There might be a ninth planet in the solar system after all — and it is not Pluto.
Two astronomers reported Wednesday that they had compelling signs of something bigger and farther away — something that would definitely 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 Brown and a fellow Caltech professor, Konstantin Batygin, have not done is actually find that planet, so it would be premature to revise mnemonics of the planets just yet.
Rather, in a paper published Wednesday in The Astronomical Journal, Brown and Batygin lay out a detailed circumstantial argument for the planet's existence in what astronomers have observed — a half-dozen small bodies in distant, highly 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, Batygin said.
A ninth planet could be gravitationally herding them into these orbits.
For the calculations to work, the planet would be quite large — at least as big as Earth, and likely much bigger — a mini-Neptune with a thick atmosphere around a rocky core, with perhaps 10 times the mass of Earth.
It would dwarf Pluto, at about 4,500 times its mass.
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 a trillion miles away. It would take from 10,000 to 20,000 years to complete one orbit around the sun.
"We have pretty good constraints on its orbit," 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 Brown has upended the map of the solar system. In January 2005, he discovered a Pluto-size object, now known as Eris, in the ring of icy debris beyond Neptune known as the Kuiper belt.
A year and a half later, the International Astronomical Union placed Pluto in a new category, "dwarf planet," because it had not "cleared the neighborhood around its orbit."
In the view of the astronomical union, a full-fledged planet must be, in essence, the gravitational bully of its orbit, and Pluto was not.
The first indication of a hidden planet beyond Pluto had come a couple 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 5 billion miles.
Beyond that, astronomers expected, was by and large empty space.
Thus they were surprised when Brown and two colleagues spotted a 600 mile-wide icy world that not only was beyond the Kuiper belt, at a distance of 8 billion miles, but remained well outside the Kuiper belt even at the closest point in its orbit.
No one could easily explain how the object, which Brown named Sedna, got there. It was too far out to have been flung by the gravitational slings of big planets like Jupiter and too close to have been nudged by the gravitational tides of the Milky Way.
The hope was that the discovery of more Sedna-like worlds would provide additional clues.
Instead, astronomers looked and found nothing, deepening the mystery.
Finally, in 2014, Chadwick Trujillo, who had worked with Brown on the Sedna discovery, and Scott S. Sheppard, an astronomer at the Carnegie Institution for Science in Washington, reported a smaller object, designated 2012 VP113, in a Sedna-like orbit, always remaining beyond the Kuiper belt.
Trujillo and Sheppard noted that several Kuiper belt objects had some similar orbital characteristics, and they laid out the possibility of a planet disturbing the orbits of these objects. "I wasn't sure," Trujillo said. "It was the best explanation we could come up with."
But the particulars of their proposed planet did not explain what was in the sky, 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. "It was really clear from their data that observationally something was happening, and it demanded some sort of explanation," Brown said.
Brown said he walked down the hall to Batygin's office, and "the two of us 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 objects that had been recently flung out by Neptune to eventually depart the solar system.
That made the picture clearer.
"What we realized is the story is much more simple and more fundamental," Batygin said. "They all point into the same overall direction. All in same quadrant. This is in stark contrast with the rest of the Kuiper belt."
Besides the long odds of this alignment being coincidental, Batygin said, this pattern would not last indefinitely, dispersing over a few hundred million years — a short time compared to the 4.5 billion-year age of the solar system.
"We're not observing a relic of a perturbation of the past," he said.
That argued for something else, something bigger, that is currently guiding Sedna and the others.
Batygin, a theorist, tried placing a planet among the six objects. That did scatter some of the Kuiper belt objects, but the orbits were not sufficiently eccentric.
Then he examined what would happen if a ninth planet were looping outward in a direction opposite to Sedna and the others. That, 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, Brown said, and Sedna will be back in the Kuiper belt, while other Kuiper belt objects will have been swept into similar elliptical orbits.
Another strange result in the simulations: A few Kuiper belt objects were knocked into orbits perpendicular to the plane of planetary orbits. Brown remembered that five objects had been found in perpendicular orbits.
"They're exactly where we predicted them to be," Brown said. "That's when my jaw hit my floor. I think this is actually right."
Trujillo said the new paper made a much more convincing argument for another planet than his own did. "We're pleasantly surprised that someone has really done a much better job than we did," he said.
Morbidelli agreed. "I think they're onto something real," he said. "I would bet money. I would bet 10,000 bucks."
Morbidelli said that the ninth planet could easily be the core of a gas giant that started forming in the early years of the solar system; a close pass to Jupiter could have flung it out. In those days, the sun was packed in a dense cluster of stars, and the gravity of those neighbors could have slowed the planet and prevented it from escaping the solar system.
Brown said he began searching for the planet a year ago, and he thought he would be able to find it within five years — perhaps sooner, with luck. Now it is likely that other astronomers will scan that swath of the sky.
If the planet exists, it would easily meet the International Astronomical Union's requirements, Brown said.
"There are some truly dominant bodies in the solar system and they are pushing around everything else," Brown said. "This is what we mean when we say planet."