Discovery of a dwarf planet candidate in an extremely wide orbit: 2017 OF201

We report the discovery of a dwarf planet candidate, 2017 OF201, currently located at a distance of 90 au. Its orbit is extremely wide and extends to the inner Oort cloud, with a semi-major axis of 830 au and a perihelion of 45 au, precisely determined from 24 observations over 20 years. Assuming a typical albedo of 0.13, we estimate a diameter about 700 km, making it the second-largest known object in this dynamical population and a dwarf planet candidate with the widest orbit. Its high eccentricity suggests that an unseen population of similar objects would total about 1% of Earth’s mass. Notably, the longitude of perihelion of 2017 OF201 lies outside the clustering observed in extreme trans-Neptunian objects, posing a challenge to the proposed dynamical evidence for the hypothetical Planet Nine.

💡 Research Summary

The authors report the discovery of a new dwarf‑planet candidate, designated 2017 OF201, currently located at a heliocentric distance of ~90 au. The object was first identified in a systematic search of the Dark Energy Camera Legacy Survey (DECaLS) data, which yielded ten detections between 2014 and 2018 in the g, r, and z bands. By cross‑matching the preliminary orbit with archival images from the Canada‑France‑Hawaii Telescope (CFHT) (2011–2012), the Sloan Digital Sky Survey (SDSS) (2004, 2009), and new follow‑up observations with the 6.5 m Magellan telescope (July 2025), the authors assembled a 20‑year observational arc comprising 24 independent detections. The long baseline enables a highly precise orbital solution using the Find_Orb least‑squares fitting package: semi‑major axis a = 830.1 ± 0.8 au, eccentricity e = 0.94589 ± 0.00005, inclination i = 16.205° ± 0.00006, longitude of perihelion ϖ = 306.32° ± 0.0013, and perihelion distance q = 44.92 au. The orbital period is ≈23,900 yr, and the aphelion reaches ~1,615 au, placing the object at the boundary between the scattered disk and the inner Oort cloud.

Photometrically, 2017 OF201 has an apparent r‑band magnitude of 22.6 mag at 85 au, corresponding to an absolute magnitude H_V = 3.72 ± 0.09 mag. Assuming a typical TNO geometric albedo of 0.13 (supported by the observed correlation between albedo and diameter for scattering‑disk objects), the authors infer a diameter of ~700 km. This size exceeds the empirical threshold (~600 km) above which icy bodies are expected to be in hydrostatic equilibrium, thus qualifying the object as a dwarf‑planet candidate. The measured colors (g − r = 0.77 ± 0.08 mag, r − i = 0.56 ± 0.07 mag, B − V = 0.96 ± 0.09 mag) place it on the red side of the TNO color distribution, consistent with other detached and scattered‑disk objects. No variability larger than 0.1 mag was detected across the DECam and CFHT epochs, suggesting a near‑spherical shape or a pole‑on orientation.

Dynamically, the authors argue that the large semi‑major axis cannot be produced by Neptune scattering alone. Using the diffusion timescale formula from Hadden & Tremaine (2024), T ≈ 0.2 Myr × exp