1998 WW31
First trans-Neptunian binary discovered after Charon in 1978.
1998 WW31 is a binary system and non-resonant Kuiper belt object, sitting beyond Neptune in the Solar System's outer reaches. It was first spotted on 18 November 1998 by American astronomers Marc Buie and Robert Millis at Arizona's Kitt Peak National Observatory. In December 2000, astronomers discovered a moon orbiting it, designated S/2000 (1998 WW31) 1. This moon, about 123 kilometers across, made 1998 WW31 the first trans-Neptunian binary found after Charon (Pluto's large satellite) in 1978, and it remains one of the most balanced binary systems known.
Orbit and classification: As a classical Kuiper belt object (cubewano), it belongs to the "hot" population—objects with higher orbital tilts. It circles the Sun at distances between 40.4 and 48.5 AU, taking roughly 297 years per orbit. Its path has a slight eccentricity of 0.09 and an inclination of 7° relative to the ecliptic.
Numbering and naming: As of 2025, the Minor Planet Center has not assigned a number or name to this object.
Physical characteristics: The object likely has a low albedo and a neutral (blue) color. Some estimates place its albedo at 0.10 or 0.16. Its V–I color index is 0.91, lower than the average for cubewanos and falling between values typical for comets and Jupiter trojans.
Satellite: The moon was discovered on 22 December 2000 by French astronomers Christian Veillet and Alain Doressoundiram, working with J. Shapiro, using the Canada–France–Hawaii Telescope on Mauna Kea, Hawaii. The discovery was announced on 16 April 2001, and the moon got the provisional designation S/2000 (1998 WW31) 1. Its orbit is highly eccentric (0.8193±0.0020), with a long orbital period of 587.27±0.18 days and a semi-major axis of 22,617±42 km. If both bodies share the same albedo, the primary is 1.2 times larger than the secondary; if they also have the same density, the secondary has about 60% of the primary's mass (or just over a third of the total system mass). Assuming equal densities of 1 g/cm³ gives the primary a diameter of 148 km and the satellite 123 km, with albedos around 0.05–0.06.
Diameter and albedo: Under the same density assumption, the primary measures 148 km across, the satellite 123 km, and the combined system diameter is 192 km.
Quick Facts
- Minorplanet
- yes
- Background
- #C2E0FF
- Discoverer
- M. W. Buie / R. L. Millis
- Discovery Site
- Kitt Peak Obs.
- Discovered
- 18 November 1998 / (first observed only)
- Mpc Name
- 1998 WW · 31
- Mp Category
- TNOcubewano (hot) / distantbinary
- Epoch
- 21 November 2025 (JD 2461000.5)
- Uncertainty
- 4
- Observation Arc
- 26.12 yr (9,540 d)
- Aphelion
- 48.550 AU
- Perihelion
- 41.339 AU
Facts from the source article.
Lore & Background
1998 WW31 was first observed on 18 November 1998 by American astronomers Marc Buie and Robert Millis at the Kitt Peak National Observatory in Arizona. It is a non-resonant classical Kuiper belt object (cubewano) of the hot population, with an orbit that takes it from 40.4 to 48.5 AU from the Sun every 297 years. Its orbit has an eccentricity of 0.09 and an inclination of 7° relative to the ecliptic.
On 22 December 2000, French astronomers Christian Veillet and Alain Doressoundiram, in collaboration with J. Shapiro, discovered that the object was a binary using the Canada–France–Hawaii Telescope at Mauna Kea, Hawaii. The discovery was announced on 16 April 2001, and the smaller body received the provisional designation S/2000 (1998 WW31) 1. The satellite has a highly eccentric orbit (eccentricity 0.8193) with an orbital period of 587.27 days and a semi-major axis of 22617 km.
As of 2025, this minor planet has not been numbered nor named by the Minor Planet Center. No rotational lightcurve has been obtained, so its rotation period, pole, and shape remain unknown. The primary is estimated at 148 km in diameter and the satellite at 123 km, assuming equal densities of 1 g/cm³, with a combined system diameter of 192 km. The object has a blue (neutral) color and a V–I color index of 0.91, lower than the mean for cubewanos.
Reader's Guide
1998 WW31 holds a significant place in the study of the outer Solar System as the first trans-Neptunian binary discovered after Charon in 1978. Its discovery by Veillet, Doressoundiram, and Shapiro in December 2000, announced in April 2001, opened the door to the subsequent discovery of many trans-Neptunian binaries. The system is noted as one of the most symmetrical binaries known, with a size ratio of 1.2 between primary and secondary (if they share the same albedo) and a mass ratio of 1.74, meaning the secondary has about 60% the mass of the primary. The satellite's highly eccentric orbit (eccentricity 0.8193) and long orbital period (587 days) provide valuable data for understanding binary formation and dynamics in the Kuiper belt. The object's physical characteristics remain uncertain, with conflicting albedo estimates (ranging from 0.04 to 0.16) and no measured rotation period. As a non-resonant classical cubewano of the hot population, it contributes to the classification of Kuiper belt objects and their orbital evolution. Its blue color and low V–I color index distinguish it from many other cubewanos, placing it between comets and Jupiter trojans in color space.
Did You Know?
- Its satellite, S/2000 (1998 WW31) 1, has a highly eccentric orbit with an eccentricity of 0.8193.
- The binary system is one of the most symmetrical known in the Solar System.
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