Astronomers Discover New 'Cosmic Fossil' in Outer Solar System: 'Ammonite'

In July 2025, an international team of astronomers announced the discovery of a new trans-Neptunian object (TNO), designated 2023 KQ₁₄ and nicknamed "Ammonite." This object was initially detected in 2023 using the Subaru Telescope in Hawaii as part of the Formation of the Outer Solar System: An Icy Legacy (FOSSIL) survey. Follow-up observations were conducted in July 2024 with the Canada-France-Hawaii Telescope, and archival data from other observatories dating back to 2005 were analyzed to track its orbit over 19 years.

Ammonite is classified as a sednoid, a rare class of distant Solar System objects characterized by highly elongated orbits and large perihelion distances. It is only the fourth known sednoid, joining Sedna, 2012 VP113, and Leleākūhonua. Ammonite's orbit ranges from approximately 66 astronomical units (AU) at perihelion to about 252 AU at aphelion, placing it far beyond Neptune's orbit. Its estimated diameter is between 220 and 380 kilometers.

Numerical simulations suggest that Ammonite has maintained a stable orbit for at least 4.5 billion years, making it a valuable "cosmic fossil" that offers insights into the early Solar System. Notably, Ammonite's current orbit differs from those of other sednoids, which may challenge existing models of the Solar System's formation and the hypothesized Planet Nine. Dr. Yukun Huang of the National Astronomical Observatory of Japan commented, "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."

The discovery of Ammonite has significant implications for our understanding of the Solar System's history and the processes that shaped its current structure. It highlights the complexity and diversity of the outer Solar System, suggesting that past events, such as the possible ejection of a planet, may have played a role in shaping the orbits of distant objects. This finding also underscores the importance of continued exploration and observation of trans-Neptunian objects to unravel the mysteries of our cosmic neighborhood.

The FOSSIL project aims to study the remnants of the early Solar System by observing distant icy bodies. The discovery of Ammonite involved collaboration among several institutions, including the National Astronomical Observatory of Japan, the Institute of Astronomy and Astrophysics at Academia Sinica, Kindai University, Chiba Institute of Technology, Kobe University, and the Japan Spaceguard Association.