Is there a planet outside of Neptune that our solar system is hiding? With a simple and plausible hypothesis—a fictitious planet called Planet Y that moves in the Kuiper Belt, gently pulling on far-off icy objects and leaving orbital anomalies that don’t match the pull of known planets—a Princeton University study explores that possibility.
As the authors emphasize, “this paper is not a discovery of a planet, but it’s certainly the discovery of a puzzle for which a planet is a likely solution… What we found is that you actually need a planet there,” says lead author Amir Siraj.
Observers track small, cold bodies beyond Neptune. Their paths’ tilts and clustering point to an ongoing, imperceptible prod. The team argues that Planet Y, which is probably smaller than Earth and larger than Mercury, may be the source of that gravitational pull.
Crucially, the idea is meant to be validated. A new astronomical observatory in Chile with the world’s largest digital camera will carry out a ten-year survey. This is the ideal time to verify or reject this potential astronomical discovery. If Planet Y is in the telescope’s field of view, the team expects a final test.
The case is currently based on those frequent orbital anomalies and a simple hypothesis: an unknown planet in the distant solar system.
The reasons planet Y makes sense
Some icy objects in the Kuiper Belt follow strangely tilted or clustered orbits. If the known worlds were the only ones forming those paths, they should be more dispersed. The writers weighed their options and chose gravity. Amir Siraj stated, “We started trying to come up with explanations other than a planet that could explain the tilt, but what we found is that you actually need a planet there.”
“Probably smaller than the Earth and probably bigger than Mercury, orbiting in the deep outer solar system” is how the proposed world is described. That size is ideal because it is both small enough to avoid detection and powerful enough to gradually alter orbits. It’s dim, slow, and obscured by a dense star field, making it difficult to see deep within the Kuiper Belt.
The research gives a logical gravitational model that precisely accounts for several orbital anomalies without the need for special pleading.
How Chile’s Telescope could help
The largest digital camera in the world will soon start a wide, deep, repeated survey with a next-generation telescope in Chile that is a part of a major astronomical observatory. This makes it ideal for locating slow, small movers in the outer solar system, like Planet Y.
“If Planet Y is in the telescope’s field of view, it will be able to find it directly,” Amir Siraj stated, adding, “I think within the first two to three years, it’ll become definitive.” A tiny point will be inching against the background stars.
Astronomers can use repeated images to calculate an orbit. If it matches predictions that explain today’s orbital anomalies, the argument becomes stronger. Otherwise, by mapping the Kuiper Belt in unprecedented detail, the survey either complements or replaces the existing theory.
What happens after confirmation?
A detection could change our understanding of the distant solar system, including how small worlds form, travel, and subtly affect each other’s paths. Furthermore, it would validate a distinct shift from confusing motions to a concrete search approach where modeling guides observations.
What happens if there are no results from the search? It would force scientists to reconsider their assumptions about the Kuiper Belt, as well as search for non-world explanations for the orbital anomalies and improve models with a more comprehensive census of icy objects.