New Sednoid “Ammonite” discovery deepens Planet Nine mystery

Astronomers have discovered a new member of a rare and mysterious class of solar system objects known as sednoids — a subset of extreme trans-Neptunian objects (ETNOs) with exceptionally distant, elongated orbits. Even at their closest approach to the Sun, these icy bodies remain far beyond Neptune’s gravitational influence, traveling on paths that defy easy explanation and remaining detached from the rest of the solar system.

The latest find, named 2023 KQ14 and nicknamed Ammonite, is only the fourth known example, and its strange path adds new complications to the debate over the possible existence of a hidden giant planet lurking in the outer solar system.

The object was first spotted in observations taken in March, May, and August 2023, using the 8.2-meter Subaru Telescope on Maunakea, Hawaii, as part of the second phase of a project called Formation of the Outer Solar System: an Icy Legacy (FOSSIL II). 

Subsequent observations with the Canada-France-Hawaii Telescope, also on Maunakea, in July 2024, along with a search through archival data from other observatories, enabled astronomers to trace the object’s orbit across 19 years. Researchers described the discovery in a paper published July 14 in Nature Astronomy.

Ammonite’s long, elongated orbit

Ammonite follows a distant and elongated orbit, with a perihelion — or closest approach to the Sun — of nearly 66 astronomical units. (1 AU is the average distance between the Sun and Earth.) That’s more than twice as far as Neptune, and far beyond the region where the giant planet’s gravity — or any other planet — dominates. Its average distance from the Sun is more than 200 AU, placing it in a region of space that few known bodies inhabit.

“The presence of objects with elongated orbits and large perihelion distances in this area implies that something extraordinary occurred during the ancient era when 2023 KQ14 formed,” said Fumi Yoshida of the University of Occupational and Environmental Health in Fukuoka, Japan, and the Chiba Institute of Technology in a July 14 press release.

With only four sednoids currently identified, scientists know little about them. And their great distances from the Sun make them dim and hard to spot. “Increasing the sample of Sedna-like objects is of great interest to better understand the history of the solar system,” the authors wrote in their Nature paper.

The Planet Nine puzzle and Ammonite’s role

The discoverers of Ammonite say the object also raises questions for the hypothesized Planet Nine.

The three other known sednoids — Sedna, 2012 VP113, and Leleākūhonua — all follow long, stretched-out orbits that keep them tens to hundreds of astronomical units from the Sun. Even more curiously, they appear to cluster in space, with their orbits oriented in similar directions. That pattern was what prompted Caltech astronomers Mike Brown and Konstantin Batygin to propose the existence of a hidden planet — a massive, distant world whose gravity could corral these icy bodies into formation.

In a 2016 article for The Astronomical Journal, Brown and Batygin suggested that this “Planet Nine” might be shepherding these objects into those clustered orbits. Brown wrote in a 2016 Astronomy article, “The signs in the sky were clear: Something was out there.” That theory has gained support from several discoveries since, but Ammonite adds a wrinkle.

Unlike Sedna, 2012 VP113, and Leleākūhonua, Ammonite’s orbit doesn’t align with the supposed direction of Planet Nine’s influence. Simulations show that Ammonite would likely be ejected by a planet in the predicted orbit, casting doubt on the simplest versions of the Planet Nine model. 

“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,” said Yukun Huang of the National Astronomical Observatory of Japan. “It is possible that a planet once existed in the solar system but was later ejected, causing the unusual orbits we see today,” Huang concluded.

The debate over orbital clustering

Pushback to Brown and Batygin’s Planet Nine theory isn’t new, and not everyone agrees the clustering is real. As David Chandler noted in a January 2025 Astronomy article, some astronomers argue that the apparent alignment might be an artifact of the small number of known objects or the result of telescopic blind spots. 

For example, Samantha Lawler, astronomer at the University of Regina, Saskatchewan, and co-author of a study that discovered additional ETNOs, recently stated that “data for these most extreme objects is completely consistent with a random distribution,” adding, “I really don’t think there is any clustering.” Similarly, the orbit of recently discovered trans-neptunian object 2017 OF201 stands out as an outlier, weakening the apparent clustering that underpins the Planet Nine hypothesis.

But the story is far from over, and more discoveries are on the horizon. At present, the Subaru Telescope is among the few telescopes on Earth capable of making such discoveries. “I would be happy if the FOSSIL team could make many more discoveries like this one and help draw a complete picture of the history of the solar system,” said Yoshida.

The Vera C. Rubin Observatory, which began operations earlier this year, will also play an important role. The observatory will scan nearly the entire southern sky every few nights on average with unprecedented depth. This powerful new survey could uncover dozens or even hundreds of distant objects, helping to clarify the outer solar system’s structure and possibly reveal the elusive Planet Nine.