The demotion of Pluto to dwarf planet didn't come about because of any discovery about Pluto itself. Rather, it was triggered by the discovery that Pluto was one of what's likely to be a large number of bodies that orbit well beyond Neptune. These Kuiper Belt objects (KBOs), some of which are larger than Pluto, tend to have unusual orbits, passing outside of the plane of the Solar System, with their ellipses stretched out on one side, while passing closer to the Sun on the other.

But in recent years, scientists have been noting some odd patterns in the orbits of KBOs. For many of them, their closest approach to the Sun comes as they cross the orbital plane of the inner planets. Now, the researcher who helped identify some of the first KBOs has published a paper in which he identifies a possible cause of these patterns: a distant, Neptune-sized body that would restore our Solar System's planetary total to nine.

Undiscovered planets have a long history, dating back to the prediction of Neptune's existence based on oddities in Uranus' orbit. That success, however, led to a couple of fruitless searches, one for an inner planet that could get Mercury's orbit to behave, and a second for something beyond Neptune. While the latter search turned up Pluto, it was too small to influence Neptune's orbit, which further observations indicated was just fine without any additional fixes.

Since then, even sketchier claims have been made for distant planets or even stars, often based on the relative regularity of mass extinction events. But few scientists have taken these seriously.

Unlikely similarities

This new claim is likely to be different, based on both how it was generated and who did the generating. The who in question is a team of Caltech astronomers, Konstantin Batygin and Mike Brown. Brown, who won a Kavli prize for his work in determining that Pluto was just was one of many KBOs, is perhaps best known for his book How I Killed Pluto and Why It Had It Coming.

Batygin and Brown's new work was inspired by a 2014 paper, which noted a number of the KBOs that had been discovered shared an odd pattern in their orbits: their closest approaches to the Sun occurred at roughly the same time as they crossed the plane of the Solar System's inner planets. That's odd, because gravitational interactions with inner planets should gradually push any of these small KBOs out of alignment over time. That, in turn, implies something else forcing them back in line. The researchers who discovered this suggested an unobserved planet could do it.

The new paper starts by trying to determine if the orbital clustering is unusual in any way. To do so, the researchers started with 13 bodies that orbit beyond 30 Astronomical Units (1 AU is the typical Earth-Sun distance) and ran simulations to determine which of them were influenced by gravitational interactions with Neptune at any point in the four billion years the Solar System has existed. That left six planets for further analysis.

The researchers estimate that orbits as tightly clustered as these should only occur at random 0.007 percent of the time. That represents a statistical significance of 3.8 sigma, meaning that the orbits are highly unlikely to occur by chance.

We'll fix it with a planet

Since this alignment is unusual, the two decided to test whether a planet could in fact produce it. Most of the rest of the work involved simulations: start with a collection of KBOs, put a planet in different orbits, run them for four billion years, and see if they could produce this sort of alignment.

Over a large series of simulations, they gradually narrowed down the options. They found a relatively massive body—they used something ten times the Earth's mass for their simulations—works best to stabilize the KBO orbits. It probably orbits in or close to the plane of the Solar System, but its orbit is highly eccentric—it's relatively close to the Sun on one side of its orbit but quite distant on the other.

The key feature is that the planet's closest approach to the Sun occurs on the opposite side of the Solar System as the closest approach of the KBOs. This orbit allows the planet to continuously herd the KBOs back into a narrow range of relatively stable orbits.

Or at least not all of them. In their simulations, the planet would shift a few KBOs into extremely distant orbits—they'd "disappear from view" in the simulations, only to reappear in orbits nearly perpendicular to the plane of the Solar System. Strikingly, a few KBOs have actually been found in these orbits, providing some independent support for the existence of this planet.

Planet hunting

So, is it time to start looking for the Solar System's new planet IX? Not yet. "The precise range of perturber parameters required to satisfactorily reproduce the data is at present difficult to diagnose," the authors write. In other words, there are a lot of potential orbits that are capable of herding KBOs into their current locations, and we haven't been able to narrow down the properties of these orbits all that much. Plus there's the issue that the work was done based on only six KBOs.

Adding more KBO orbits to the simulations, including those in the perpendicular orbits, could provide some more constraints. And identifying more KBOs would help as well—either by telling us where to look for the planet, or by telling us that the orbital alignments were spurious in the first place. (In the latter case, we'd never have to begin our search for the planet, since it's likely not to be there.)

Assuming the planet's there, there's a rather big question: how did the planet get into a location where it takes 20,000 years to complete an orbit? The Kuiper Belt would have had very little mass compared to the inner Solar System, and it would have been spread over vast distances. So it's completely unlikely to have formed there. Instead, Batygin and Brown favor the idea that it formed in with the other planets and was a potential core of a giant planet, but gravitational interactions with the others ejected it to a distant orbit.

If the results hold up, Brown says that, unlike Pluto, there will be no debate over its planetary status. With a mass likely to be 5,000 times Pluto's and a gravitational influence that dominates an enormous orbit, Brown says it's "the most planet-y of the planets in the whole Solar System."

The Astronomical Journal, 2015. DOI: 10.3847/0004-6256/151/2/22 (About DOIs).