Planet-9 | 13th July 2021 | Virtual Wire
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.
That’s the conclusion of a new study.“We can’t rule it out,” says Kevin Napier of Planet Nine. He’s a physicist at the University of Michigan in Ann Arbor. “But there’s not necessarily a reason to rule it in.”
Previous work had suggested that a number of far-out objects in the solar system clump in the sky as if shepherded by an unseen giant planet. That planet would have to have 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 clumping. The study knocks down the primary reason to believe in Planet Nine. Napier and his colleagues shared their findings on February 10 at arXiv.org. The paper will appear later in the Planetary Science Journal.
IS EVERY HEAVENLY BODY, A PLANET?
Chad Trujillo is an astronomer at Northern Arizona University in Flagstaff. Scott Sheppard is an astronomer at the Carnegie Institution for Science in Washington, D.C. In 2014, they revived interest in the idea of a distant planet lurking far beyond Neptune.
At the time, they reported a collection of distant bodies with strangely bunched-up orbits near the edge of our solar system. Those distant bodies are called trans-Neptunian objects.
In 2016, planetary scientists Mike Brown and Konstantin Batygin reported new findings. They work at Caltech in Pasadena, Calif. This pair used six trans-Neptunian objects to refine the possible properties of Planet Nine.
They pinned it to an orbit between 500 and 600 times as far from the sun as Earth’s is. But those earlier studies all relied on just a handful of objects. Those objects may not have represented all that’s out there, says Gary Bernstein. He’s an astronomer at the University of Pennsylvania in Philadelphia who works with Napier.
The objects might have seemed to show up in certain parts of the sky only because that’s where astronomers were looking. “It’s important to know what you couldn’t see, in addition to what you did see,” he says.
There are other mysteries of the solar system that Planet Nine would have neatly explained, says Samantha Lawler. She’s an astronomer in Canada who did not take part in this study. She works at the University of Regina in Saskatchewan.
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, she adds, it might explain where proto-comets called centaurs to come from. That was part of the appeal of the Planet Nine hypothesis.
“But the entire reason for it was the clustering of these orbits,” she adds. “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.”
Batygin, an author 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, he adds. But unstable objects have “strongly contaminated” the new study, he says. These bodies may have been nudged by Neptune and lost their position in the cluster.
They also may be on their way to leaving the solar system entirely. Lawler says there’s not strong agreement yet among people who study trans-Neptunian objects about which ones are stable and which are not.
Everyone does agree, though, that astronomers need to find more trans-Neptunian objects if they hope to prove whether Planet Nine’s exists. The Vera Rubin Observatory in Chile should find hundreds more once it begins surveying the sky in 2023. “There always maybe some gap in our understanding,” Napier says. “That’s why we keep looking.”