2017 OF201

2017 OF₂₀₁

2017 OF201 imaged by the Canada–France–Hawaii Telescope on 31 August 2011
Discovery
Discovered by	Sihao Cheng, Jiaxuan Li, Eritas Yang
Discovery site	Cerro Tololo Inter-American Observatory
Discovery date	23 July 2017 (date of first observation)
Designations
MPC designation	2017 OF201
Minor planet category	TNO, SDO and eTNO
Orbital characteristics (barycentric)
Epoch 5 May 2025 (JD 2460800.5)
Uncertainty parameter 3
Observation arc	14.11 yr (5,153 days) (using 25 observations)
Earliest precovery date	21 Sep 2004
Aphelion	1632±14 AU 1700±60 AU (heliocentric)
Perihelion	44.9 AU
Semi-major axis	840 AU
Eccentricity	0.946
Orbital period (sidereal)	24200 years 24300 years 26000±1300 yr (heliocentric)
Mean anomaly	1.3°
Mean motion	0° 38m 7.8s / day
Inclination	16.2°
Longitude of ascending node	329°
Time of perihelion	November 1930 ± 1 month
Argument of perihelion	338°
Physical characteristics
Mean diameter	≈ 550 to 850 km (calc. for a typical TNO albedo of 0.15)
Spectral type	B–V = 0.99±0.11 g–r = 0.77±0.11 r–z = 0.80±0.07
Apparent magnitude	22.8
Absolute magnitude (H)	3.52±0.45

2017 OF₂₀₁ is an extreme trans-Neptunian object and dwarf planet candidate, estimated to be at least 550 kilometres (340 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 to 2018. With an absolute magnitude of between 3 and 4, 2017 OF₂₀₁ may be the brightest known object in the Solar System that does not have a directly estimated size. The orbit of 2017 OF₂₀₁ is extremely large and elongated, bringing it from 45 to 1,630 astronomical units (0.00071 to 0.02577 ly) away from the Sun.

Discovery

The path of 2017 OF₂₀₁ in the sky from 2012 to 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. 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 2017 OF₂₀₁ 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 2017 OF₂₀₁ in archived Canada–France–Hawaii Telescope (CFHT) images from 2011 and 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.

Cheng's team reported their DECaLS and CFHT detections to the Minor Planet Center, which gave the TNO its provisional designation 2017 OF₂₀₁ 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 2017 OF₂₀₁. Additional precovery observations of 2017 OF₂₀₁ were later found in Sloan Digital Sky Survey images from 2004 and 2009.

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 840 astronomical units (AU), taking around 24,000 years 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. 2017 OF₂₀₁ has an orbital inclination of 16.2° with respect to the ecliptic. It last passed perihelion around November 1930. As of 2025, its current distance from the Sun is 90.5 AU, making it one of the most distant Solar System objects observed. 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.

The orbit of 2017 OF₂₀₁ has a perihelion distance of 44.9 AU and aphelion distance of 1,630 AU (0.0258 light-years), which places this object near the estimated boundary of the scattered disc region and the inner Oort cloud. As a result, the orbit of 2017 OF₂₀₁ is shaped by both Neptune and the galactic tide over billions of years. 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. In terms of semi-major axis and eccentricity, the orbit of 2017 OF₂₀₁ is similar to that of extreme TNO 2013 SY99.

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. Simulations run by Cheng's team suggest that Planet Nine would have ejected 2017 OF₂₀₁ from its current orbit within 100 million years, though 2017 OF₂₀₁'s current orbit could be a temporary state. 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. Cheng, however, notes that 2017 OF₂₀₁ "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 2017 OF₂₀₁'s orbit, but later conceded that the results were not definitive, jokingly stating that "it's 49 percent killed."

Physical characteristics

The diameter of 2017 OF₂₀₁ is estimated to be somewhere between 550–850 km (340–530 mi) in diameter (average 700 km if assuming a geometric albedo of 0.15), placing it within the range of possible dwarf planets. For reference, Pluto is 2,377 km (1,477 mi) in diameter and Ceres is 939 km (583 mi) in diameter.

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 0.1 magnitudes, indicating its shape is likely close to spherical.

External links

• 2017 OF201 at the JPL Small-Body Database