Ninth Planet May Exist Beyond Pluto, Scientists Report
By KENNETH CHANG

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

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.

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

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.

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.

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.

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

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.

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.

“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.”

Alessandro Morbidelli of the Côte 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.

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.

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.

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.

Astronomers expected that beyond lay mostly empty space.

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.

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.

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

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.

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.

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

“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.”

Still, the peculiarities of the orbits appeared genuine.

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

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

That made the picture clearer.

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

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

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

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

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.”

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.

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.

“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.”

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.

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

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.

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

“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.”

第九行星再現 美有證據

冥王星2006年自9大行星中除名後，太陽系僅剩8大行星。美國加州理工學院的天文學家19日宣布，外太陽系（the outer solar system）中可能有一顆質量為地球10倍且尚未被觀測到的遙遠行星，能夠填補冥王星的遺缺，而且絕對不會被除名。

美國加州理工學院天文學家巴提金（Konstantin Batygin）和布朗（Michael Brown）發現，庫伯帶五個天體運行的軌道、近日點的距離或角度，都與2003年發現的小行星「賽德娜」（Sedna）相似。

6個天體近日點相符的機率僅0.7%，角度吻合的機率僅1%；研究人員認為這絕非巧合。此外，這6個天體全在太陽系的同一象限內，軌道傾斜角度大致相同。巴提金說，這種巧合發生的機率只有0.007%。

巴提金和布朗暱稱這顆尚未被直接觀測到的新行星為「第9行星」（Planet 9），該行星可能以引力將這6個星體拉進這些軌道。巴提金和布朗利用數學模型運算和電腦模擬，演算出這6個天體應該是繞著一顆尚未發現的行星運行，這顆行星不但質量是地球的10倍，繞行太陽時與太陽的平均距離是海王星的20倍、地球的50倍以上。

第9行星最近可能距太陽200億英里，最遠可能達1000億英里；繞行太陽一周需要一萬到兩萬年。天文學家還不清楚第9行星為何如此遙遠，推測可能是太陽系行星形成時，被其他行星的引力推開，吸引太陽系中其餘的塵埃和氣體，形成現在的規模。

法國尼斯天文台專家莫比德里認為，第9行星的理論相當可信，但科學家必須提出觀測到第9行星的證據，否則這篇20日發表於「天文學期刊」的研究結果將僅止於理論。

這已是布朗第二次對太陽系提出重大異動的相關研究，他2005年在庫伯帶發現冥王星大小的矮行星「鬩神星」（Eris）。翌年，冥王星被國際天文學會從行星降級為矮行星。

原文參照：
http://www.nytimes.com/2016/01/21/science/space/ninth-planet-solar-system-beyond-pluto.html

Video：An animation explains the case for the presence of a ninth planet-size object deep in the solar system, beyond the orbit of Neptune.
http://nyti.ms/1Phs52a

2016-01-22．聯合報．A17．國際．編譯陳韻涵