(82158) 2001 FP185
A scattered-disc object with a highly eccentric orbit and potential Planet Nine co-orbital.
(82158) 2001 FP185 is a scattered-disc object in the outer Solar System, roughly 330 kilometers across, known for its highly elongated orbit. American astronomer Marc Buie discovered it on March 26, 2001, using the Kitt Peak National Observatory in Arizona.
Its orbit takes it between 34.3 and 420.2 AU from the Sun, completing one revolution every 3,425 years and four months. The orbit has an eccentricity of 0.85 and is tilted 31° relative to the ecliptic. A precovery image from the Sloan Digital Sky Survey, taken in March 1999, extends the known observation arc by two years before the official discovery.
If the hypothetical Planet Nine exists, 2001 FP185 might share its orbit as a co-orbital object.
Its color has been studied extensively, placing it between the very blue BB and very red RR spectral classes. Astronomer Michael Brown, using radiometric data, estimates a diameter of 336 kilometers and an assumed albedo of 0.05. Observations from the Herschel Space Observatory’s PACS instrument, part of a survey of scattered-disc objects, give a similar diameter of 332 kilometers with an albedo of 0.046. The Collaborative Asteroid Lightcurve Link, assuming a higher albedo of 0.10, calculates a smaller diameter of 222 kilometers based on an absolute magnitude of 6.38.
No rotational lightcurve has been obtained, so its rotation period, pole orientation, and brightness variation remain unknown.
The Minor Planet Center numbered this object on May 4, 2004. As of 2025, it has not been named.
- Discovery date
- 26 March 2001
- Discoverer
- Marc Buie
- Discovery site
- Kitt Peak National Observatory, Arizona, United States
- Diameter
- approximately 330 kilometers (range: 222–336 km depending on assumed albedo)
- Orbital period
- 3425 years and 4 months (1,251,096 days)
- Eccentricity
- 0.85
- Inclination
- 31°
Lore & Background
2001 FP185 is a scattered-disc object, belonging to the most distant and coldest objects in the Solar System, thought to be a source of most periodic comets. It orbits the Sun at a distance of 34.3–420.2 AU once every 3425 years and 4 months, with an eccentricity of 0.85 and an inclination of 31° relative to the ecliptic. A first precovery was taken by the Sloan Digital Sky Survey in March 1999, extending the observation arc by two years prior to its official discovery.
Reader's Guide
The significance of 2001 FP185 lies in its extreme orbit and its potential role in the search for the hypothetical Planet Nine. If a massive trans-Neptunian object such as Planet Nine exists, 2001 FP185 may be co-orbital with it, making it a key object for testing that hypothesis. Its size is uncertain, with estimates ranging from 222 to 336 kilometers depending on assumed albedo, reflecting the challenges in characterizing distant objects. The object's color has been extensively measured, with BR and IR spectra intermediate between very blue BB and very red RR classes. No rotational lightcurve has been obtained, so its rotation period and pole axis remain unknown. It was numbered by the Minor Planet Center on 4 May 2004 but, as of 2025, has not been named.
Did You Know?
- 2001 FP185 was discovered on 26 March 2001 by American astronomer Marc Buie at Kitt Peak National Observatory.
- Its orbit has an eccentricity of 0.85 and an inclination of 31°, taking it from 34.3 to 420.2 AU from the Sun.
More in Trans-Neptunian Objects, Part 2 1-24
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