The strongest candidate yet for an undiscovered outer solar system world has an orbit that contradicts the Planet Nine hypothesis, raising the possibility that the theory's architects were looking for the wrong planet all along.
The strongest candidate yet for an undiscovered planet in the outer solar system has emerged from decades-old infrared survey data, but the object’s orbit is so misaligned with the Planet Nine hypothesis that one of the theory’s architects says it would, if confirmed, effectively disprove the original prediction.
That is the current state of one of modern astronomy’s most closely watched open questions, following analysis that has received renewed attention this week as teams continue searching for a hypothetical large world far beyond Neptune.
The Planet Nine hypothesis was proposed in 2016 by Caltech astronomers Konstantin Batygin and Mike Brown. Analysing the orbital paths of extreme trans-Neptunian objects (eTNOs) - small bodies in the far outer solar system well beyond Neptune’s influence - they found that a subset of these orbits appeared to cluster in a way that ordinary gravitational effects could not explain. Their most parsimonious interpretation was a planet five to ten times the mass of Earth, orbiting the Sun at roughly 400 to 800 astronomical units, once every ten thousand years or more, according to NASA’s summary of the hypothesis.
No such planet has been directly observed. The search continues.
A team led by Terry Long Phan of National Tsing Hua University in Taiwan set out to find it in existing data. The researchers cross-referenced two far-infrared all-sky surveys conducted 23 years apart: NASA’s Infrared Astronomical Satellite (IRAS), which mapped the sky in 1983, and Japan’s AKARI satellite, active from 2006 to 2007. A genuinely distant, slowly orbiting planet should drift detectably between surveys that far apart while remaining too faint for most visible-light searches.
One candidate survived scrutiny. An infrared point source visible in 1983 IRAS data appeared to have shifted 47.4 arcminutes by the time of AKARI observations - a displacement consistent with the expected motion of a distant planet over 23 years, according to the team’s paper published in the Publications of the Astronomical Society of Australia. The candidate is estimated at roughly the size of Neptune. Its implied orbital period falls between 10,000 and 20,000 years.
On its face, this is exactly the kind of signal the Planet Nine search was designed to find.
Mike Brown reviewed the candidate’s orbit and reached an uncomfortable conclusion. The object, if real, appears to occupy an orbit tilted roughly 110 degrees relative to the solar system’s plane. Planet Nine, as predicted by Batygin and Brown, is expected at a far lower inclination. A planet at 110 degrees of orbital tilt would be dynamically incompatible with the predicted Planet Nine location - the two could not coexist without destabilising each other’s orbits.
“This mismatch doesn’t mean it’s not there, but it means it’s not Planet Nine,” Brown told Science magazine. He added: “It’s kind of fun that a paper that purports to find a candidate for Planet Nine is really finding something that would basically say that we were wrong the entire time.” He suggested calling such a potential find “Planet 8.5.”
The candidate therefore presents a paradox. If confirmed, the detection would simultaneously represent the best evidence yet of an unknown outer solar system planet and a refutation of the orbital clustering argument that originally motivated the Planet Nine search.
A separate discovery added further uncertainty in 2025. An object designated 2023 KQ14, nicknamed “Ammonite,” was identified by researchers using the 8.2-metre Subaru Telescope in Hawaii as part of the FOSSIL II survey project. The results were published in Nature Astronomy. Sednoids - a rare subclass of extreme trans-Neptunian objects whose closest approach to the Sun is so distant that Neptune’s gravity cannot account for their paths - number only four known examples. 2023 KQ14 is the fourth.
Ammonite’s orbit does not align with those of the other three known sednoids. Numerical simulations indicate the object has maintained this path for approximately 4.5 billion years, roughly the age of the solar system. A misaligned sednoid complicates any model that relies on a single shepherding planet in a fixed predicted position.
Both Batygin and Brown have pointed to the Vera Rubin Observatory - the wide-field survey telescope in Chile that completed commissioning and began scientific operations in 2025 - as the instrument most likely to settle the debate. Its sky coverage, survey cadence, and depth make it far better suited than any predecessor to detecting a faint, slowly moving, distant object. Batygin said recently that he expected new relevant discoveries “basically starting this summer, starting summer twenty twenty-six.”
Whether those discoveries confirm a planet, rule one out, or reveal something that matches neither the original prediction nor the current IRAS-AKARI candidate, the outer solar system has proven more complex than the 2016 hypothesis assumed. The mystery has, by the assessment of its own architects, deepened rather than resolved.
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