The astronomical object2017 OF201 is an extreme trans-Neptunian object and dwarf planet candidate, estimated to be around 500 to 900 kilometres (300 to 600 miles) in diameter. It was announced in 2025 by Sihao Cheng, Jiaxuan Li, and Eritas Yang, who discovered the object in archived telescope images from 2011–2018.[1] With an absolute magnitude between3~4, 2017 OF201 is one of the brightest known objects in the Solar System that does not have a directly measured size. The orbit of 2017 OF201 is extremely large and elongated, bringing it from 45–1,610 astronomical units (0.00071–0.02546ly) away from the Sun. 2017 OF201 is not considered a sednoid because its perihelion or closest point to the Sun is below 60AU, which means that the object comes close enough to Neptune that the planet's gravitational influence affects the orbit of 2017 OF201.[7]
2017 OF201 has not yet been imaged by high-resolution telescopes, so it has no known moons. The Hubble Space Telescope is planned to image it in 2026, which should determine if it has significantly sized moons,[8] whose orbital motion would allow reliable determination of its mass, and possibly its size as well.
Discovery
The path of 2017 OF201 in the sky from 2012–2018. Black circles mark the time and location of 2017 OF201 when it was imaged by DECaLS and CFHT (inset images shown).
2017 OF201 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.[9][10][11][12][1] Cheng stated that he became inspired to survey the deep Solar system after attending a 2005 lecture by Michael E. Brown, the discoverer of Eris and Sedna and co-author of the Planet Nine hypothesis.[10] Cheng's team were able to find ten DECaLS detections of 2017 OF201 from 2014–2018, 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 2017 OF201 in archived Canada–France–Hawaii Telescope (CFHT) images from 2011–2012, which further improved calculations of 2017 OF201's orbit. Cheng's team attempted to search for 2017 OF201 in more archived images from the Subaru and Gemini North telescopes, but were unable to find any detections.[1]
Cheng's team reported their DECaLS and CFHT detections to the Minor Planet Center, which gave the TNO its provisional designation2017 OF201 and announced it to the public on 21May 2025.[13] Following the announcement, the institutions of Cheng's team published a press release and a paper detailing the discovery of 2017 OF201.[11][12] Additional precovery observations of 2017 OF201 were later found in Sloan Digital Sky Survey images from 2004 and 2009.[2]
Orbit
Orbit diagram of 2017 OF201 (red) compared to other extreme TNOs. The right plot shows the orientation (longitude of perihelion) and average distance (semi-major axis) of extreme TNO orbits, with 2017 OF201 plotted in red.
2017 OF201 orbits far beyond Neptune at an average distance (semi-major axis) of 840astronomical units (AU), taking around 24,000years to complete an orbit around the Sun. 2017 OF201 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 (TNO).[1]2017 OF201 has an orbital inclination of 16.2° with respect to the ecliptic. Fortuitously for its discovery, it recently passed through its closest approach in mid-November1930.[4]As of 2025[update], its current distance from the Sun is 90.5 AU, making it one of the most distant observed Solar System objects.[1] Because of 2017 OF201'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 2017 OF201 means that there are likely thousands of similar objects that are currently too distant and faint from Earth to be observed.[14]
The orbit of 2017 OF201 has a perihelion distance of 44.9AU and aphelion distance of 1,610AU (0.0255 light-years), which places this object near the estimated boundary of the scattered disc region and the inner Oort cloud.[1] As a result, over billions of years, the orbit of 2017 OF201 is shaped by both Neptune and the galactic tide.[1][14] Specifically, 2017 OF201 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.[1] Yang has not stated which planetary body the team thinks scattered 2017 OF201, and argued that it might have been scattered multiple times.[15] Specifically, Yang stated the team believes that 2017 OF201 had been scattered into the Oort cloud by some "large planet" and something from the Oort cloud ejected it again into its current orbit.[16] In terms of semi-major axis and eccentricity, the orbit of 2017 OF201 is similar to that of extreme TNO 2013 SY99.[1]
Implications for the Planet Nine hypothesis
The orientation or longitude of perihelion of 2017 OF201'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.[15] Simulations run by Cheng's team suggest that Planet Nine would have ejected 2017 OF201 from its current orbit within 100million years, though 2017 OF201's current orbit could be a temporary state.[10][1][12] Cheng stated
"the existence of 2017 OF201 as an outlier to [TNOs] clustering could potentially challenge [the Planet Nine] hypothesis."[17]
Konstantin Batygin, the co-author of the Planet Nine hypothesis, argued that the discovery of 2017 OF201 means nothing in relation to the hypothesis, because the object's orbit is significantly influenced by Neptune.[14] Cheng, however, notes that 2017 OF201 "is right at the boundary between being stable and unstable".[14] Nevertheless, Cheng's team agree that their simulations do not disprove Planet Nine.[10] 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 "Okay, this kills Planet Nine" upon seeing 2017 OF201's orbit, but later conceded that the results were not definitive, jokingly stating that "it's 49percent killed".[10] Yang also reiterated that the discovery of 2017 OF201 means it is likely there are many more similar objects in similar orbits that just have not been detected yet.[15] 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".[9]
Physical characteristics
The diameter of 2017 OF201 has not been measured with observational techniques, but it can be estimated from its brightness by assuming it has the same geometric albedo as other, similar, large scattered disc objects. Cheng, Li, and Yang estimate that 2017 OF201 is most likely 700km in diameter, with a predicted albedo of 0.15.[1] This places 2017 OF201 within the diameter range of possible dwarf planets.[12][14] For reference, Pluto is 2,377km (1,477mi) in diameter[12] and Ceres is 939km (583mi) in diameter.[14]
Observations of 2017 OF201 in different light filters show that it has a red color similar to Sedna. 2017 OF201 is slightly redder than the average color of TNOs on scattered disc and detached orbits. The brightness of 2017 OF201 does not show variability over 0.1magnitudes, suggesting that its shape is likely close to spherical.[1]
See also
2023 KQ14 – sednoid whose orbit is not aligned with other extreme TNOs
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