Overview

• Ammonite is the fourth known Sednoid with a perihelion beyond 60 AU and one of the most elongated orbits among Trans-Neptunian Objects.
• The FOSSIL Survey identified it in 2023 using Subaru’s Hyper Suprime-Cam, and follow-up observations in 2024 with the Canada–France–Hawaii Telescope plus archival data confirmed a 19-year orbital path.
• Its orbit features a 66 AU perihelion and a semi-major axis over 200 AU, filling a persistent q-gap in the distribution of distant Solar System bodies.
• Numerical simulations show its trajectory has remained stable for roughly 4.5 billion years and implies external gravitational forces shaped its path.
• The object’s misaligned orbit tightens the possible region for a hypothetical Planet Nine and upcoming surveys like the Vera Rubin Observatory’s LSST will refine outer Solar System models.