Pluto’s replacement? R. Hurt (IPAC)/Caltech

A small world leads the way. There’s another tantalising hint that an undiscovered ninth planet lurks in the outer reaches of the solar system.

In January, Konstantin Batygin and Mike Brown at the California Institute of Technology inferred Planet Nine’s existence after studying six objects in the Kuiper Belt, the region of icy bodies that circle the sun beyond Neptune’s orbit.

All of these objects’ orbits take them slingshotting out to the most distant reaches of the solar system before swooping back in. What’s odd is that their eccentric orbits all point in roughly the same direction, when they should be oriented randomly in space. Such an alignment suggests that some unseen planet – 10 times as massive as Earth and four times its size – is shepherding them.


Smack-exactly

While the culprit has still not officially been discovered, Batygin and Brown argued that the chance this was a random occurrence was as low as 0.007 per cent. Although those seem like pretty good odds, the team suspected the number could be much lower and that there were more Kuiper Belt Objects with similar alignments out there. It was only a matter of time before they found another one.

And last week they did. It was spotted in an outer solar system survey, and announced quietly, on a single slide in a talk at the SETI Institute given by Michele Bannister of the University of Victoria, Canada. The object is so new it doesn’t have an official catalogue name yet, but it’s easy to see that its orbit is exactly like the other ushered Kuiper Belt objects.

“It’s smack-exactly where we predicted it should be,” says Brown.

For Brown, that’s a huge relief. He worried he was seeing patterns in the sky that weren’t really there, but the latest object strengthens his case for Planet Nine. “At least in my mind, it removes all doubts that the pattern that we’re seeing is real,” he says.

Although he has yet to run in-depth statistics on this latest object, Brown argues that it is likely to reduce the odds of the alignment occurring by chance to about 0.001 per cent.

Orbital inclination

But Greg Laughlin at the Lick Observatory in California isn’t convinced yet. “In addition to being clustered in their angle, they also should be pointing the same direction in three-dimensional space,” he says. In other words, they should all have the same tilt above or below the plane of the solar system. “And what this image doesn’t show is whether this new Kuiper Belt Object is doing that or not. It’s just impossible to tell.”

Still, if the object has that orbital inclination, which Laughlin and Brown expect it will, then “that makes it more likely that there’s some kind of dynamical explanation,” Laughlin says. “And among the dynamical explanations that have been offered, the Planet Nine hypothesis seems to make the most sense.”

Brown, whose discovery of the dwarf planet Eris earned him the nickname “Pluto Killer”, couldn’t be more excited. “It’s fun to see that first one,” he says. “We were pretty sure that it was going to be true, but you still keep your fingers crossed.”