The predicted position of Planet Nine is intended to explain the irregular orbits of some Kuiper Belt objects in the outer solar system. However, some researchers are unconvinced that these orbits are caused by an undiscovered planet, and the scientific community as a whole still awaits direct observational evidence of Planet Nine’s existence. If Planet Nine does exist, researchers predict it to be much larger than Earth and to orbit in an unusual orbit billions of miles from Neptune, the eighth and currently last planet in our solar system. Planet Nine’s proposed orbit is so far from the Sun that it would be difficult for Earth-based scientists to detect it, making it difficult to prove or disprove its existence.
Researchers have identified a candidate for the hypothetical Planet Nine, which may be an undiscovered giant planet located far beyond our Solar System. However, like any study of planet ninths, the new findings have been met with some skepticism.
Astronomers searching for a ninth planet in our solar system claim to have found a “good” candidate far beyond Neptune, according to a preliminary study.
This could be humanity’s first glimpse of the long-discussed Planet Nine, which—if it exists—is a gigantic, undiscovered world orbiting our Sun in distant orbits. However, the Planet Nine hypothesis is controversial, and new discoveries are unlikely to change anything.
Researchers discovered this new planetary candidate while studying old satellite data. At this stage, the candidate is just a curious dot in a few infrared images. However, this dot appears to be moving in a manner consistent with a large, distant planet.
An illustration of the hypothetical Planet Nine in the Solar System. The existence of Planet Nine could explain the unusual orbits of some objects in the Kuiper Belt. Shutterstock
Lead author Terry Fan, a graduate student in astronomy at National Tsing Hua University in Taiwan, and his colleagues published their results on the preprint server arXiv on April 24, 2025, and they were accepted for publication in the journal Publications of the Astronomical Society of Australia. However, the preprint was met with some skepticism.
Mike Brown, an astronomer at the California Institute of Technology who first proposed the “Planet Nine” hypothesis with a colleague in 2016, isn’t convinced that the infrared dots correspond to the elusive planet. Brown wasn’t involved in the study, but he calculated the orbit of the infrared signal and found that the object would be tilted approximately 120 degrees relative to the plane of the solar system, Science reports. This tilt is significantly greater than the predicted tilt of 15–20 degrees for “Planet Nine,” and also means that the object would orbit in a direction different from that of the known planets, which all orbit roughly in the same plane.
This discrepancy “doesn’t mean it’s not there, but it does mean it’s not Planet Nine,” Brown told Science magazine. “I don’t think this planet could have any of the effects we see on the solar system.”
As reported by Space.com, the team conducting the new study searched for Planet Nine candidates in the data archives of two decommissioned satellites: the Infrared Astronomy Satellite (IRAS) (1983) and the AKARI satellite (2006–2011). They looked for distant objects slowly moving between data sets, similar to how the putative Planet Nine would move as it orbited the Sun.
After eliminating known objects from the data, the researchers focused on a short list of candidates. They then carefully examined images of these potential objects and ultimately selected what they described in the study as “one good candidate.” The candidate appears as a point with similar colors and brightness in the two sets of images, suggesting it is the same object, captured by both satellites. According to the study, further observations are needed to determine the object’s full orbit.
Brown told Science magazine that if the infrared signal corresponds to a planet, then, according to his calculations, it could not exist near the original Planet Nine hypothesis without making their orbits unstable. Therefore, the new Planet Nine candidate could be another planet, capable of disproving the Planet Nine hypothesis.
Another study suggests the existence of Planet Y, an alternative candidate for Planet Nine that is smaller and closer to Earth than the hypothetical Planet X that astronomers have been hunting for nearly a decade. However, the evidence for this new theory is “not conclusive.”
Astronomers believe they have discovered evidence of a previously unknown ninth planet lurking in the distant reaches of our solar system. This hypothetical world, dubbed “Planet Y,” could be roughly the same size as Earth and exist alongside another popular candidate for Planet Nine, potentially bringing the number of planets in our cosmic neighborhood to ten if both proposals are confirmed.
However, despite researchers’ claims that this new theoretical world could be discovered within three years, there is no evidence of its existence, and some experts are skeptical of the new discoveries.
The search for a ninth world in the solar system began after the discovery of Neptune in 1846 and temporarily halted in the 1930s with the discovery of Pluto, which was subsequently downgraded to dwarf planet status in 2006. But the search resumed in 2016, when Caltech astronomers Mike Brown and Konstantin Batygin proposed their “Planet Nine” hypothesis, which proposed that the unusual orbits of about a dozen large objects beyond Neptune were caused by the gravitational pull of a massive, hidden object.
Researchers believe the newly commissioned Vera K. Rubin Observatory could help find Planet Y or Planet X in the next few years. Hernan Stockebrand
This hypothetical world, also known as Planet X, is likely lurking somewhere in the Kuiper Belt—the giant disk of asteroids, comets, and dwarf planets (including Pluto) orbiting the Sun beyond the eight known planets—but has so far eluded detection.
In a study published on August 21, 2025, in the journal Monthly Notices of the Royal Astronomical Society, another team of researchers claims to have discovered evidence of an entirely new candidate for Planet X (P9), which they have dubbed Planet Y. This hypothetical world, also located in the Kuiper Belt, could be approximately twice as close to Earth as Planet X and much closer in size to our planet. The researchers noted that its potential existence does not disprove the Planet X theory.
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The team reached this conclusion after analyzing the trajectories of 50 Kuiper Belt Objects (KBOs) and finding that they are tilted by approximately 15 degrees compared to the rest of the planets in the Solar System. They argued that the only explanation for this tilt is the existence of a hidden world.
“We started looking for explanations for the tilt other than a planet, but we found that there actually must be a planet there,” lead study author Amir Siraj, an astrophysicist at Princeton University, recently told CNN. “This work is not the discovery of a planet,” he added. “But it is certainly the discovery of a mystery for which a planet could be a likely solution.”
Based on their calculations, the research team believes that Planet Y is likely a rocky planet with a mass somewhere between that of Mercury and Earth. This is significantly smaller than the estimated mass of Planet X, which is thought to be a gas giant up to 10 times larger than Earth.
Researchers proposed the Planet Y theory after noticing a strange tilt in about 50 objects in the Kuiper Belt, beyond the orbit of Neptune. Getty Images
Researchers also suggest that Planet Y is somewhere between 100 and 200 times farther from the Sun than Earth. This is only slightly further than the orbit of Neptune, which orbits the Sun at a distance 30 times greater than Earth’s distance from the Sun, and much closer to us than Planet X, which is likely at least 400 times farther from the Sun than Earth. (At such distances, both planets reflect very little light back to Earth, making their direct detection difficult.)
The researchers write that if Planet Y exists, it would be tilted 10 degrees compared to the orbital plane of the eight known planets—another factor that would make it difficult to detect.
However, not everyone agrees with the new proposal.
Samantha Lawler, an astronomer at the University of Regina in Saskatchewan and a critic of the Planet X hypothesis, told CNN that the results are “not conclusive,” largely due to the small sample size of Kuiper Belt objects included in the study. Patrick Sophia Likavka, an astronomer at Kindai University in Japan who specializes in Kuiper Belt objects, added that the existence of Planet Y is “likely,” but requires further observations to consider it a credible possibility.
To fully prove the existence of Planet Y or Planet X, researchers will either be very lucky and directly observe the elusive worlds, or they will find many more Kuiper Belt objects that match the patterns they have discovered.
This could happen in no time at all, thanks to the recently opened Vera K. Rubin Observatory in Chile, which has begun scanning the night sky with the world’s largest digital camera. Many experts believe this research will uncover thousands more Kuiper Belt objects in the coming years, potentially providing the data needed to confirm these theories.
Planet Y isn’t the only alternative candidate for P9 to emerge in recent years. Several other hypothetical worlds have been proposed, but all have been largely disproven.
In May 2025, researchers detected a faint infrared spot that they believed could be Planet Nine. However, this discovery was largely dismissed because the object responsible for these readings would have to orbit the Sun in a plane perpendicular to the other planets, which is highly unlikely.
There’s also the possibility that several rogue planets—worlds ejected from distant star systems and subsequently captured by the Sun’s gravity—may lurk even farther from our star. Siraj’s calculations, conducted in 2023, showed that the outermost reaches of the Solar System could harbor five rocky, Earth-like planets.
Some astronomers have also proposed alternative explanations for Planet X’s existence, including the suggestion that the gravitational anomalies are caused by a young black hole or are the result of some alternative form of gravity we don’t yet understand. Others suggest that this vanished world may be smaller than we think, possibly surrounded by dozens of moons.
On the other hand, some experts believe that the recent discovery of several dwarf planet candidates in the Kuiper Belt—including 2017 OF201 and 2023 KQ14, announced in May and July 2025, respectively—reduces the likelihood of Planet X’s existence, as these objects do not behave as predicted by this theory.
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