Is the Mysterious Planet Nine Just a Swarm of Asteroids?

There’s something funky about objects in the Kuiper Belt—the ring of rock and ice beyond Neptune. The orbits of some of these space rocks don't follow computer-predicted paths. Some scientists have suggested that the source of this weirdness is an undiscovered planet, dubbed Planet X or Planet 9, whose gravity warps the orbits of detached Kuiper Belt objects.

The problem is, despite several years of searching, Planet 9 has not appeared. Now, reports Jay Bennett at Popular Mechanics, a new analysis suggests the mysterious Planet 9 may not exist at all—and the weird orbits can instead be explained by swarms of asteroids.

As Bennett reports, the search for Planet 9 began in 2014 with the publication of study showing that two objects that orbit the sun beyond Neptune (Sedna and 2012 VP₁₁₃) had almost circular orbits. The rest of the objects in the solar system, however, are pulled into elliptical orbits by the gravity of giant planets like Jupiter and Neptune. Something, it seemed, had “detached” them from the gravitational clutches of Neptune. A later study showed other objects had oddly tilted orbits.

The most likely culprit, the researchers concluded, was the tug of gravity from a large, undiscovered planet. According to NASA, that planet would have to be substantial, with a mass 10 times that of Earth, orbiting the sun every 10,000 to 20,000 years, circling 20 times farther out than Neptune.

In the latest study, presented this week at a meeting of the American Astronomical Society, astronomers looked at new models depicting how the massive swarm of objects that make up the Kuiper Belt orbit the sun. What they found is that smaller asteroids and icy bits swirl around the sun faster than larger objects, like Sedna, other minor planets and large asteroids. Those small objects tend to bunch up on one side of the sun with their collective gravity tugging on the larger, slower objects.

“These orbits crash into the bigger body, and what happens is those interactions will change its orbit from an oval shape to a more circular shape,” Jacob Fleisig, an undergraduate at CU Boulder and lead author of the study, tells Bennett.

Not everyone is convinced that this “self-gravity” story explains the motion of detached objects. Caltech's Konstantin Batygin, for one, does not think the Kuiper Belt objects have enough mass to pull minor planets and asteroid out of their orbit.

But as Ann-Marie Madigan, co-author of the study also from CU Boulder, tells Nola Taylor Redd at Space.com, the new model does show that there’s enough mass to create the odd orbits. Modeling such a complex system as the Kuiper Belt is enormously expensive, and past researchers did not add in the masses of as many objects. In this study, Madigan says they added in the mass of 400 of the 2,300 documented objects. That, she says, was enough to distort the orbits of the largest detached objects.

Even so, as Redd reports, the model can't explain everything. For example, it still can't account for the tilted orbits.

Even if the study doesn’t strike down the concept of Planet 9, it could help figure out another big mystery. Some researchers hypothesize that there is a mass extinction cycle in which comets rain down on Earth every 26 million years or so, wiping out a large percentage of living creatures each time. As Madison tells Redd, it’s possible that if gravity interactions among objects in the Kuiper Belt can sling minor planets outward, they could also redirect comets toward the inner solar system.

While we await our cosmic doom, however, there’s still a chance that Planet 9 is out there. Last year, NASA asked citizen scientists to help comb through images from the Wide-field Infrared Survey Explorer to see if it caught any signs of the elusive body orbiting the far reaches of our planetary neighborhood.

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