2017 OF201: A potential Dwaf Planet with an extremely wide orbit.

The majority of the known sub-planetary bodies of the Solar System are found within the Main Asteroid Belt, but the total mass of the bodies here is thought to add up to only about 0.04% of the mass of the Earth. The Kuiper Belt, located between 30 and 50 AU from the Sun (i.e. between 30 and 50 times as far from the Sun as the Earth) is thought to contain a total mass of about 2% that of the Earth, including large icy bodies such as the Dwarf Planet Pluto. Beyond this, at a distance of between about 2000 and 200 000 AU from the Sun, the Oort Cloud may contain several Earth masses of material, largely in the form of icy comets, but also including dwarf planets, and possibly unidentified planets.

Other than the Dwarf Planet Pluto, which was discovered in 1930, all of the 5000 plus known trans-Neptunian objects (Solar System bodies outside the orbit of the Planet Neptune) have been discovered in the past three decades. Most of these have been discovered by surveys concentrating on the elliptic plane of the Solar System, with higher latitudes very poorly mapped. The limitations of the instruments used also mean that few objects beyond 60 AU from the Sun have been discovered.

Cosmological surveys (i.e. surveys of the deeper cosmos, outside the Solar System) have also detected trans-Neptunian objects, most notably the Dark Energy Survey, which has already discovered about 800 such bodies.

In a paper published on the arXiv database at Cornell University on 22 May 2025, Sihao Cheng of the Institute for Advanced Study and the Perimeter Institute, and Jiaxuan Li and Eritas Yang of the Department of Astrophysical Sciences  at Princeton University, detail the discovery of  a large and exotic trans-Neptunian object from data collected by the Dark Energy Camera Legacy Survey.

Cheng et al. searched data collected by thee Dark Energy Camera Legacy Survy, which utilises the Dark Energy Camera on the 4-meter Blanco telescope at Cerro Tololoin Chile, discovering the same object had been detected at three wavelength bands on ten occasions between 2014 and 2018, and that it was possible to connect these sightings and calculate a tentative orbit for the object. This object, identified as 2017 OF201 (a name which implies it was the 5031st object discovered in the second half of July 2017) had an extremely wide and excentric orbit, was about 85 AU from the Sun was detected, and had an apparent magnitude of about 22.6, making it the second brightest object yet discovered with an orbital distance greater than 80 AU.

Armed with this data, Cheng et al. searched the data archives of the 3.6 m Canada-France-Hawaii Telescope, the Subaru Telescope, and Gemini-North Telescope, recovering images of 2017 OF201 at the predicted positions in  nine 3.6 m Canada-France-Hawaii Telescope images from 2011 and 2012, but not detecting it in data from the Subaru or Gemini-North telescopes.

Trajectory of 2017 OF201 on the sky from 2011 to 2018. Individual detections from 13 nights are shown on top of the predicted trajectory based on the best-fit orbit, which describes the detections very well with a scatter of 0.13 for the Dark Energy Camera (DECam) and 0.03 for the Canada-France-Hawaii Telescope (CFHT) arcsec in each component, consistent with the estimated astrometric error. The insets show example images from DECam (r-band on 2017-09-17) and CFHT (r-band on 2011-08-31). Cheng et al. (2025).

2017 OF201 is calculated to have an orbital period of 24 256 years, with a perihelion distance (closest approach to the Sun) of 44.9 AU, an aphelion distance (furthest distance from the Sun) of 1632 AU, and a semi-major axis (average distance from the Sun) of 838.3 AU. The last perihelion of 2017 OF201 was in 1930, the year in which Pluto was discovered, however, even at perihelion 2017 OF201 would have been about four orders of mangnitude fainter than Pluto (i.e. roughly a ten thousandth as bright), quite beyond detection by the telescopes of the day. The orbit of 2017 OF201 is tilted at 16.2° to the plane of the Solar System.

The orbits & current positions of Neptune, Pluto, and 2017 OF201. Jiaxuan Li & Sihao Cheng/Institute for Advanced Study.

2017 OF201 has a longitude of perihelion of 306° (i.e. it reaches perihelion at an angle of 306° relative to the First Point of Aries, taken as a celestial reference point). This is noteworthy, as many previously discovered trans-Neptunian objects have longitudes of perihelion clustered around 60°, something which has been postulated to imply the presence of a ninth planet (termed 'Planet X') in the Outer Solar System, the gravity of which is pushing the orbits of trans-Neptunian objects towards a similar trajectory. The orbit of 2017 OF201 not only shows no signs of such influence, it appears to be incompatible with such an object existing at all. This suggests that the similarity seen in the orbits of trans-Neptunian objects discovered to date is due to sampling bias - we have discovered more objects with longitudes of perihelion close to 60° because we have been looking at that part of the sky.

Plan view of the orbits of trens-Neptunian objects (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 is shown in black. Cheng et al. (2025).

Analysis of light from 2017 OF201 suggests that it has a reddish hue, within the colour range of other trans-Neptunian objects, but possibly one of the redder objects. 2017 OF201 is calculated to be about 700 km in diameter, at which size it is presumed that it would be roughly spherical in shape. It is estimated to have a density of about 1.7 grams per cm squared, which would give it a total mass of about 300 000 000 000 000 megatons, or roughly one twenty thousandth the mass of the Earth.

2017 OF201 forms part of the Scattered Disk, an area between the Kuiper Belt considered to contain far less mass than either. However, if 2017 OF201, and other Scattered Disk objects, such as 90377 Sedna, represent an examples of a population of similar objects (which is a more likely explanation than all such objects currently being on the inner part of their orbits where we can detect them), then it is likely that the total mass contained in the Scattered Disk may be as high as 10% of that of the Earth, compared to 1-2% for the Kuiper Belt.

The orbits and positions of 2017 OF201 and 90377 Sedna. Wikimedia Commons.

2017 OF201 is unlikely to have formed on its current, highly eccentric, orbit. Rather, Cheng et al. estimate that it formed closer in to the Sun, on a more circular orbit, and has been moved onto its current orbit by encounters with other bodies. This orbit isnot consistent with the 'Planet X' hypothesis which has been used to explain the highly eccentric orbits of other trans-Neptunian objects.  Instead, Cheng et al. suggest that 2017 OF201 was initially knocked onto a less eccentric orbit by one or more encounters with the planet Neptune, and that that orbit has subsequently been further modified by the action of the Galactic Tides, and possibly close encounters with other steller systems.

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