Astronomers have proposed Planet Y, a hypothetical world that may very well be orbiting unseen within the far reaches of our photo voltaic system. Discover out extra on this brief video from EarthSky’s Will Triggs.
Is Planet Y hiding in our photo voltaic system?
For a century not less than, when astronomers have spoken of searching for an unknown planet, they’ve referred to it as Planet X. The X stands for “unknown.” So, for instance, Pluto was referred to as Planet X previous to its discovery in 1930.
Over time, there’ve been a number of Planet Xs. The most recent one is extra extensively often known as Planet 9, a theoretical world first proposed in January 2016 by Caltech astronomers Konstantin Batygin and Mike Brown. Batygin and Brown imagine this unknown world – which they estimate to be between 5 and 10 occasions the mass of Earth – is hiding someplace within the depths of our photo voltaic system.
Now, a staff of researchers from Princeton has printed a examine suggesting there is likely to be a 2nd massive physique hiding within the outer photo voltaic system. They’re calling it Planet Y. If it exists, Planet Y is a bit nearer in and lower-mass than Planet 9.
The lead creator of the brand new paper is Amir Siraj, an astrophysicist and Ph.D. candidate at Princeton College. You may already acknowledge his title for the papers he co-authored with Avi Loeb whereas he was at Harvard.
Siraj, together with Christopher Chyba and Scott Tremaine, each of Princeton, printed their new peer-reviewed paper on August 21, 2025, within the Month-to-month Notices of the Royal Astronomical Society.
A view of our photo voltaic system, with the pink orbits representing 46 Kuiper belt objects and the lighter-colored orbit representing the attainable Planet Y. Picture through Tony Dunn. Used with permission.
The make-up of our photo voltaic system
Our photo voltaic system fashioned when a spinning cloud of fuel and dirt collapsed because of gravity. The solar was the middle of this collapse, and the flattened disk round it fashioned the planets and different assorted our bodies, reminiscent of asteroids and comets. So our photo voltaic system is basically oriented in a single flat, disklike aircraft of area. There are exceptions, after all, particularly the farther away we get from the solar. Pluto, for instance, has a extra inclined orbit across the solar.
Pluto is what astronomers name a Kuiper belt object (or a trans-Neptunian object). The Kuiper belt lies past Neptune in a donut-shaped disk. These area rocks could be from about 30 astronomical items away (30 AU is the common distance of Neptune) as much as so far as 1,000 AU away.
And when you go means, means far out from the solar, you attain the still-hypothetical Oort Cloud. This cloud is the place scientists imagine many comets come from. It’s not a disk form however a spherical area surrounding the solar some 2,000 to 200,000 AU away.
So, the place do Planet X and Planet Y slot in? Theoretically, Planet X would lie about 600 AU away, whereas Planet Y could be nearer, at about 80 to 200 AU.
Is Planet Y warping the Kuiper belt?
The researchers analyzed the orbits of greater than 150 Kuiper belt objects. They needed to look past Neptune’s affect, which has a giant pull – actually – on objects in its neighborhood. So first they examined the orbits of the objects between 50 and 80 AU, and people largely adopted the aircraft of our photo voltaic system.
However after they checked out objects orbiting the solar between 80 and 200 AU, they discovered their paths have been warped. They ran numerous fashions to attempt to account for the warped path of those objects. What they discovered was {that a} planet 25 to 450 occasions extra huge than Pluto was the probably answer. And that object, a attainable Planet Y, would orbit someplace round 100 to 200 AU away.
So, Planet Y would have a mass someplace between that of Mercury and Earth. Might we ever affirm and possibly even spot such an object? The paper stated:
The Vera C. Rubin Observatory’s Legacy Survey of House and Time (LSST) is predicted to substantiate or deny the existence of the warp reported right here, and may detect the planet which will produce it.
A brand new planet for our photo voltaic system?
The final new planet to affix our photo voltaic system was Neptune, when a couple of astronomers visually confirmed its existence in 1846. After all, Clyde Tombaugh found Pluto in 1930, and on the time – and for a few years afterward – it was categorized as a planet. However in 2006, the Worldwide Astronomical Union (IAU) reclassified it as a dwarf planet. Based on the IAU’s new definition of planethood, Pluto hadn’t “cleared its orbit.” So I requested Amir Siraj through e-mail if Planet Y, far out within the Kuiper belt, would actually be thought-about a planet and have cleared its orbit. Siraj responded:
A roughly Mars-mass physique within the 100 to 200 AU semimajor axis vary would have the ability to clear its orbit, so it will be categorized as a planet. ‘Clearing the orbit,’ after all, is a considerably subjective (and flawed) criterion. However to place issues in perspective, one Mars is roughly 50 Plutos, so I don’t assume there could be a lot debate, as there was with Pluto, on whether or not the physique could be huge sufficient to be a planet.
I additionally requested Siraj which hypothetical planet would conceivably be found first, Planet X/9 or Planet Y. Siraj answered:
It’s arduous to say whether or not Planet X/9 or Planet Y could be found first, in the event that they each exist. Planet Y is nearer to the solar, however smaller, whereas Planet X/9 is farther, however greater.
So, the Vera Rubin Observatory is on the case to find if Planet X or Planet Y may certainly exist. And I requested Siraj if there may nonetheless be different attainable planets past X and Y. He stated:
It’s actually attainable. For instance, if a free-floating planet have been captured by the photo voltaic system early on, it will most likely reside within the Oort cloud and sure stay undetectable even with LSST.
Backside line: A staff of astronomers have found some warped orbits for distant our bodies in our photo voltaic system. They stated the presence of a Planet Y would finest clarify these orbits.
Supply: Measuring the imply aircraft of the distant Kuiper belt
