Signs of a hidden Planet Nine in the solar system may not hold up

Planet Nine might be a mirage. What once looked like evidence for a massive planet hiding at the solar system’s edge may be an illusion, a new study suggests.

“We can’t rule it out,” says Kevin Napier, a physicist at the University of Michigan in Ann Arbor. “But there’s not necessarily a reason to rule it in.”

Previous work has suggested that a number of far-out objects in the solar system cluster in the sky as if they are being shepherded by an unseen giant planet, at least 10 times the mass of Earth. Astronomers dubbed the invisible world Planet Nine or Planet X.

Now, a new analysis of 14 of those remote bodies shows no evidence for such clustering, knocking down the primary reason to believe in Planet Nine. Napier and colleagues reported the results February 10 at arXiv.org in a paper to appear in the Planetary Science Journal.

The idea of a distant planet lurking far beyond Neptune received a surge in interest in 2014, when astronomers Chad Trujillo of Northern Arizona University and Scott Sheppard of the Carnegie Institution for Science reported a collection of distant solar system bodies called trans-Neptunian objects with strangely bunched-up orbits (SN: 11/14/14).

In 2016, Caltech planetary scientists Mike Brown and Konstantin Batygin used six trans-Neptunian objects to refine the possible properties of Planet Nine, pinning it to an orbit between 500 and 600 times as far from the sun as Earth’s (SN: 7/5/16).

But those earlier studies all relied on just a handful of objects that may not have represented everything that’s out there, says Gary Bernstein, an astronomer at the University of Pennsylvania. The objects might have seemed to show up in certain parts of the sky only because that’s where astronomers happened to look.

“It’s important to know what you couldn’t see, in addition to what you did see,” he says.

To account for that uncertainty, Napier, Bernstein and colleagues combined observations from three surveys — the Dark Energy Survey, the Outer Solar System Origins Survey and the original survey run by Sheppard and Trujillo — to assess 14 trans-Neptunian objects, more than twice as many as in the 2016 study. These objects all reside between 233 and 1,560 times as far from the sun as Earth.

The team then ran computer simulations of about 10 billion fake trans-Neptunian objects, distributed randomly all around the sky, and checked to see if their positions matched what the surveys should be able to see. They did.

“It really looks like we just find things where we look,” Napier says. It’s sort of like if you lost your keys at night and searched for them under a streetlamp, not because you thought they were there, but because that’s where the light was. The new study basically points out the streetlamps.

“Once you see where the lampposts really are, it becomes more clear that there is some serious selection bias going on with the discovery of these objects,” Napier says. That means the objects are just as likely to be distributed randomly across the sky as they are to be clumped up.

That doesn’t necessarily mean Planet Nine is done for, he says.

“On Twitter, people have been very into saying that this kills Planet Nine,” Napier says. “I want to be very careful to mention that this does not kill Planet Nine. But it’s not good for Planet Nine.”

There are other mysteries of the solar system that Planet Nine would have neatly explained, says astronomer Samantha Lawler of the University of Regina in Canada, who was not involved in the new study. A distant planet could explain why some far-out solar system objects have orbits that are tilted relative to those of the larger planets or where proto-comets called centaurs come from (SN: 8/18/20). That was part of the appeal of the Planet Nine hypothesis.

“But the entire reason for it was the clustering of these orbits,” she says. “If that clustering is not real, then there’s no reason to believe there is a giant planet in the distant solar system that we haven’t discovered yet.”

Batygin, one of the authors of the 2016 paper, isn’t ready to give up. “I’m still quite optimistic about Planet Nine,” he says. He compares Napier’s argument to seeing a group of bears in the forest: If you see a bunch of bears to the east, you might think there was a bear cave there. “But Napier is saying the bears are all around us, because we haven’t checked everywhere,” Batygin says. “That logical jump is not one you can make.”

Evidence for Planet Nine should show up only in the orbits of objects that are stable over billions of years, Batygin adds. But the new study, he says, is “strongly contaminated” by unstable objects — bodies that may have been nudged by Neptune and lost their position in the cluster or could be on their way to leaving the solar system entirely. “If you mix dirt with your ice cream, you’re going to mostly taste dirt,” he says.

Lawler says there’s not a consensus among people who study trans-Neptunian objects about which ones are stable and which ones are not.

Everyone agrees, though, that in order to prove Planet Nine’s existence or nonexistence, astronomers need to discover more trans-Neptunian objects. The Vera Rubin Observatory in Chile should find hundreds more after it begins surveying the sky in 2023 (SN: 1/10/20).

“There always may be some gap in our understanding,” Napier says. “That’s why we keep looking.”