Sky & Telescope: “Another Dwarf Planet in Our Solar System?”

The newfound object, which for now bears the unwieldy name of 2017 OF₂₀₁, is approximately 700 kilometers (400 miles) wide and follows an extremely elliptical orbit around the Sun that takes an estimated 25,000 years to complete. Its size puts it in the category of dwarf planets, along with Pluto, the asteroid Ceres, and other objects. It’s one of only a half-dozen or so (depending on exactly which definitions are used) dwarf planets now known in the outer solar system.

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The orbits of some other outer solar system objects, specifically those whose orbits are larger than that of Neptune (known as trans-Neptunian objects, or TNOs) appear to cluster, leading some astronomers to suggest that they’re feeling the gravitational influence of an unknown far-out major planet, dubbed Planet X or Planet 9. But this new dwarf planet discovery doesn’t seem to fit that pattern. Cheng and his team say this puts the Planet X hypothesis into question, since if that planet existed, 2017 OF₂₀₁’s orbit would be unstable.

David L. Chandler

The ‘Planet 9’ hypothesis is almost 10-years old by now, and has inspired many additional studies, some going as far as attributing this orbital clustering to a small-sized black hole in the outer solar system. Alas, the most likely explanation was always that the effect is simply an artefact of observational bias, and the orbit of this newly found dwarf planet strongly contradicts the clustering hypothesis that gave birth to the hunt for Planet 9. Moreover, as the paper notes, the high elongation of 2017 OF201’s orbit makes this object extremely hard to detect with current surveys, suggesting that a substantial population of similar objects – with large sizes, wide orbits, and high eccentricities – could exist but have not been detected due to their extremely large distances. Finding more of them with a wider variety of perihelion angles would dispel the idea of a hidden, massive Planet 9 for good.

Figure 2. Left: Plan view of the orbits of TNOs with extremely wide orbits, including our newly discovered 2017 OF201, which has a distinct orbit is an outlier to the apsidal clustering of the others. For reference, the most probable orbit of Planet X from Siraj et al. (2025) is shown in black. Right: Distribution of longitude of perihelion ϖ of TNOs from Siraj et al. (2025). The purple shaded region indicates the clustering of ϖ around 60◦. Objects with (meta)stable orbits are shown as solid (open) symbols. The newly discovered 2017 OF201, highlighted in red, does not belong to the apsidal clustering.