Is Planet Y hiding in our solar system?
For a century at least, when astronomers have spoken of seeking an unknown planet, they’ve referred to it as Planet X. The X stands for “unknown.” So, for example, Pluto was called Planet X prior to its discovery in 1930.
Over the years, there’ve been multiple Planet Xs. The latest one is more widely known as Planet 9, a theoretical world first proposed in January 2016 by Caltech astronomers Konstantin Batygin and Mike Brown. Batygin and Brown believe this unknown world – which they estimate to be between five and 10 times the mass of Earth – is hiding somewhere in the depths of our solar system.
Now, a team of researchers from Princeton has published a study suggesting there might be a 2nd large body hiding in the outer solar system. They’re calling it Planet Y. If it exists, Planet Y is a bit closer in and lower-mass than Planet 9.
The lead author of the new paper is Amir Siraj, an astrophysicist and Ph.D. candidate at Princeton University. You might already recognize his name for the papers he co-authored with Avi Loeb while he was at Harvard.
Siraj, along with Christopher Chyba and Scott Tremaine, both of Princeton, published their new peer-reviewed paper on August 21, 2025, in the Monthly Notices of the Royal Astronomical Society.
A view of our solar system, with the pink orbits representing 46 Kuiper belt objects and the lighter-colored orbit representing the possible Planet Y. Image via Tony Dunn. Used with permission.
The makeup of our solar system
Our solar system formed when a spinning cloud of gas and dust collapsed due to gravity. The sun was the center of this collapse, and the flattened disk around it formed the planets and other assorted bodies, such as asteroids and comets. So our solar system is largely oriented in one flat, disklike plane of space. There are exceptions, of course, especially the farther away we get from the sun. Pluto, for example, has a more inclined orbit around the sun.
Pluto is what astronomers call a Kuiper belt object (or a trans-Neptunian object). The Kuiper belt lies beyond Neptune in a donut-shaped disk. These space rocks can be from about 30 astronomical units away (30 AU is the average distance of Neptune) up to as far as 1,000 AU away.
And once you go way, way far out from the sun, you reach the still-hypothetical Oort Cloud. This cloud is where scientists believe many comets come from. It’s not a disk shape but a spherical region surrounding the sun some 2,000 to 200,000 AU away.
So, where do Planet X and Planet Y fit in? Theoretically, Planet X would lie about 600 AU away, while Planet Y would be closer, at about 80 to 200 AU.
Is Planet Y warping the Kuiper belt?
The researchers analyzed the orbits of more than 150 Kuiper belt objects. They wanted to look beyond Neptune’s influence, which has a big pull – literally – on objects in its vicinity. So first they examined the orbits of the objects between 50 and 80 AU, and those largely followed the plane of our solar system.
But when they looked at objects orbiting the sun between 80 and 200 AU, they found their paths were warped. They ran various models to try to account for the warped path of these objects. What they found was that a planet 25 to 450 times more massive than Pluto was the most likely solution. And that object, a possible Planet Y, would orbit somewhere around 100 to 200 AU away.
So, Planet Y would have a mass somewhere between that of Mercury and Earth. Could we ever confirm and maybe even spot such an object? The paper said:
The Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) is expected to confirm or deny the existence of the warp reported here, and might detect the planet that may produce it.
A new planet for our solar system?
The last new planet to join our solar system was Neptune, when a few astronomers visually confirmed its existence in 1846. Of course, Clyde Tombaugh discovered Pluto in 1930, and at the time – and for many years afterward – it was classified as a planet. But in 2006, the International Astronomical Union (IAU) reclassified it as a dwarf planet. According to the IAU’s new definition of planethood, Pluto hadn’t “cleared its orbit.” So I asked Amir Siraj via email if Planet Y, far out in the Kuiper belt, would really be considered a planet and have cleared its orbit. Siraj responded:
A roughly Mars-mass body in the 100 to 200 AU semimajor axis range would be able to clear its orbit, so it would be classified as a planet. ‘Clearing the orbit,’ of course, is a somewhat subjective (and flawed) criterion. But to put things in perspective, one Mars is roughly 50 Plutos, so I don’t think there would be much debate, as there was with Pluto, on whether the body would be massive enough to be a planet.
I also asked Siraj which hypothetical planet would conceivably be discovered first, Planet X/9 or Planet Y. Siraj answered:
It’s hard to say whether Planet X/9 or Planet Y would be discovered first, if they both exist. Planet Y is closer to the sun, but smaller, while Planet X/9 is farther, but bigger.
So, the Vera Rubin Observatory is on the case to discover if Planet X or Planet Y might indeed exist. And I asked Siraj if there might still be other possible planets beyond X and Y. He said:
It’s certainly possible. For example, if a free-floating planet were captured by the solar system early on, it would probably reside in the Oort cloud and likely remain undetectable even with LSST.
Bottom line: A team of astronomers have discovered some warped orbits for distant bodies in our solar system. They said the presence of a Planet Y would best explain these orbits.
Source: Measuring the mean plane of the distant Kuiper belt