An Astronomer Has Found the Hardest Evidence Yet for the Elusive Planet Nine

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• Scientists have been searching for a ninth planet to explain the orbital eccentricities of Uranus and Neptune (and dwarf planets like Sedna and Pluto) for more than a century.
• For the past decade, one of the leading theories attempting to explain these oddities is that an extremely far-flung planet that we have yet to detect could be orbiting the Sun.
• Now, one of Planet Nine's leading proponents has extensively compared models—both including the planet and not including the planet—and reportedly found strong statistical evidence for its existence.

Over the course of millennia, scientists have made some pretty big strides in figuring out how our Solar System ticks. Putting the Sun in the center was a big help (thanks Copernicus), and discovering distant planets such as Uranus and Neptune—while discounting other phantom planets such as Vulcan—helped describe some of our system's strange orbital anomalies.

But while there's certainly been progress, the work is far from over—especially because wobbles in the orbits of our far flung ice giants (along with dwarf planets like Pluto and Sedna) suggest there's something out there that we're still missing.

A fair number of explanations have been put forth to try and understand this unexplained "wobble," including undiscovered belts, a grapefruit-sized primordial black hole, or even (more controversially) a misunderstanding of astrophysics. However, the leading theory is that far beyond the orbit of Neptune lies another planet that is causing these orbital perturbations—Planet Nine.

One of the main proponents of this idea is Caltech's Konstantin Batygin, who (along with colleague Mike Brown) revealed what was described as a "road map" for a finding a proposed ninth planet that's roughly five times the size of an Earth—an icy super-Earth or a mini-Neptune—in 2016. Fast forward eight years, and Batygin is back with even more evidence that a ninth planet is the most likely explanation for the orbital data gathered throughout the Solar System.

"What we show in this paper is that not only is Planet Nine up to the task, moreover it's that the … orbital distribution that the Planet Nine model predicts is perfectly consistent with what we see in the data," Batygin said on the podcast Event Horizon. "Conversely, a Solar System without a Planet Nine can be ruled out with a confidence of five sigma [a.k.a. a statistical discovery]."

In the latest paper, currently uploaded to the preprint server arXiv, the researchers plugged in known celestial forces—from planets, stars, and the Milky Way itself—and ran multiple simulations, some of which included a Planet Nine (P9) and some of which did not. They found that the P9-inclusive simulations more accurately reflected what astronomers see in the Solar System, which means that some planetary body (possibly around 400 to 800 time further from the Sun than Earth) has avoided our gaze for millennia.

And for good reason. At such an astronomical distance, the presumed faintness of the planet would make it incredibly difficult to detect, even using telescopes like PanSTARRS or the upcoming Vera C. Rubin Observatory. But Planet Nine's elusive nature isn't its only problem. If the planet did exist, it would be on an extremely strange trajectory around the Sun—so strange, in fact, that some astronomers say it could challenge our understanding of planetary science.

Simply put, there is just too much about our own Solar System we still don't know to be certain of one solution over another. However, when the Vera C. Rubin Observatory goes online sometime in 2025, one of its missions will be providing unprecedented clarity into what lurks beyond Neptune, potentially revealing 10 times as many Solar System objects as are known today.

For centuries, astronomers have been shining a flashlight, hoping to stumble upon a cosmic mystery in the darkness. Hopefully when the Vera C. Rubin Observatory finally arrives, it'll be like turning on a light switch.

Darren Orf

Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough. 

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