(505478) 2013 UT15
Extreme scattered-disc object with orbit suggesting Planet Nine.
(505478) 2013 UT15 is a large object from the scattered disc, situated in the Solar System's far outer reaches. It measures roughly 260 kilometers across. Discovered on August 2, 2013, by the Outer Solar System Origins Survey at Hawaii's Mauna Kea Observatory, it follows a highly stretched orbit. Its path takes it from 43.9 AU to 348 AU from the Sun, with an orbital period of about 2,742 years and an eccentricity of 0.78, inclined 11° to the ecliptic. This orbit resembles that of (148209) 2000 CR105, though with a smaller tilt. 2013 UT15 is one of a rare set of detached objects whose perihelion exceeds 30 AU and semi-major axis surpasses 150 AU. Such orbits are difficult to explain without a perturbing body, fueling speculation about Planet Nine. Its estimated diameter ranges from 243 to 340 kilometers, depending on assumed albedo (0.09 or 0.04), with a mean of 260 kilometers based on an albedo of 0.08 and an absolute magnitude of 6.2951. The object has a bluish spectral type (BB). The Minor Planet Center assigned it the number (505478) on November 4, 2017, but as of 2025, it remains unnamed.
- Discovery date
- 2 August 2013
- Discovery site
- Mauna Kea Observatory, Hawaii, United States
- Discovery survey
- Outer Solar System Origins Survey
- Semi-major axis
- 196 AU
- Perihelion
- 43.9 AU
- Aphelion
- 348 AU
- Orbital period
- 2,742 years
- Eccentricity
- 0.78
- Inclination
- 11°
- Diameter estimate
- 260 kilometers (mean, based on albedo 0.08)
Lore & Background
Discovered on 2 August 2013 by astronomers of the Outer Solar System Origins Survey at Mauna Kea Observatory, Hawaii, 2013 UT15 orbits the Sun at a distance ranging from 43.9 to 348 AU, with a semi-major axis of 196 AU and an orbital period of 2,742 years. Its orbit has an eccentricity of 0.78 and an inclination of 11° relative to the ecliptic. The object is estimated to have a bluish spectra (BB). Its diameter has been estimated at 243 and 340 kilometers based on assumed albedos of 0.09 and 0.04, respectively; a generic magnitude-to-diameter conversion using an albedo of 0.08 and an absolute magnitude of 6.2951 gives a mean diameter of 260 kilometers. It was numbered (505478) by the Minor Planet Center on 4 November 2017, and as of 2025 has not received a name.
Reader's Guide
2013 UT15 belongs to a small number of detached objects with perihelion distances of 30 AU or more and semi-major axes of 150 AU or more. Its orbit is similar to that of (148209) 2000 CR105, except for a smaller inclination. The article notes that such objects cannot reach these orbits without some perturbing object, which leads to speculation about the existence of Planet Nine. This makes 2013 UT15 significant in the context of outer Solar System dynamics and the search for a hypothetical ninth planet. Its large size—around 260 kilometers in diameter—and bluish spectral type further characterize it as a notable member of the extreme trans-Neptunian population.
Did You Know?
- Its orbit has a semi-major axis of 196 AU and an eccentricity of 0.78.
- The object's diameter is estimated at 260 kilometers using an albedo of 0.08.
More in Trans-Neptunian Objects, Part 2 1-24
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