Ammonite is a celestial body located beyond Neptune, classified as a Trans-Neptunian Object (TNO). What makes it special is its highly elliptical orbit.
Because of this feature, it has been grouped with only three other similar objects like Sedna, known as Sednoids. Sednoids are rare objects whose closest approach to the Sun (perihelion) is well beyond 60 astronomical units (au).
Astronomers propose the existence of a large, yet undiscovered planet called Planet Nine to explain the orbits of these objects. However, Ammonite's orbit differs from the other known Sednoids, complicating the Planet Nine hypothesis even further, according to researchers.
STABLE ORBIT FOR 4.5 BILLION YEARS
According to computer simulations, Ammonite's orbit has remained stable for approximately 4.5 billion years, since the formation of the Solar System. The object was first detected in 2023 using the Subaru Telescope, but its exact identification was confirmed in 2024 through data from the Canada-France-Hawaii Telescope and archival records.
The discovery was made as part of the FOSSIL (Formation of the Outer Solar System: An Icy Legacy) observation program, which aims to study icy bodies in the distant regions of the Solar System.
THE FIRST OBJECT FILLING THE "Q-GAP"
Ammonite occupies an orbital region that had not been previously observed. This region, called the "q-gap" by astronomers, is a range of orbits that was mysteriously empty. According to the authors of the study, Ammonite fills this gap, completing an important missing link.
RAISING NEW QUESTIONS
Co-author Dr. Yukun Huang stated that Ammonite's orbit not matching the other Sednoids might challenge the existence of Planet Nine. Huang suggested that the formation of these objects could have been influenced by a planet that once existed in the Solar System but was later ejected.
Additionally, some theories propose that such distant objects may have formed through interactions with other stars in the early cluster where the Sun was born, or could even be captured bodies from other star systems.
COULD PLANET NINE STILL BE FOUND?
Ammonite's discovery narrows down the possible orbit and hiding places of Planet Nine. Although astronomers have not yet observed this hypothetical planet, hopes remain high for new-generation telescopes.
The Vera Rubin Observatory will soon begin the LSST (Legacy Survey of Space and Time) project, a decade-long mission designed to observe the darkest and hardest-to-detect objects in the Solar System. If Planet Nine truly exists, Rubin Observatory is considered the strongest candidate to reveal this unknown giant.
