Quaoar
Quaoar is a dwarf-planet candidate in the Kuiper belt, roughly half the size of Pluto. In 2023 astronomers discovered it has a ring — surprisingly far outside the distance where rings were thought able to survive. It has one small moon, Weywot. No spacecraft has visited; everything we know comes from telescopes and stellar occultations — and those have been generous: 37 occultations over 14 years (Margoti et al., ApJ, 2026) pin its shape to a slightly flattened spheroid, 1,132 km across the equator and 1,022 km pole to pole, with no detectable atmosphere. Its density, from Weywot's orbit, is close to what a body of its size and spin would settle into under its own gravity — which is the test for dwarf-planet status.
Key facts
Science
Physical
- Mass
- 1.21 × 10²¹ kg · 0.0165 Moon masses
- Braga-Ribas et al. 2025, Phil. Trans. R. Soc. A 383, 20240200 — system mass from Weywot's orbit (±5%), as adopted by Margoti et al. 2026 View source ↗
- Mean radius
- 547 km · 9% of Earth's radius
- Margoti et al. 2026, ApJ (arXiv:2607.06450) — volume-equivalent radius of the oblate spheroid fitted to 37 stellar occultations, 2011–2025 (a = b = 566.1 km, c = 511.2 km; ±2.3 km) View source ↗
- Density
- 1,760 kg/m³ · 1.8× water
- Computed from mass and radius
- Surface gravity
- 0.269 m/s² · 3% of Earth's gravity
- Computed from mass and radius
- Escape velocity
- 543 m/s · 5% of Earth's escape velocity
- Computed from mass and radius
Orbital
- Distance from the Sun
- 42.6 AU
- Computed from the position at the selected time
- Semi-major axis
- 43.2 AU
- Computed from the cached state vector
- Eccentricity
- 0.03495
- Computed from the cached state vector
- Inclination
- 15.59 °
- Computed from the cached state vector
- Orbital period
- 283.63 years
- Computed from the cached state vector
- Perihelion
- 41.7 AU
- Computed from the orbital elements
- Aphelion
- 44.7 AU
- Computed from the orbital elements
- Mean orbital velocity
- 4.53 km/s
- Computed from the orbital elements
Rotation
- Rotation period
- 17.68 hours
- Rotation
- Ambiguous: the light curve gives 17.68 h if it has two peaks per turn (a triaxial body) or 8.84 h if one (an oblate body with surface markings) — Ortiz et al. 2003. The 2026 occultation shape is oblate, which favours 8.84 h, but the authors note that reading also moves the ring off its expected resonance, so they leave both open. We show the double-peaked value.
Atmosphere
- Composition
- None detected — stellar occultations put a 1σ upper limit of 0.15 nbar on the surface pressure of a methane atmosphere (Margoti et al. 2026)
Composition
- Bulk composition
- Near-infrared spectroscopy shows crystalline water ice with traces of methane and other ices on a reddish surface; the crystalline (rather than amorphous) ice hints at past internal warming. The bulk rock/ice interior is unconstrained — no spacecraft has visited.
- Surface imagery
- No real surface imagery exists — this object has never been visited; color is approximate, inferred from spectroscopy.
Radiation & temperature
- Insolation
- 0.000537 S⊕ · 1/1,864 of the sunlight Earth receives
- Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
- Geometric albedo
- 0.12
- V-band geometric albedo 0.125 ± 0.038 — from the occultation shape and an absolute magnitude H = 2.79 ± 0.35 (Margoti et al. 2026); the Bond albedo is not directly measured.
- Equilibrium temperature
- 41 K (-232 °C)
- Computed from albedo + distance from the Sun (airless black body); uses the geometric albedo as an estimate
Discovery & history
- Discovered
- June 4, 2002
- By
- Chad Trujillo and Mike Brown
- Method
- Photographic plates
- Name
- Quaoar, the creation force of the Tongva people native to the Los Angeles basin; its moon Weywot is named for Quaoar's son.
Quaoar was discovered on 4 June 2002 by Chad Trujillo and Mike Brown at Palomar Observatory (provisional designation 2002 LM60). About half the size of Pluto, it was among the largest Kuiper-belt finds of its era and helped make the case that the outer Solar System holds many Pluto-like worlds. Its surface shows crystalline water ice and traces of methane, hinting at past internal warming. A moon, Weywot, was found in 2007, and in 2023 astronomers discovered a ring around Quaoar orbiting far beyond the distance where rings were thought able to survive — a puzzle still being studied. It is named for the creation force of the Tongva people of southern California.
Images
Images from open archives · credit shown per image