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

TL;DR

This study reports the discovery of 2017 OF201, a dwarf-planet–candidate TNO on an exceptionally wide, highly eccentric orbit extending toward the inner Oort cloud. Using an archival DECaLS search plus CFHT, SDSS, and Magellan follow-up, the authors determine a precise orbit with $a ≈ 830$ au, $q ≈ 44.9$ au, and $e ≈ 0.946$, and estimate a diameter near 700 km assuming an albedo of about 0.13. Dynamical simulations indicate the object is metastable over Gyr timescales under the known planets and Galactic tide, with a likely origin requiring external torques after an initial Neptune-scattering stage. The object’s orbital geometry, particularly its longitude of perihelion $ ext{varpi} = 306^ ext{°}$, challenges simple Planet Nine configurations, as simulations including proposed P9 parameters show a high probability of ejection for 2017 OF201. The results imply a widespread population of such extreme TNOs contributing ~1% of Earth’s mass, and motivate future size–albedo measurements (e.g., ALMA) and occultation studies to refine the physical and dynamical picture.

Abstract

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.

Figures (5)

• Figure 1: Trajectory of 2017 OF201 on the sky from 2011 to 2025, which is a combination of parallax (elliptic) and proper motion (straight) components. Individual detections from 16 nights are shown with the predicted trajectory based on the best-fit orbit, with a small residue of 0.13 (DECam) and 0.03 (CFHT) arcsec in each component, consistent with their astrometric error. The insets show example images from Magellan, DECam, and CFHT.
• Figure 2: Left: Plan view of the orbits of TNOs with extremely wide orbits. The newly discovered 2017 OF201, highlighted in red, is an outlier to the apsidal clustering of the others. For reference, the most probable orbit of Planet 9 / Planet X from Siraj2025 is shown in black. Right: Distribution of longitude of perihelion $\varpi$ of TNOs from Siraj2025. The purple shaded region indicates the clustering of $\varpi$ around 60$^{\circ}$. Objects with (meta)stable orbits are shown as solid (open) symbols.
• Figure 3: Left: TNOs with wide orbits ($a>$150 au). 2017 OF201 is the second brightest in this population and has the widest orbit among all dwarf planet candidates. Right: Schematic of a possible migration pathway for 2017 OF201. The object may first have been scattered to a large $a$ while maintaining a low $q$ and then galactic tidal torques slowly increase $q$, detaching the orbit from Neptune.
• Figure 4: Typical clones of dynamical evolution simulation of 2017 OF201, with and without P9. In the absence of P9, 2017 OF201 remains stable over 4 Gyr under the influence of the known giant planets and the Galactic tide (the x-axis is truncated for clarity). By contrast, when P9 is included, 2017 OF201 undergoes close encounters with Neptune and is ejected from the Solar System within $\sim$0.3 Gyr.
• Figure 5: Cutout images of all 22 detections from CFHT, DECam, and Magellan. Each image is 20 arcsec on a side. The detection time corresponds to the middle of each exposure. CFHT data are deeper and have a much better seeing than the DECam data. The detection spans a long time baseline from Aug 2011 to Jul 2025, enabling precise determination of its orbit.