A small icy world found far beyond Pluto has made the case for some proposed versions of Planet Nine look weaker, not stronger.
On 14 July 2025, Nature Astronomy published the discovery of an object provisionally designated 2023 KQ14 and nicknamed “Ammonite” by the research team. It was first identified using the Subaru Telescope on Maunakea as part of a survey called FOSSIL.
Ammonite is only the fourth known “sednoid.” Sednoids are icy bodies orbiting far beyond Neptune. Their orbits are extremely elongated, and even at their closest approaches they remain so distant that their present paths cannot readily be explained by Neptune under the Solar System’s current configuration.
Ammonite’s orbit carries it from about 66 astronomical units from the Sun at perihelion to roughly 438 au at aphelion. One astronomical unit is the average distance between the Earth and the Sun. The object was discovered at a distance of about 71 au, relatively close to its nearest approach. The other three known sednoids are Sedna, 2012 VP113 and Leleakuhonua.
Objects this distant move so slowly across the sky that determining their orbits accurately can take years. The researchers combined recent Subaru observations with precovery detections in archival DECam images from 2014 and 2021. They then carried out follow-up observations with the Canada-France-Hawaii Telescope in July 2024, producing a formal observational arc of 10.16 years.
Ying-Tung Chen, the study’s first author at Academia Sinica, said: “Finding Ammonite is like discovering a missing piece of the puzzle at the Solar System’s frontier.”
What the earlier sednoids seemed to show
To understand why a fourth sednoid matters, it helps to look at what the earlier objects appeared to show. The idea of an unseen planet in the outer Solar System is old, but its modern form emerged in 2016, when Caltech astronomers Konstantin Batygin and Michael Brown proposed a giant planet orbiting far beyond Neptune.
The original model considered a world around ten times the mass of Earth. Later modelling generally favoured a somewhat lighter planet of roughly six Earth masses, although its possible mass and orbit remain uncertain.
No such planet has ever been directly observed. The case for it instead rests on patterns in the orbits of a wider group of distant trans-Neptunian objects. Some of these objects appeared to have clustered orbital orientations rather than being distributed randomly.
One important measurement is the longitude of perihelion: in simple terms, the direction in which an elongated orbit points. The three previously known sednoids had roughly clustered longitudes of perihelion, fitting one part of the broader orbital pattern cited in support of Planet Nine.
A massive hidden planet could, in principle, shepherd or confine such orbits through its gravity over billions of years. However, astronomers continue to debate how statistically significant the apparent clustering is, because surveys do not observe every part of the sky equally.
Where Ammonite breaks from the pattern
Ammonite does not share the same orbital orientation. Its longitude of perihelion lies roughly opposite those of the other three known sednoids. It does not orbit the Sun backwards; rather, the long axis of its orbit points in a different direction.
The researchers describe Ammonite as the first known anti-clustered Sedna-like object. The feature that made the previous three sednoids look potentially shepherded is precisely the feature Ammonite does not share.
The team then tested how Ammonite would behave under several previously proposed Planet Nine configurations. Under some of the closer orbital models, most simulated versions of Ammonite became unstable and were eventually ejected from the Solar System.
That makes Ammonite’s present stable orbit difficult to reconcile with those particular versions of the hypothesis. Yukun Huang of the National Astronomical Observatory of Japan, who led the orbital simulations, said: “The fact that 2023 KQ14’s current orbit does not align with those of the other three sednoids lowers the likelihood of the Planet Nine hypothesis.”
What the misalignment rules out, and what it doesn’t
This is not a verdict against Planet Nine. Ammonite challenges some proposed versions of the planet’s orbit, but it does not eliminate every possible configuration.
The researchers found that Ammonite remained considerably more stable when the hypothetical planet was placed farther from the Sun. If Planet Nine exists, Ammonite therefore appears to favour a more distant orbit than some earlier models predicted.
That is more cautious than saying Planet Nine must lie farther out. The simulations tested selected versions of the hypothesis, not every possible combination of planetary mass and orbit.
The study also examined Ammonite’s history over billions of years. In simulations without a disruptive nearby Planet Nine, its orbit remained broadly stable for the age of the Solar System.
More intriguingly, backward integrations suggested that the longitudes of perihelion of all four known sednoids may have been more closely clustered around 4.2 billion years ago. That result is tentative rather than conclusive, but it raises the possibility that the objects once shared a stronger orbital alignment before gradually drifting apart.
If that primordial clustering is real, it may point to a disturbance early in the Solar System’s history rather than continuous shepherding by a planet that remains nearby today.
Fumi Yoshida, who leads the FOSSIL survey, said that the existence of objects such as Ammonite “implies that something extraordinary occurred during the ancient era when 2023 KQ14 formed.”
A temporary planet, a passing star, or something else
If the simplest version of Planet Nine is no longer the neat answer, several other possibilities remain. None has been confirmed.
One possibility is a temporary rogue planet: a planet-sized body that once travelled through the outer Solar System before being ejected. Such an object could have altered the orbits of distant icy bodies and then disappeared, leaving only the gravitational imprint of its former presence.
Huang raised that possibility carefully, saying: “It is possible that a planet once existed in the solar system but was later ejected, causing the unusual orbits we see today.”
Encounters with passing stars are another proposed way of producing detached objects in the distant Solar System. The young Sun was probably born among many other stars, and a sufficiently close stellar passage could have pulled small bodies into stretched, remote orbits.
However, the particular ancient clustering reconstructed in this study may be more consistent with a temporary planetary perturber than with the stellar-flyby models examined by the researchers. A passing star remains a possible part of the broader story, but it does not directly explain every feature reported in the paper.
Only four sednoids are currently known. Three have longitudes of perihelion clustered in one broad direction, while Ammonite points roughly the other way. That is far too small a sample from which to reconstruct the Solar System’s full history with confidence.
Finding more objects will be crucial. The next generation of wide-field surveys should reveal whether Ammonite is an outlier, part of a second orbital group or evidence that the apparent clustering was never as strong as it first seemed.
For now, Planet Nine remains where it has always been: unseen, neither confirmed nor ruled out, and increasingly constrained by each new frozen world found at the edge of the Solar System.