2017 OF201

is an extreme trans-Neptunian object and dwarf planet candidate, estimated to be at least in diameter. It was announced in 2025 by Sihao Cheng, Jiaxuan Li, and Eritas Yang, who discovered the object in archived telescope images from 2011 to 2018. With an absolute magnitude of between 3 and 4, may be the brightest known object in the Solar System that does not have a directly measured size. The orbit of is extremely large and elongated, bringing it from away from the Sun.

Discovery

was discovered by a team at the Institute for Advanced Study led by Sihao Cheng and two Princeton University students, Jiaxuan Li, and Eritas Yang, in a search for trans-Neptunian objects (TNOs) in archived Dark Energy Camera Legacy Survey (DECaLS) images in the hope of finding the hypothetical Planet Nine. Cheng claimed that he became inspired to survey the deep solar system after attending a 2005 lecture by Dr. Michael E. Brown, the discoverer of Eris and Sedna and co-author of the Planet Nine hypothesis, at California Institute of Technology. Cheng's team were able to find ten DECaLS detections of from 2014 to 2018, which allowed them to determine that this TNO had an unusually distant and eccentric orbit. Following the advice of Mike Alexandersen of the Minor Planet Center, Cheng's team found nine additional detections of in archived Canada–France–Hawaii Telescope (CFHT) images from 2011 and 2012, which further improved calculations of 's orbit. Cheng's team attempted to search for in more archived images from the Subaru and Gemini North telescopes, but were unable to find any detections.

Cheng's team reported their DECaLS and CFHT detections to the Minor Planet Center, which gave the TNO its provisional designation and announced it to the public on 21 May 2025. Following the announcement, the institutions of Cheng's team published a press release and a paper detailing the discovery of . Additional precovery observations of were later found in Sloan Digital Sky Survey images from 2004 and 2009.

Orbit

orbits far beyond Neptune at an average distance (semi-major axis) of 840 astronomical units (AU), taking around 24,000 years to complete an orbit around the Sun. has an extremely elongated orbit with an eccentricity of 0.95, which together with its large orbital distance makes it an extreme trans-Neptunian object. has an orbital inclination of 16.2° with respect to the ecliptic. It last passed perihelion in mid-November 1930. , its current distance from the Sun is 90.5 AU, making it one of the most distant Solar System objects observed. Because of 's extremely distant and eccentric orbit, it is only visible to Earth during perihelion, which spans less than 1% of its orbit. Co-discoverer Eritas Yang stated that the discovery of means that there are likely thousands of similar objects that are currently too distant and faint from Earth to be observed.

The orbit of has a perihelion distance of and aphelion distance of , which places this object near the estimated boundary of the scattered disc region and the inner Oort cloud. As a result, the orbit of is shaped by both Neptune and the galactic tide over billions of years. Specifically, is thought to have been gravitationally scattered into a high semi-major axis and low perihelion orbit by Neptune first, and then galactic tides and stellar encounters raised its perihelion. Yang has not stated which planetary body the team thinks scattered , and argued that it might have been scattered multiple times. Specifically, Yang stated the team believes that had been scattered into the Oort cloud by some "large planet" and something from the Oort cloud ejected it again into its current orbit. In terms of semi-major axis and eccentricity, the orbit of is similar to that of extreme TNO .

Implications for the Planet Nine hypothesis

The orientation or longitude of perihelion of 's orbit does not align with other extreme TNOs like Sedna, whose orbits are thought to be clustered because of the gravitational influence of a distant massive planet, dubbed Planet Nine. Simulations run by Cheng's team suggest that Planet Nine would have ejected from its current orbit within 100 million years, though 's current orbit could be a temporary state. As such, Cheng has stated that "the existence of 2017 OF201 as an outlier to NOsclustering could potentially challenge he Planet Ninehypothesis." Konstantin Batygin, the co-author of the Planet Nine hypothesis, argued that the discovery of means nothing in relation to the hypothesis because the object's orbit is significantly influenced by Neptune. Cheng, however, notes that "is right at the boundary between being stable and unstable." Nevertheless, Cheng's team agree that their simulations do not disprove Planet Nine. In an interview with ''The New York Times'', Cheng "still thought Planet Nine was possible" while Yang "was neutral on Planet Nine’s existence." Li initially thought "OK, this kills Planet Nine" upon seeing 's orbit, but later conceded that the results were not definitive, jokingly stating that "it's 49 percent killed." Yang also reiterated that the discovery of means it is likely there are many more similar objects in similar orbits that just have not been detected yet. In response to the discovery, astronomer Samantha Lawler from the University of Regina stated that "the original argument for Planet Nine is getting weaker and weaker".

Physical characteristics

The diameter of is estimated to be somewhere between in diameter (average if assuming a geometric albedo of 0.15), placing it within the range of possible dwarf planets. For reference, Pluto is in diameter and Ceres is in diameter.

Observations of in different light filters show that it has a red color similar to Sedna. is slightly redder than the average color of TNOs on scattered disc and detached orbits. The brightness of does not show variability over 0.1 magnitudes, indicating its shape is likely close to spherical.

See also

* – sednoid whose orbit is not aligned with other extreme TNOs

Notes

References

External links

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Inner Oort cloud
Minor planet object articles (unnumbered)
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Astronomical objects discovered in 2017