A new celestial object has been uncovered on the edge of our solar system, revealing another piece of the vast puzzle that is our cosmic neighborhood. Discovered using the Subaru Telescope at Mauna Kea Observatory in Hawaii, this mysterious world, which has been given the nickname “Ammonite,” is estimated to be between 220 and 380 kilometers in diameter. It adds to a group of peculiar small bodies called sednoids, which follow unusual orbits around the solar system’s outermost regions.

Ammonite: A Fossil from the Past

Ammonite is being described as a “fossil” of the solar system, preserving clues from its infancy. Scientists believe that this object holds “memories of the solar system’s infancy.” The object was first detected by the Subaru Telescope in 2023, with follow-up observations confirming its orbit and its significant distance from the Sun.

This discovery is particularly notable as it marks the fourth member of the sednoids, a class of objects with highly eccentric orbits that circle the Sun from the outer edges of the solar system.

The presence of Ammonite sheds light on a little-known part of the solar system, one that remains largely unexplored. Dr. Fumi Yoshida, leader of the Formation of the Outer Solar System: An Icy Legacy (FOSSIL) project, emphasized the importance of such findings, stating, “Most of the vast Solar System remains unexplored. Wide-field observations with the Subaru Telescope are steadily pushing back the frontier.”

The Sednoids and Their Peculiar Orbits

The discovery of Ammonite adds to the growing understanding of the sednoids, a group of objects first identified in 2003. That year, NASA researchers discovered Sedna, the most distant object in the solar system at the time, located some eight billion miles from Earth. These objects are distinct due to their odd, elongated orbits, which do not align with the typical paths of other solar system bodies. As such, they have been subjects of intense study.

The orbit of Ammonite is particularly intriguing because it differs from the other sednoids. This irregularity challenges existing theories about the presence of a “Planet Nine,” a hypothetical planet believed to be influencing the orbits of distant bodies in the outer solar system. Dr. Yukun Huang, from the Center for Computational Astrophysics at NAOJ, conducted simulations of Ammonite’s orbit and pointed out, “The fact that Ammonite’s current orbit does not align with those of the other three sednoids lowers the likelihood of the Planet Nine hypothesis.”

Meet ‘Ammonite’ — A New World Just Found In The Solar System

Was found by astronomers in Japan using its Subaru Telescope in Hawaii. Announced in a paper published today in Nature Astronomy, the object is not a planet but a sednoid. It’s only the fourth sednoid ever discovered. pic.twitter.com/mJj7bYx7Os

— Henrique Pereira (@Hickgeography) July 15, 2025

The Implications for the Planet Nine Hypothesis

For years, astronomers have speculated that a ninth planet, located far beyond Neptune, could be responsible for the strange orbits of distant icy objects. This idea, known as the Planet Nine hypothesis, has been a subject of debate among scientists. However, the discovery of Ammonite provides a new twist to this theory. The unique orbit of Ammonite suggests that the outer solar system might be far more complex than initially thought.

Dr. Yoshida also highlighted the unusual characteristics of the region where Ammonite was found, noting, “Ammonite 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 Ammonite formed.”