It’s been nearly 2 centuries since a planet was discovered in the solar system. But now scientists think they’ve uncovered evidence of a newcomer that just might usurp that honor from Neptune. Following an analysis of the orbits of bodies in the Kuiper Belt, a team has proposed that an unseen planet at least 25 times more massive than Pluto might reside there. These results were published in Monthly Notices of the Royal Astronomical Society.
The Kuiper Belt is loosely defined as a doughnut-shaped swath of space beginning just beyond the orbit of Neptune and extending to roughly 1,000 times the Earth-Sun distance. It’s home to untold numbers of icy, rocky objects, including Pluto and other so-called Kuiper Belt objects such as Arrokoth.
Everything in the Kuiper Belt can be thought of as cosmic debris, said Amir Siraj, an astrophysicist at Princeton University and lead author of the new paper. “It represents some of the leftovers from the formation of our solar system.”
And most of those leftovers are small: Pluto is the most massive known Kuiper Belt object, and it’s just 0.2% the mass of Earth.
But over the past decade, scientists have hypothesized that something substantially larger than Pluto might be lurking in the Kuiper Belt. Evidence of that unseen world—a so-called Planet Nine or Planet X—lies in the fact that six Kuiper Belt objects share curiously similar orbital parameters and are associated in physical space. A nearby, larger planet could have shepherded those worlds into alignment, researchers have proposed.
Planes, Planes, Everywhere
Siraj and his colleagues recently took a different tack to look for a massive resident of the Kuiper Belt: They analyzed a much larger sample of Kuiper Belt objects and focused on their orbital planes. One would naively expect the average orbital plane of Kuiper Belt objects to be the same as the average orbital plane of the planets in the solar system, said Siraj. But a planet-mass body in the Kuiper Belt would exert a strong enough gravitational tug on its neighboring Kuiper Belt objects to measurably alter the average orbital plane of the Kuiper Belt, at least in the vicinity of the planet. Siraj and his collaborators set out to see whether they could spot such a signal.
“Neptune has a really strong grasp on the outer solar system.”
The researchers extracted information about the orbits of more than 150 Kuiper Belt objects from the JPL Small-Body Database managed by NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Of the several thousand known Kuiper Belt objects, the team honed in on that subset because those objects aren’t gravitationally influenced by Neptune. Neptune is the playground bully of the outer solar system, and the orbits of many Kuiper Belt objects are believed to be literally shoved around by gravitational interactions with the ice giant. “Neptune has a really strong grasp on the outer solar system,” said Siraj.
The team calculated the average orbital plane of their sample of Kuiper Belt objects. At distances of 50 to 80 times the Earth-Sun distance, they recovered a plane consistent with that of the inner solar system. But farther out, at distances between 80 and 200 times the Earth-Sun distance, the researchers found that their sample of Kuiper Belt objects formed a plane that was warped relative to that of the inner solar system. There was only a roughly 4% probability that that signal was spurious, they calculated.
Meet Planet Y
Siraj and his collaborators then modeled how planets of different masses at various orbital distances from the Sun would affect a simulated set of Kuiper Belt objects. “We tried all sorts of planets,” said Siraj.
By comparing those model results with the observational data, the researchers deduced that a planet 25–450 times more massive than Pluto with a semimajor axis in the range of 100–200 times the Earth-Sun distance was the most likely culprit. There’s a fair bit of uncertainty in those numbers, but the team’s results make sense, said Kat Volk, a planetary scientist at the Planetary Science Institute in Tucson, Ariz., not involved in the research. “They did a pretty good job of bracketing what kind of object could be causing this signal.”
To differentiate their putative planet from Planet X, Siraj and his colleagues suggested a new name: Planet Y. It’s important to note that these two worlds, if they even exist, aren’t one and the same, said Siraj. “Planet X refers to a distant, high-mass planet, while Planet Y denotes a closer-in, lower-mass planet.”
“This is really expected to be a game changer for research on the outer solar system.”
There’s hope that Planet Y will soon get its close-up. The Legacy Survey of Space and Time (LSST)—a 10-year survey of the night sky that will be conducted by the Vera C. Rubin Observatory in Chile beginning as soon as this fall—will be supremely good at detecting Kuiper Belt objects, said Volk, who is a member of the LSST Solar System Science Collaboration. “We’re going to be increasing the number of known objects by something like a factor of 5–10.”
It’s entirely possible that Planet Y itself could be spotted, said Volk. But even if it isn’t, simply observing so many more Kuiper Belt objects will better reveal the average orbital plane of the Kuiper Belt. That will, in turn, shed light on whether it’s necessary to invoke Planet Y at all.
Even if his team’s hypothesis is proven wrong, Siraj says he’s looking forward to the start of the LSST and its firehose of astronomical data. “This is really expected to be a game changer for research on the outer solar system.”
—Katherine Kornei (@KatherineKornei), Science Writer
Citation: Kornei, K. (2025), A survey of the Kuiper Belt hints at an unseen planet, Eos, 106, https://doi.org/10.1029/2025EO250344. Published on 16 September 2025.
Text © 2025. The authors. CC BY-NC-ND 3.0
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