In the icy, dark expanse beyond Neptune, a cosmic fossil has surfaced that threatens to upset decades-old assumptions about the solar system’s secret structure. The new discovery of “Ammonite,” a sednoid object that was detected by Japan’s National Astronomical Observatory with the Subaru Telescope, is not merely another distant body it is a possible game-changer in the discussion regarding the presence of the hypothetical Planet Nine.
Ammonite, formally named 2023 KQ14, is merely the fourth sednoid discovered to date, and it forms a uncommon group of objects having very stretched, eccentric orbits well outside Neptune. Its orbit brings it as close as 66 astronomical units (AU) to the Sun and as far as 252 AU, a trajectory so distant that Neptune’s gravitational pull is inconsequential. The Subaru Telescope’s sophisticated wide-field imaging capacity, complemented by 19 years of archival data, allowed astronomers to accurately reconstruct Ammonite’s orbit. As planetary scientist Fumi Yoshida explains, “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”.
However, Ammonite’s orbit differs from that of the other sednoids. This oddity hits at the very center of the Planet Nine hypothesis, constructed from the fact that a number of extreme trans-Neptunian objects (eTNOs) have a striking orbital clumping, as though corralled by a giant, unseen world. “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,” wrote Yukun Huang of the National Astronomical Observatory of Japan (Forbes). This discovery indicates either that Planet Nine, should it exist, is orbiting even further out than initially believed, or that a cataclysmic event most likely the ejection of a planet reorganized the outer reaches of the solar system.
Ammonite is by no means the only one to add complexity to the story. The recent confirmation of dwarf planet candidate 2017 OF201 adds yet more intrigue. Discovered in 19 images spread across seven years with the Blanco and Canada-France-Hawaii telescopes, 2017 OF201 is approximately 700 kilometers in diameter, the second-largest known object in its dynamical class (arXiv). Its orbit is even more unusual: a semi-major axis of 838 AU, perihelion at 44.9 AU, and an aphelion reaching well over 1,600 AU into the inner Oort Cloud. One solar orbit takes 25,000 years, and it only spends 1% of that time close enough to be observable. “It must have experienced close encounters with a giant planet, causing it to be ejected to a wide orbit,” explained Eritas Yang of Princeton University. Sihao Cheng, lead discoverer, added, “The presence of this single object suggests that there could be another hundred or so other objects with similar orbit and size; they are just too far away to be detectable now.”
Orbits of both Ammonite and 2017 OF201 are exceptional outliers to clustering pattern that initially fueled the hunt for Planet Nine. Numerical simulations, including those by Batygin and Brown, have indicated that the orbital structure of clustered eTNOs closely matches models involving a distant, large planet (arxiv.org). In these models, Planet Nine’s gravitational pull can shepherd TNOs into tight orbits, creating the observed clustering. The addition of objects like 2017 OF201 and Ammonite causes problems for the models, though. Simulations show that the presence of 2017 OF201, with its non-clustered longitude of perihelion, is challenging to accommodate with the gravitational shepherding of one far-off planet (arxiv.org).
Other explanations have been in the vanguard. Certain scientists suggest that the seen orbital variety may be a product of old stellar flybys, whereby a traveling star disrupted the outer solar system, or early chaotic interactions between giant planets that ejected objects to wide orbits. Recent simulations estimate that as many as 40% of solar systems such as our own might accrete wide-orbit planets during their creation in dense star clusters.
The success of the Subaru Telescope in discovering Ammonite demonstrates the authority of contemporary wide-area surveys and deep imaging, a process within sight of being outdone by the new Vera Rubin Observatory. As Fumi Yoshida has said, “Understanding the orbital evolution and physical properties of these unique, distant objects is crucial for comprehending the full history of the Solar System”.
With every new find, the far reaches of the outer solar system become more complex. Whether Ammonite and 2017 OF201 mark the end of the Planet Nine hypothesis or simply initiate a rethinking of its parameters, their presence serves to highlight an immutable fact: the frontier of the solar system is still an arena of great mystery and scientific possibility.
