Planet Nine might not actually be a planet

Written by | Science

There’s something bizarre lurking around the outskirts of the solar system. It’s 10 times more massive than Earth, about 10 to 20 times farther away than Pluto, and powerful enough to gravitationally affect other objects residing in the area. Scientists continue to stumble upon evidence of the existence of this supposed Planet Nine. But how can something so gargantuan just remain hidden from our eyes for so long?

Perhaps, according to new findings published in Astronomical Journal on Sunday, we need to stop thinking about Planet Nine as, well, a planet—and instead consider that it might actually just be a bunch of smaller objects, working in concert to exert the sort of gravitational forces one would expect from a large, single body.

“I like the paper,” says Caltech astronomer Mike Brown, who was not involved with the study but has worked very extensively over the last several years to characterize and find Planet Nine. “It’s the first time someone has proposed something other than Planet Nine that actually explains the phenomena that we are seeing. I don’t think the explanation is actually correct, but it looks to me like the physics actually works to actually explain what we see.”

The Kuiper Belt (the orbital region of the universe beyond Neptune) is covered in small bodies of ice and rock originating from the early days of the solar system. Many of these “trans-Neptunian Objects,” or TNOs, move in very unusual orbits around the sun. Some of this can be explained by the influence of larger celestial bodies like Neptune. But some of these orbits need another explanation—some other unseen object capable of disrupting gravity.

“Planet Nine could be the hidden “shepherd” of these TNOs, guiding their movements and spatial orientations. But Jihad Touma, a researcher from the American University of Beirut and a coauthor of the new study, explains that there are a few shortcomings to the hypothesis that a ninth planet is at play—namely that it doesn’t account for the gravitational pull of other planets in the solar system, and that some of the TNO orbits in question are a bit too eccentric to be influenced by a single object. In addition, we’re still totally stumped on where a giant planet like this came from and how it ended up in such a remote region of the solar system.

Ultimately, Touma and his graduate student, Antranik Sefilian were inclined to wonder, what if the mass of Planet Nine was spread out into a disk? “It seemed like the sound thing to do,” he says. As a result, the pair came up with what’s basically a disk of thousands of icy TNOs on the scale of less than 100 kilometers in diameter (including a small fraction of dwarf planets like Sedna), all lying in the same orbital plane as the true planets of the solar system.

It is not a question of believing or not in Planet Nine,” says Touma. “It is more a question of what one can reasonably expect as a byproduct of the formation of the solar system, all the while allowing for observational constraints, and whether what can be reasonably expected explains the observed clustering of orbits of TNOs naturally—without appealing to an additional planet.”

Last modified: February 8, 2019