In July 2025, astronomers revealed an exciting new discovery in the far reaches of our solar system: a Sednoid object, nicknamed Ammonite. This icy body, found far beyond the orbit of Neptune, is stirring fascination in the scientific community because of what it might reveal about the solar system’s origins and the mysterious forces shaping its outer edges.
Sednoids are a rare class of trans-Neptunian objects (TNOs) whose highly elongated orbits suggest that something—or perhaps someone—is tugging on them. The most famous Sednoid, Sedna, was discovered in 2003, and since then only a handful of others have been confirmed. These objects orbit so far away that they never come close enough to Neptune to be gravitationally influenced, leaving scientists puzzled about how they ended up on such strange trajectories.
The discovery of Ammonite adds to this mystery. Early observations suggest it is about 200 kilometers across, smaller than Sedna but still significant enough to be considered a dwarf planet candidate. Its orbit takes it hundreds of astronomical units (AU) from the Sun—so distant that sunlight there is thousands of times dimmer than what we see on Earth. At its farthest, Ammonite will spend centuries in the cold, dark fringes of the solar system before slowly drifting back inward.
What makes this discovery important is its implications for the “Planet Nine” hypothesis. Some astronomers believe an undiscovered giant planet—ten times the mass of Earth—lurks in the outer solar system, shepherding Sednoids like Ammonite into their eccentric orbits. Others argue that instead of one large planet, a series of smaller gravitational nudges from passing stars or collective forces of many small icy bodies might be responsible. The addition of Ammonite to the Sednoid family gives researchers more data to test these competing theories.
The nickname Ammonite was inspired by the fossilized marine mollusks that spiral inward in a coil. Just as ammonite shells contain layers of history, this icy world preserves clues about the earliest days of the solar system. Because Sednoids likely formed closer to the Sun before being scattered outward, their composition could help astronomers understand the chemistry of the solar nebula from billions of years ago.
For now, telescopes around the globe are tracking Ammonite to refine its orbit and gather data about its brightness and color. The upcoming Vera Rubin Observatory in Chile, set to begin operations soon, will be instrumental in identifying more Sednoids and potentially uncovering whether “Planet Nine” truly exists.
In a universe filled with mysteries, the discovery of Ammonite reminds us of how little we know about our own cosmic backyard. Every new object we find at the solar system’s edge adds a piece to the puzzle of where we came from—and perhaps where we are going.
