Ninth Planet May Exist Beyond Pluto, Scientists Report
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.
“我们相当确定有那么一颗行星，”加州理工学院(California Institute of Technology)的行星天文学教授迈克尔·E·布朗(Michael E. Brown)说。
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.
布朗和巴蒂金在《天文学杂志》(The Astronomical Journal)上联合发表了一篇论文，提出详细的旁证，认为天文学家们观测到的现象——在遥远的椭圆轨道上运行的六个小天体——证明了这颗行星的存在。
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.
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.
法国蔚蓝海岸天文台(Côte d’Azur Observatory)的太阳系动力学专家亚历山德罗·莫比德利(Alessandro Morbidelli)觉得可信。“我认为现在的目标是找到这颗行星，”他说。
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.
第二年，国际天文学联合会(International Astronomical Union)就把冥王星归入名为“矮行星”的一个新类别，因为在该学会看来，一颗名副其实的行星必须能够清空轨道上的其他天体，而冥王星做不到这一点。
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.
最后，2014年，曾和布朗共同发现赛德娜的查德威克·特鲁希略(Chadwick Trujillo)与位于华盛顿的卡内基科学研究所(Carnegie Institution for Science)的天文学家斯科特·S·谢泼德(Scott S. Sheppard)通报，发现了一颗较小的天体有着与赛德娜类似的轨道，一直运行在柯伊伯带以外。
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.
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.”