Planet 9: Scientists Have a New Theory About Mysterious Objects in the Outer Solar System

An artist's representation of Sedna, a dwarf planet orbiting far beyond Neptune. NASA/JPL-Caltech

The outer solar system is a weird place full of weird things displaying weird behavior. Scientists are still trying to figure out how all those mysteries have fit together, leading to hypotheses like the existence of a so-called Planet 9 wreaking havoc on otherwise orderly orbits.

But one key piece of the outer solar system puzzle doesn't need a ninth planet to make the numbers work, according to research presented at the annual conference of the American Astronomical Society being held this week in Denver. (That doesn't mean it proves Planet 9 doesn't exist, however.)

That research focused on what scientists call "detached objects," large bodies that hang out way beyond Neptune, nearly 8 billion miles away from the sun. Usually, even stuff that wanders so far away still swings by the sun now and then. But detached objects have such giant, circular orbits that they never do. That's one of the mysteries that inspired the Planet 9 hypothesis in the first place.

The detached objects scientists have spotted so far are all pretty big—smaller than the moon, but still large enough to form a fairly regular sphere, rather than the odd lumpy shapes of asteroids. We haven't found very many yet, just a handful, which makes studying them a little trickier.

They're surrounded by a huge number of much, much smaller companions called trans-Neptunian objects. No one had really looked at all those objects' movement at once. "This seems like a really obvious thing to do," Ann-Marie Madigan, an astrophysicist at the University of Colorado Boulder who led the research, told Newsweek. "It should have been done before, but they're really expensive computer calculations."

So she and an undergraduate student, Jacob Fleisig, did it, modeling about 400 objects. And when Fleisig watched models of their movements, he saw something weird: All those tiny trans-Neptunian objects have orbits scientists call highly elliptical, stretched out to a long, thin oval, rather than circular. In each of those paths, the sun is off to the side of center.

As the object travels in its orbit along the edge of that spinner, the orbit itself also orbits around the sun—if you had a birds'-eye view of the whole spectacle and plenty of time to watch, you'd see it gradually trace out a flower-like shape as the object isn't quite able to retrace its path from last time around the sun.

And it turns out that as the small trans-Neptunian objects make this long dance, the gravity of the comparatively huge objects ends up tugging them along in such a way that the orbits themselves end up piled together on one side, like a flower that lost most of its petals except for one little patch.

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The model suggested that when that celestial imbalance builds up strongly enough, the total gravity of all the tiny objects can be strong enough to influence a large one, kicking it out into the super-large, super-circular orbit and making it a detached object.

But it's a little more complicated than that: The model also showed the collective gravity effect can do the reverse, pulling a detached object in off its giant orbit and back to a path that carries it reasonably close to the sun.

Despite the intriguing model results, Madigan said it's far too early to be confident that this is what's really happening out at the edge of our solar system. That's because astronomers just haven't identified enough detached objects yet to build particularly authoritative models.

"It's a very small number that we've actually detected," Madigan said. We can never know how many are actually out there until someone can put a number on just how hard they are to spot. But she's suspicious that it's a matter of how to find these detached objects, not whether there are more to be found.

"By definition, they are extremely hard to detect," she said. "That suggests to me that there's a large reservoir of detached objects out there, and we'll detect more and more of them as our telescopes improve." If that holds up, she may someday have the data to know for sure.

Although explaining the detached orbits that this mechanism tackles is one of the key motivations of the Planet 9 hypothesis, Madigan was careful to say her team's results don't shed any light on whether there's another planet lurking out there that we haven't spotted yet. "We can't explain everything in the outer solar system," Madigan said, including some of the other mysteries Planet 9 attempts to solve. "This is just a really natural way of explaining detached objects."