Scientists detect signs of hidden planet beyond Neptune
Astronomers may have uncovered evidence of a hidden world — “Planet Y” — whose gravity could be warping the Kuiper Belt. If confirmed, it would represent a new addition to the solar system, distinct from the debated Planet Nine.
Image credit: Credit: NASA, ESA, and G. Bacon (STScI)
- A new study suggests a hidden planet, informally called Planet Y, may be warping the orbits of icy bodies in the Kuiper Belt.
- Researchers from Princeton University and the Institute for Advanced Study found a 15° orbital tilt between 80–200 AU that cannot be explained by chance or bias.
- Simulations indicate Planet Y could have a mass between Mercury and Earth, orbiting at 100–200 AU with an inclination of at least 10°.
- The signal differs from the Planet Nine hypothesis, which proposes a larger super-Earth farther out at 300–600 AU.
- Upcoming observations by the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) may confirm or refute the existence of Planet Y.
Could there be more planets in our solar system? A new study published in Monthly Notices of the Royal Astronomical Society seems to have reignited the debate.
Researchers from Princeton University and the Institute for Advanced Study have reported signs of a mysterious planetary body that may be warping the orbits of icy objects far beyond Neptune.
The proposed world, informally referred to as Planet Y, could represent a completely new category of planet—smaller and closer than the long-hypothesized Planet Nine.
The new study led by Amir Siraj, Christopher Chyba, and Scott Tremaine, presents a refined way of measuring the average orbital orientation of distant trans-Neptunian objects (TNOs).
Their analysis shows that while the Kuiper Belt’s mean plane aligns with the solar system’s invariable plane at 50–80 AU and 200–400 AU, it departs significantly in the 80–200 AU range.
This deviation appears as a “warp” of about 15°. The authors report confidence levels of ~96% for the 80–200 AU bin and ~98% for 80–400 AU. In other words, the false alarm probability that the warp is due to chance is just 2–4%.
If confirmed, the finding would show the gravitational influence from a previously unknown body, possibly a new planet. The researchers argue that neither chance nor observational bias is sufficient to explain the observed tilt.
Measuring the Kuiper Belt’s mean plane
The Kuiper Belt, a reservoir of icy objects beyond Neptune, that orbits the sun in a disc-like structure. In theory, the average orbital orientation of these bodies — their “mean plane” — should coincide with the invariable plane, defined by the solar system’s total angular momentum. Any sustained deviation signals an external force at work.
Previous attempts to measure the mean plane often suffered from observational bias. Telescopes survey the sky unevenly, skewing the average if not corrected. The new method uses a statistical approach designed to cancel out survey-selection effects and better reflect the true underlying distribution.
The study examined 154 non-resonant TNOs with semimajor axes between 50–400 AU, excluding objects locked in resonances with Neptune. This sample ensures the result is not distorted by Neptune’s strong gravitational influence.
Their results confirm that the mean plane is consistent with the invariable plane at 50–80 AU and 200–400 AU. However, between 80–200 AU the deviation reaches 2.5σ, and when extended to 80–400 AU, it rises to 2.7σ. These values exceed the typical threshold for statistical noise.
What’s up with the warp?
Differential precession, the slow shift of orbital nodes, would erase any primordial warp in less than 100 million years. This means that the observed deviation must be sustained by an ongoing gravitational force.
To test possible explanations, the team ran N-body simulations including a hypothetical additional planet. They found that a body with a mass between Mercury and Earth (~0.06–1 Earth masses), orbiting at ~100–200 AU with an inclination of at least 10°, could reproduce the observed warp.
Larger planets exceeding ~1 Earth mass tend to warp regions closer than 80 AU, inconsistent with observations. Pluto-mass bodies (~0.002 Earth masses) are less favored because their gravitational influence is too weak, though the authors note they cannot be entirely ruled out.
Control simulations show that known dwarf planets such as Pluto, Eris, and Sedna cannot account for the measured deviation.
Planet Y vs Planet Nine
The search for unseen planets beyond Neptune has been ongoing for years. In 2016, Konstantin Batygin and Mike Brown proposed “Planet Nine,” a super-Earth of 5–10 Earth masses orbiting 300–600 AU away, invoked to explain the clustering of extreme TNOs.
Planet Y is a different hypothesis. It involves a smaller, closer planet that would explain a local warp in the mean orbital plane rather than apsidal clustering. If real, Planet Y would represent a separate addition to the solar system’s inventory of major bodies.
The possibility that both Planet Nine and Planet Y exist remains speculative. The new results do not confirm or refute Planet Nine but instead highlight an additional, independent anomaly.
Earlier studies, including Matheson and Malhotra (2023), measured the Kuiper Belt’s mean plane up to ~150 AU and found it consistent with the invariable plane, with deviations no larger than ~0.7°.
The current signal relies on just 46 non-resonant TNOs in the 80–400 AU range. The authors acknowledge that small-number statistics are a limitation, though they argue the warp is unlikely to be a statistical fluke. Previous claims of a warp in the 50–80 AU region, reported in 2017, later disappeared as more objects were discovered.
Even with bias-robust methods, the uneven distribution of known TNOs means that undiscovered objects could shift the apparent average. Critics caution that the observed warp may fade with more complete surveys.
Detection prospects
“Finally, we note that a hypothetical Planet Y as described in this work would likely be detectable by the upcoming Legacy Survey of Space and Time (LSST) on the Vera C. Rubin Observatory,” the authors state.
The Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST), scheduled to begin operations in 2026, is expected to transform outer solar system studies. By detecting tens of thousands of new TNOs, LSST will provide the statistical power needed to confirm or refute the warp.
If Planet Y exists, Rubin may also detect it directly. At ~100–200 AU — about 15–30 billion km (9.3–18.6 billion miles) from the Sun — a Mercury-to-Earth-mass planet would be faint but potentially observable, depending on its albedo and position in the sky.
Infrared surveys could help detect its thermal emission, while improved orbital measurements of known TNOs will refine the signal. Beyond detection, the implications are wide-reaching. A confirmed Planet Y would reshape models of solar system formation and suggest that planetary migration left more survivors than previously thought.
References:
1 Measuring the mean plane of the distant Kuiper Belt – A. Siraj, C. F. Chyba, & S. Tremaine – MNRAS – August 19, 2025 – DOI:https://doi.org/10.1093/mnrasl/slaf091 – OPEN ACCESS
I am an Assistant Editor and Severe Weather & Science Journalist at The Watchers, specializing in real-time severe weather coverage, geophysical event reporting, and research-driven scientific analysis. You can reach me at rishav(at)watchers(.)news.
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