Every now and then the outer solar system reminds us that we have only discovered a negligible part of it, and that the vast majority is still full of surprises. The latest example of this is a trans-Neptunian object (TNO), first spotted by the Subaru Telescope in Hawaii in 2023.
The object was formally designated 2023 KQ14 by an international team of astronomers as part of the FOSSIL (Formation of the Outer Solar System: An Icy Legacy) project and became known as ‘Ammonite’. But it is neither a planet nor a dwarf planet. Instead, it belongs to a rare category of trans-Neptunian objects called sednoids.
At its closest point in its orbit (perihelion), Ammonite is about 66 astronomical units (AU) from the Sun (more than twice Neptune’s perihelion). One AU is the approximate distance from the Sun to Earth, about 93 million miles. On average, the cosmic body remains about 252 AU away from the Sun (about 23.4 billion miles). Based on the sunlight it reflects, the sednoid’s diameter is estimated to be between 210 and 236 kilometers (less than one-tenth the diameter of Pluto).
Due to its extreme distance, Ammonite takes about 4,000 Earth years to complete one revolution around the Sun in its highly elongated and tilted orbit. It is only the fourth sednoid ever discovered in the far reaches of our solar system, after Sedna (found in 2003), 2012 VP113 and 541132 Leleākūhonua.
The discovery of another sednoid reignites the Planet Nine hypothesis
On the other hand, Ammonite does not really follow the observed pattern of other sednoids. Compared to the others, his trajectory points in the other direction. According to a Subaru telescope press releaseYukun Huang, who performed simulations of Ammonite’s orbit, said: “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.”
Huang still believes that a ninth planet “once” existed in our solar system, but was later ejected, resulting in the unusual sednoid orbits. The claim stems from estimates of closer sednoid orbits some 4.2 billion years ago, and how the gravity of a passing star or the ejection of the solar system’s ninth planet could have pushed the small icy bodies into their current, detached orbit.
Studying such objects, even though they are located on the far edge of our solar system, gives scientists the opportunity to learn about the history of our cosmic environment; the fourth sednoid is just a small piece of the giant puzzle.
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