Planet Nine can be a mirage. A new study suggests that what once seemed like evidence of a massive planet hidden on the edge of the solar system may be an illusion.
“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 out."
Previous work has suggested that various objects far from the solar system cluster in the sky as if they were being grazed by an invisible giant planet, at least 10 times the mass of Earth. Astronomers have nicknamed the invisible world Planet Nine or Planet X.
Now, a new analysis of 14 of those remote bodies shows no evidence of such a grouping, tearing down the main reason for believing in Planet Nine. Napier and colleagues reported the results Feb. 10 on arXiv.org in an article that will appear in the Planetary Science Journal.
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The idea of a distant planet hiding far beyond Neptune received a surge of 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 transneptunian objects with strangely objects. grouped orbits (SN: 14/11/14).
In 2016, Caltech planetary scientists Mike Brown and Konstantin Batygin used six trans-Neptunian objects to refine the possible properties of planet Nine, setting it in an orbit 500 to 600 times farther from the sun than Earth (SN: 7/5 / 16).
But those previous studies relied only on a handful of objects that might not represent everything there is, says Gary Bernstein, an astronomer at the University of Pennsylvania. Objects may appear to appear in certain parts of the sky just because that was where astronomers looked.
“It’s important to know what you couldn’t see, in addition to what you saw,” he says.
To account for this uncertainty, Napier, Bernstein, and colleagues combined observations from three surveys – the Dark Energy Survey, the Outer Solar System Origin Survey, and the original Sheppard and Trujillo survey – to evaluate 14 transneptunian objects, more than twice as many as in the 2016 study. These objects reside 233 to 1,560 times farther from the sun than Earth.
The team then conducted computer simulations of about 10 billion fake trans-Neptunian objects, randomly distributed across the sky, and checked whether their positions matched what the surveys should be able to see. They did.
“We seem to find things where we look,” Napier says. It’s like you lose your keys at night and look for them under a streetlight, not because you thought they were there, but because the light was there. The new study basically points to streetlights.
“Once you see where the headlights really are, it’s clearer that there’s some serious selection bias with the discovery of these objects,” Napier says. This means that objects are likely to be randomly distributed across the sky as they clump together.
That doesn’t necessarily mean Planet Nine is made.
“On Twitter, people have been very convinced that this is killing planet Nine,” Napier says. "I want to be very careful to mention that this doesn't kill planet nine. But it's not good for planet nine."
There are other mysteries in the solar system that Planet Nine would have explained in an orderly fashion, says astronomer Samantha Lawler of Regina University in Canada, who did not participate in the new study. A distant planet could explain why some distant objects in the solar system have inclined orbits relative to those of larger planets or where proto-comets called centaurs come from (SN: 18/8/20). That was part of the appeal of the Planet Nine hypothesis.
“But the whole reason was the grouping of these orbits,” he says. "If that clustering isn't real, then there's no reason to believe there's a giant planet in the distant solar system that we haven't yet discovered."
Batygin, one of the authors of the 2016 article, is not ready to give up. “I’m still pretty optimistic about Planet Nine,” he says. Compare Napier’s argument with seeing a group of bears in the woods: 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 him everywhere,” Batygin says. "That logical leap you can't make."
Evidence from Planet Nine should only appear in the orbits of stable objects for billions of years, Batygin adds. But the new study, he says, is "heavily contaminated" by unstable objects: bodies that Neptune may have pushed and lost their position in the cluster or could be on their way to completely abandoning the solar system. “If you mix trash with your ice cream, you’ll mostly try trash,” he says.
Lawler says there is no consensus among people studying transneptunian objects about which are stable and which are not.
However, everyone agrees that in order to prove the existence or non-existence of Planet Nine, astronomers need to discover more trans-Neptunian objects. The Vera Rubin Observatory in Chile should find hundreds more after it begins inspecting the sky in 2023 (SN: 1/10/20).
“There can always be some gap in our understanding,” Napier says. "That's why we're still looking."