Evidence is mounting that Planet Nine is lurking out there somewhere

The supposed Planet Nine is a distant, massive planet in our solar system. Credit: NASA

Recent papers by Michael Brown and Konstantin Batygin, both astronomers at Caltech, provide a whole new line of evidence supporting the existence of the supposed Planet Nine in our solar system.

First evidence

This distant, massive world was first predicted in 2014 by astronomers who noticed the unusual orbits of several bodies in the outer Solar System, called extreme trans-Neptunian objects. Trans-Neptunian objects (TNOs) have orbits farther from the Sun than Neptune. Extreme TNOs, or ETNOs, have even longer, distant orbits that are unaffected by interactions with any of the known planets. The dwarf planet Sedna, discovered in late 2003, was the first known ETNO.

After Sedna’s discovery, it was speculated that its strange orbit must imply the existence (and influence) of a large unknown planet or perhaps a nearby passing star. Then, the discovery of the second ETNO in 2014 strongly pointed toward an unknown planet as the more likely explanation, as described by astronomers Chad Trujillo and Scott Sheppard in a paper. Nature paper. Meanwhile, other astronomers suggested that two large planets in orbital resonance with each other would be needed to explain the similarities in the orbits of these extreme objects, now classified as Sednoids.

In 2016, Batygin and Brown outlined in detail for the first time how the orbits of six of these objects could be explained by a specific Planet Nine. They predicted that this world should have a mass at least ten times that of Earth, and an orbit about 400 to 800 times the distance between Earth and the Sun. That is about 13 to 26 times further away than the orbit of Neptune, the outermost planet.

Now, Brown says, they have studied the orbits of thirteen major ETNOs. And the clustering of some elements of their orbits “opened our eyes and made us say, ‘What the hell is going on?’” Brown says. Astronomy.

Natural consequences

What’s going on, he explains, is that they see that they “not only cluster in the direction the orbits point, but also that they are all on average about 15 degrees from the plane of the ecliptic (the plane of the solar system) have tilted. °. So both of those things together are pretty strange.

The researchers have seen similar clustering at the perihelia (the point of their closest approach to the Sun) of these distant objects that range up and down, sometimes diving close to Neptune’s orbit and sometimes sweeping further away. “That appears to be another natural consequence of Planet Nine,” says Brown. “We didn’t think about it when we first imagined the planet, it wasn’t in our minds. But we quickly realized that this was it.”

Then a third unexpected line of evidence emerged: a “very strange population of objects that no one had been able to explain,” says Brown. They are objects whose orbits are “rotated approximately 90° with respect to the plane of the solar system and are in highly eccentric orbits.” These unusual orbits, he says, are “almost impossible to explain without the existence of Planet Nine.”

Now the latest results have been accepted for publication in The astronomical journal lettersyet another line of evidence has emerged. It turns out that there is another population of objects much closer to the Sun, between the orbits of Jupiter and Neptune, whose orbits also extend beyond Neptune.

Brown says these are likely objects that were originally part of a very distant population “that move into closer orbits and stay there for a long time.” The team’s analysis showed a gradual decline in the population of objects with perihelia closer to the Sun than Neptune, rather than the sharp boundary just inside Neptune’s orbit that would occur without the influence of Planet Nine’s gravity. “I can’t think of any other explanation that could produce this result,” he says.

In a separate article, Brown, Batygin and Matthew Holman of the Center for Astrophysics | Harvard & Smithsonian describe their detailed search for the parts of the sky where Planet Nine is expected to lie, using the vast database of images from the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) on Haleakala, Maui. This search, combined with previous searches using other databases and the team’s own observations, has now ruled out the planet’s presence in 78 percent of the predicted area.

New tools

But there is new hope. Next year, the Vera C. Rubin Observatory in Chile will come online and scan the entire southern sky every four days with the largest camera ever built. The team’s analysis shows that there is a very good chance that if Planet Nine exists, the Rubin System will be able to find it within a year or two, proving its existence once and for all.

“What’s great about this,” says Thomas Levenson, professor of science journalism at MIT and author of The Hunt for Vulcan (Random House, 2015), “is that they have continued to refine that prediction since the original proposal. They used further observations of additional groups of these objects to make the prediction more explicit, precise and therefore falsifiable.”

He adds that “this is the way science is supposed to work, and often it doesn’t work this way.” And as a fan of astronomy, he says: “The exciting thing is that they offer the prospect that this can be tested in an ordinary human life… With the right observatory we can see things that will help us confirm or deny, and that observatory is almost in reach, it’s about to go, and that’s very exciting.

“I think the odds are good,” Brown said. But a detection “is dependent on the observatory choosing to push further into the Northern Hemisphere than the nominal plan does,” a possibility that has yet to be decided. And it also depends on whether Planet Nine is not “at the very ends of its range” in terms of location and distance.

“It could be further away than our initial predictions, in which case (the Rubin Observatory) won’t be able to detect it either, and I’ll be depressed,” Brown says. But he adds that “if Planet Nine isn’t there, we need a separate explanation for each of these other things, all of which can be explained by a single Planet Nine. So it’s a very elegant solution to a lot of different problems, which is a good sign that it’s probably out there.”

“Until there is a better explanation for all these phenomena, I still think Planet Nine probably really exists,” he says.