Would one frozen world, just visible on the rim of the Sun’s kingdom, turn decades of speculation on the concealed layout of our Solar System on its head? The identification of 2023 KQ14, dubbed Ammonite, by the FOSSIL program on the Subaru Telescope is challenging astronomers to reconsider how the most distant objects ended up in their strange orbits, and what, if anything, may yet be hiding in the darkness beyond Neptune.
The search for Ammonite started with Hyper Suprime-Cam, a wide-field, 870-megapixel camera on the 8.2-meter Subaru Telescope, within the FOSSIL (Formation of the Outer Solar System: An Icy Legacy) survey. The target of the survey: to systematically record faint, slow-moving objects in the outer Solar System, extending the detection limits to a limiting magnitude of mr ≃ 25.2. Ammonite first showed up in Subaru’s photos in March, May, and August 2023, but verification needed a thorough combing of almost two decades’ worth of archival records, including photos from the Dark Energy Camera and Kitt Peak National Observatory. By July 2024, with fresh observations from the Canada-France-Hawaii Telescope, astronomers had mapped Ammonite’s orbit along an impressive 19-year arc an unprecedented achievement considering the faintness and glacial motion of such remote objects.
What distinguishes Ammonite is not only its remoteness, but also its belonging to the elite club of Sednoids Sednoidal Trans-Neptunian Objects (TNOs) with highly elongated orbits, perihelia far beyond Neptune’s orbit, and semi-major axes greater than 200 astronomical units (au). Ammonite’s orbit is constrained by a semi-major axis of 251.9 ± 0.3 au, eccentricity of 0.738, inclination close to 11°, and perihelion of 65.9 au making it the fourth discovered Sednoid and the first one to fall into the so-called ‘q-gap’, a hitherto unexplained gap between 50 and 75 au in which no other object had yet been discovered. As the Nature Astronomy paper authors observe, “The orbit of Ammonite does not align with those of the other Sedna-like objects and fills the previously unexplained ‘q-gap’ in the observed distribution of distant Solar System objects.”
This oddity in orbit carries significant consequences. Astronomers have long argued whether the unusual clustering of Sednoid orbits indicates the gravitational pull of an unseen giant Planet Nine or ancient cosmic history like stellar flybys or rogue planets. Ammonite’s current orbit, which is different from Sedna, 2012 VP113, and Leleākūhonua, refutes the dominant Planet Nine scenario. As National Astronomical Observatory of Japan Dr. Yukun Huang declared, “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. It is possible that a planet once existed in the Solar System but was later ejected, causing the unusual orbits we see today.” Numerical simulations affirm that Ammonite’s orbit has been stable at least since 4.5 billion years ago, implying it is a primordial relic from the beginning of the Solar System’s epoch.
However, Ammonite’s discovery does more than make Planet Nine doubtful it gives us a vital test point for orbital evolution models. The FOSSIL team’s N-body simulations demonstrate that Sednoids with a > 200 au and q > 60 au are highly resistant to Neptune’s gravitational perturbations and need an external force to account for their isolated orbits. Theories vary from encounters with stars during the Sun’s natal cluster to interstellar capture and transient planetary bodies. Backward integrations of Sednoid orbits, and now including Ammonite, indicate a loose clustering event approximately 4.2 billion years ago, indicating a radical reconfiguration of the outer Solar System in its early days.
Ammonite’s discovery as a technical achievement is as extraordinary as its scientific consequence. The Hyper Suprime-Cam’s capability to image 25 square degrees to record-breaking depths, along with an advanced cadence of repeated exposures and cross-matching to archival surveys, allowed the FOSSIL team to detect and verify this hidden world. These techniques are the precursors of what is to come: soon, the Vera C. Rubin Observatory’s Legacy Survey of Space and Time will sweep the sky with a 3.2-gigapixel camera, to promise to find tens of thousands of new TNOs and map out the Solar System’s population in unprecedented detail.
As Dr. Fumi Yoshida noted, “2023 KQ14 was found in a region far away where Neptune’s gravity has little influence. 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.” The existence of bodies with extended orbits and big perihelion distances in this region suggests that an unusual event took place during the ancient times when 2023 KQ14 began to exist. With each succeeding find, astronomers come closer to piecing together the Solar System’s turbulent start and possibly, long last, to solving if a secret planet continues to influence the farthest corners of our cosmic community.
