Ninth Planet May Exist Beyond Pluto, Scientists Report

Thus, they were surprised when Dr. Brown and two colleagues spotted a 600-milemile-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 using computer simulations, 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 that he had begun searching for the planet, and that 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.”

/By The New York Times/