Planet Nine from outer space?
Evidence points to orb’s existence at edge of solar system
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 satisfy the current definition of a planet, where Pluto falls short.
“We are pretty sure there’s one out there,” said Michael 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 start revising mnemonics of the planets.
In a paper published 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 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 at least an equal to Earth, and most likely much bigger — perhaps a mini-Neptune with a small but thick atmosphere surrounding a rocky core and 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,” 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 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.
Icy world spotted
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 5 billion miles.
Astronomers expected that beyond lay mostly empty space.
Thus they were surprised when Brown and two colleagues spotted a 600-mile-wide icy world at a distance of 8 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 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 Brown on the Sedna discovery, and Scott 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.
Trujillo and 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,” Trujillo said.
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.
Core of a gas giant
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 on to something real,” he said. “I would bet money. I would bet 10,000 bucks.”
Brown said he has begun searching for the planet, 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, 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.”