Something Other Than Planet 9 Might Be Shaping the Outer Solar System

Far beyond Neptune, small celestial bodies circle the sun in strange orbits. The paths of these “trans-Neptunian” objects aren’t the usual round or elliptical shape, suggesting that it isn’t just the sun that directs their movement. One of the leading theories for their wacky orbits is the existence of Planet Nine, a giant object at the edge of the solar system that exerts its gravitational pull on the smaller bodies. A paper released in The Astronomical Journal Monday, however, posits a far less dramatic explanation.

There are at least 23 of these trans-Neptunian objects with “eccentric and inclined” orbits, first author Antranik A. Sefilian, a Ph.D. candidate at Cambridge University, writes alongside American University of Beirut physics professor Jihad R. Touma, Ph.D. In astronomy, the term “eccentric” technically refers to any deviation from a perfect circle (but is also eccentric in the figurative sense). Rather than blame those orbits on the existence of a massive unseen planet that we’ve never seen, these researchers think a giant disc of smaller objects might have the same effect.

“Here, we go precisely after the dynamical impact of an extended and relatively massive disk of TNOs, and demonstrate that this alone can provide a fair amount of shepherding,” they write, “perhaps obviating the need for an extra planetary member in the solar system pantheon, but surely complementing it.”

New objects in the outer edges of the solar system could help the search for Planet Nine.
Some of the "wacky" trans-Neptunian orbits that could be explained by the disk theory. The tiny yellow dot in the middle is our sun.

This “less daring” hypothesis rests on the existence of a lopsided, eccentric, disk of trans-Neptunian debris that has the mass of about 10 Earths. In the paper, they show how this massive disk could account for the weird orbits for the 23 objects orbiting the sun way past Neptune without any need for a hypothetical super-massive planet.

The Carnegie Institute for Science’s Scott Shephard, Ph.D., who has discovered many of those trans-Neptunian objects in his team’s hunt for Planet Nine and wasn’t involved with the research, tells Inverse the new work is “interesting, but is not really all that new.” Previous researchers have posited the disc explanation, though this study appears to be the first to account for both the strange orbits as well as the mass and gravity for the eight planets we’re familiar with.

“The problem is there is no evidence for a massive disk in the outer solar system,” says Shephard. Like Planet Nine itself, this massive disk remains theoretical and, for now, unseen. Shephard maintains that, in the hunt for an explanation for the weird orbits, the thing to keep in mind is that “something massive” must be at play, and it’s likelier that our surveys of the outer solar system would have missed a single massive planet rather than a larger, more diffuse massive disk of objects.

Sefilian and Touma admit in the paper that they’re operating purely in the hypothetical space, but do not mind “entertaining, as we like to do, the possibility of a massive trans-Neptunian disk.”

“Ultimately, though, we do not have secure and direct observational evidence for our proposed disk,” they write, “in much the same way we do not have full proof arguments against Planet Nine.”