The possibility that there might be a large, unknown planet lurking in the outer Solar System far beyond Pluto has long been a staple of science fiction.

But for the past decade, astronomers have been finding it increasingly likely that something big – often called Planet X – might actually be out there. And a new study by a team from Princeton University has substantially raised the likelihood that it really exists.

The evidence comes from an apparently non-random distribution in the orientation of the orbits of smaller outer Solar System bodies known as trans-Neptunian objects or Kuiper Belt objects (KBOs).

“An asymmetry in the orbits of these distant objects would be a reason to think a planet is potentially there,” says Amir Siraj, a graduate student in astrophysics and first author on the pre-print paper, which has been accepted for publication by The Astrophysical Journal.

“A planet can shepherd the orbits into a confined configuration.”

Prior studies, Siraj told Cosmos, focused on the most distant of these objects, under the assumption that because it would take billions of years for a planet to shepherd their orbits into a clustered configuration, any that venture too deep into the Solar System would get their orbits perturbed by Neptune, erasing any effect from Planet X.

But that doesn’t have to be the case, Siraj says. His team’s computer models show that some KBOs are on orbits that would not be destabilised by Neptune, even over the course of billions of years. By including them in the analysis, he says, it’s possible to increase the number of KBO orbits that might bear detectable imprints of Planet X’s influence from the 11 used in previous studies to 51.

This allowed his team to do a more powerful statistical analysis than prior studies, finding that there is only a one-in-a-thousand chance that the alignment of their orbits is random. “That’s not necessarily a home run,” he says, “but it’s definitely suggestive of a planet being there”.

The extra data also made it easier to hone in on what type of planet could have produced the apparent shepherding of orbits. “We ran 300 combinations of mass and orbital parameters for a possible Planet X across a really wide range,” he says.

Compared to prior studies, he says, “we find a planet mass that is lower and an orbit that is closer in, more elliptical, and more aligned with the orbits of the known planets of the Solar System”—all clues that might help astronomers know where to look for it. “[It’s] like a treasure-hunt map of where the planet is most likely to be found in the sky,” he says.

Though, he notes, it could be anywhere in its orbit, meaning astronomers still have to search a 360° swath of sky.

Physically, the best guess is that it is 4.4 times the mass of the Earth—something that could make it a super-Earth (like some exoplanets), or a mini-Neptune. Either way, it’s likely to be very dim.

Mike Brown, a planetary astronomer at California Institute of Technology, USA, who has long spearheaded the quest to find Planet X, is happy to see the new paper come out. “It’s always nice when someone else takes a fresh look at the data and comes up with the same [basic] answer,” he tells Cosmos.

At the moment, he says, the quest to actually find Planet X is on hiatus, awaiting the opening of the Vera C. Rubin Observatory, a half-billion-dollar (or more) project being built on a mountaintop in northern Chile. When it becomes fully operational, about a year from now, this telescope will use the world’s largest camera to survey the entire visible sky every few days, looking, among other things, for anything that moves.

“I am pretty confident that within the first 18 months, we will have either found [it], or will have made the evidence so irrefutable that no other possibilities exist,” Brown says.

Siraj agrees. “It likely won’t be detected immediately,” he says, “but if it exists, the chances are quite good in the first year or two.”