out there,” said Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology.

“我們相當確定有那麼一顆行星，”加州理工學院(California Institute of Technology)的行星天文學教授邁克爾·E·布朗(Michael E. Brown)說。

What Dr. Brown and a fellow Caltech professor, Konstantin Batygin, have not done is actually find that planet, so it would be premature to start revising mnemonics of the planets.

布朗博士和他的同事康斯坦丁·巴蒂金(Konstantin Batygin)尚未真正找到那顆行星，所以現在修改行星名單還爲時過早。

In a paper published in The Astronomical Journal, Dr. Brown and Dr. Batygin lay out a detailed circumstantial argument for the planet’s existence in what astronomers have observed — a half-dozen small bodies in distant elliptical orbits.

布朗和巴蒂金在《天文學雜誌》(The Astronomical Journal)上聯合發表了一篇論文，提出詳細的旁證，認爲天文學家們觀測到的現象——在遙遠的橢圓軌道上運行的六個小天體——證明了這顆行星的存在。

What is striking, the scientists said, is that the orbits of all six loop outward in the same quadrant of the solar system and are tilted at about the same angle. The odds of that happening by chance are about 1 in 14,000, Dr. Batygin said.

這兩位科學家稱，一個突出的現象是，這六個天體的軌道全部位於太陽系的同一象限內，並且傾斜角度大致一樣。巴蒂金表示，這種情況純屬巧合的概率大約是1.4萬分之一。

A ninth planet could be gravitationally herding them into these orbits.

可能有第九顆行星通過引力將這些天體拉入這樣的軌道。

For the calculations to work, the planet would be at least an equal to Earth, and most likely much bigger — perhaps a mini-Neptune with a mass about 10 times that of Earth. That would be 4,500 times the mass of Pluto.

要想理論成立，這顆行星至少要和地球尺寸相當，很可能要大得多——大概相當於一顆小海王星，質量是地球的10倍、冥王星的4500倍。

Pluto, at its most distant, is 4.6 billion miles from the sun. The potential ninth planet, at its closest, would be about 20 billion miles away; at its farthest, it could be 100 billion miles away. One trip around the sun would take 10,000 to 20,000 years.

冥王星運行到遠日點時，距離太陽46億英里。這顆有望成爲第九大行星的天體距離太陽最近時是200億英里，最遠時可能是1000億英里。繞行太陽一週需要1萬到2萬年。

“We have pretty good constraints on its orbit,” Dr. Brown said. “What we don’t know is where it is in its orbit, which is too bad.”

“我們對它的軌道參數有了不少了解，”布朗說。“我們尚不知道的是它在軌道上的位置，這太糟糕了。”

Alessandro Morbidelli of the d’Azur Observatory in France, an expert in dynamics of the solar system, said he was convinced. “I think the chase is now on to find this planet,” he said.

法國蔚藍海岸天文臺( d’Azur Observatory)的太陽系動力學專家亞歷山德羅·莫比德利(Alessandro Morbidelli)覺得可信。“我認爲現在的目標是找到這顆行星，”他說。

This would be the second time that Dr. Brown has upended the map of the solar system. In January 2005, he discovered a Pluto-size object, now known as Eris, in the Kuiper belt, the ring of icy debris beyond Neptune.

如若證實，這將是布朗第二次顛覆太陽系的圖譜。2005年1月，他發現了一顆大小相當於冥王星的天體，位於海王星以外的充滿冰結碎塊的柯伊伯帶(Kuiper Belt)。它如今命名爲鬩神星(Eris)。

The next year, the International Astronomical Union placed Pluto in a new category, “dwarf planet,” because in its view, a full-fledged planet must be the gravitational bully of its orbit, and Pluto was not.

第二年，國際天文學聯合會(International Astronomical Union)就把冥王星歸入名爲“矮行星”的一個新類別，因爲在該學會看來，一顆名副其實的行星必須能夠清空軌道上的其他天體，而冥王星做不到這一點。

The first indication of a hidden planet beyond Pluto had come a couple of years earlier. The Kuiper belt extends outward from Neptune’s orbit, about 2.8 billion miles from the sun, to a bit less than twice Neptune’s orbit, about five billion miles.

在此之前兩年，科學家就首次發現了冥王星以外可能存在隱藏行星的跡象。柯伊伯帶從海王星的軌道向外延伸，一直到接近海王星軌道兩倍的地方，也就是分佈在距離太陽約28億英里到50億英里處。

Astronomers expected that beyond lay mostly empty space.

天文學家認爲，那以外應當幾乎空無一物。

Thus they were surprised when Dr. Brown and two colleagues spotted a 600-mile-wide icy world at a distance of eight billion miles that remained well outside the Kuiper belt even at the closest point in its orbit.

所以，當布朗和他的兩名同事在80億英里遠的地方發現了一片直徑600英里的冰雪世界時，科學家們頗爲意外。就連該天體運行軌道的近日點也在柯伊伯帶以外。

No one could convincingly explain how the object, which Dr. Brown named Sedna, got there, and the hope was that the discovery of more Sedna-like worlds would provide enlightening clues.

布朗把它命名爲賽德娜(Sedna)。沒有人能夠對這顆天體爲何出現在那裏作出令人信服的解釋。科學家希望，將來發現更多的類似天體，從而提供能帶來啓發的線索。

Instead, astronomers looked and found nothing, deepening the mystery.

但事與願違，天文學家的搜尋一無所獲，讓此事變得更加撲朔迷離。

Finally, in 2014, Chadwick Trujillo, who had worked with Dr. Brown on the Sedna discovery, and Scott S. Sheppard, an astronomer at the Carnegie Institution for Science in Washington, reported a smaller object in a Sedna-like orbit, always remaining beyond the Kuiper belt.

最後，2014年，曾和布朗共同發現賽德娜的查德威克·特魯希略(Chadwick Trujillo)與位於華盛頓的卡內基科學研究所(Carnegie Institution for Science)的天文學家斯科特·S·謝潑德(Scott S. Sheppard)通報，發現了一顆較小的天體有着與賽德娜類似的軌道，一直運行在柯伊伯帶以外。

Dr. Trujillo and Dr. Sheppard noted that several Kuiper belt objects had similar orbital characteristics, and they laid out the possibility of a planet disturbing the orbits of these objects. “It was the best explanation we could come up with,” Dr. Trujillo said.

特魯希略和謝潑德注意到，柯伊伯帶的幾個天體有類似的軌道特徵。他們提出了一種假設：有可能存在一顆行星，正是它影響了這些天體的軌道。“這是我們能想到的最佳解釋了，”特魯希略博士說。

But the particulars of their proposed planet did not explain what was in the sky, Dr. Brown said.

不過，布朗稱，他們提出的這顆行星的參數卻無法解釋太空中的現象。

“The theorists didn’t really take it seriously,” he said. “They figured it was all some observational effect. The observers didn’t take it seriously, because they figured it was all some theoretical thing they couldn’t understand.”

“理論領域的學者沒當回事，”他說。“他們認爲這都是某種觀測效應。觀測領域的學者也沒當回事，因爲他們認爲這都是他們無法理解的某種理論。”

Still, the peculiarities of the orbits appeared genuine.

然而，它們軌道的特殊性看來是真實的。

Dr. Brown said he and Dr. Batygin “sat down and beat our heads against the wall for the last two years.”

布朗表示，他和巴蒂金“過去兩年坐在那裏，想破了腦袋。”

First, they focused on the six objects in stable orbits and disregarded others that had been recently flung out by Neptune.

他們開始把重點放在處於穩定軌道的六個小型星體上，不再考慮近期被海王星拋出的其他天體。

That made the picture clearer.

局面豁然開朗。

“They all point into the same overall direction,” Dr. Batygin said. “This is in stark contrast with the rest of the Kuiper belt.”

“他們都指向大致一樣的方向，”巴蒂金說。“這和柯伊伯帶的其他天體截然不同。”

Besides the long odds of this alignment being coincidental, Dr. Batygin said, this pattern would disperse over time.

且不說這種一致性純屬巧合的機率有多小，巴蒂金表示，這樣的運行模式也本應隨時間的流逝而消散。

That argued for the force of some unseen body guiding Sedna and the others.

他們認爲，這是未曾發現的星體在吸引賽德娜和其他天體。

Dr. Batygin, a theorist, tried placing a planet among them, which scattered some Kuiper belt objects, but the orbits were not sufficiently eccentric.

作爲理論天文學家，巴蒂金試圖將一顆行星安放其中。這打散了柯伊伯帶中的不少天體，但形成的橢圓軌道的離心率依然不夠大。

Then he examined what would happen if a ninth planet were looping outward in the opposite direction. That, Dr. Batygin said, gave “a beautiful match to the real data.”

隨後，他假設有第九大行星在相反方向向外繞行。巴蒂金表示，結果呈現出“與真實數據完美的匹配”。

The computer simulations showed that the planet swept up the Kuiper belt objects and placed them only temporarily in the elliptical orbits. Come back in half a billion years, Dr. Brown said, and Sedna will be back in the Kuiper belt, while other Kuiper belt objects will have been pushed into elliptical orbits.

計算機模擬顯示，該行星將柯伊伯帶的天體全部重新洗牌，只是暫時被安置在橢圓形軌道中。巴蒂金表示，五億年後賽德娜會回到柯伊伯帶，而其他天體也會被推入橢圓形軌道中。

Another strange result in the simulations: A few Kuiper belt objects were knocked into orbits perpendicular to those of the planets. Dr. Brown remembered that five objects had been found in perpendicular orbits.

模擬中另一奇怪的結果是：小部分柯伊伯帶天體被撞向了與其他行星相垂直的軌道。根據巴蒂金回憶，在垂直軌道中發現了五個天體。

“They’re exactly where we predicted them to be,” he said. “That’s when my jaw hit my floor. I think this is actually right.”

“它們完全處在我們預測的位置上，”他稱，“我驚得下巴都掉地了。我認爲這應該是對的。”

Dr. Morbidelli said a possible ninth planet could be the core of a gas giant that started forming during the infancy of the solar system; a close pass to Jupiter could have ejected it. Back then, the sun resided in a dense cluster of stars, and the gravitational jostling could have prevented the planet from escaping to interstellar space.

莫比德利表示，可能存在的第九大行星，也許是一個氣態巨行星的核心，自太陽系初期就開始形成；可能當它經過木星時被引力拋遠了。當時的太陽處在密集的恆星羣中，而引力的推擠可能阻止了這顆行星逃到星際空間。

“I think they’re onto something real,” he said. “I would bet money. I would bet 10,000 bucks.”

“我覺得他們這樣下去有戲，”他說，“我敢打賭。我賭一萬塊。”

Dr. Brown said he began searching for the planet a year ago, and he thought he would be able to find it within five years. Other astronomers will most likely also scan that swath of the night sky.

巴蒂金稱，他早在一年前就開始尋找這顆行星，他認爲自己可以在五年內找到它。其他天文學家很有可能也會去搜索那一片夜空。

If the planet exists, it would easily meet the definition of planet, Dr. Brown said.

他說如果這顆行星真的存在，它應該能輕易吻合對行星的定義。

“There are some truly dominant bodies in the solar system and they are pushing around everything else,” Dr. Brown said. “This is what we mean when we say planet.”

“在太陽系有一些格外顯要的星體，它們會肆意擺佈其他一切天體，”巴蒂金說，“這就是我們說的‘行星’的意思。”