Quaoar
A dwarf-planet candidate about 1,110 km across on a nearly circular orbit beyond Pluto, with a moon, Weywot, and two rings where none were expected.
At a glance
- Status
- Candidate: likely large enough to be round, not formally classified[3]Not among the five dwarf planets the International Astronomical Union recognizes
- Size
- About 1,110 km across[1]Equivalent radius 555 km, give or take 2.5, from stellar occultations; its surface reflects about 11 per cent of sunlight
- Orbit
- 41.6 to 44.7 au from the Sun, once every 285 years or so[5]Nearly circular; semi-major axis 43.2 au, tilted 8° to the plane of the planets
- Day
- About 8.8 hours[5]From its light curve
- Moons
- One, Weywot, about 81 km across[6]81 km, give or take 11, assuming it reflects light like Quaoar
- Rings
- Two, about 4,057 km and 2,520 km from the center[2]Both lie outside the distance within which rings were thought able to survive without clumping into a moon
- Density
- About 1.99 g/cm³[1]From the occultation size and the mass given by Weywot's orbit
Timeline
- June 4, 2002Discovered[5]Chad Trujillo and Mike Brown at the Palomar Observatory, California
- July 5, 2002Hubble measures its size[4]The first direct size for a Kuiper belt object, about 1,300 km, in images taken on 5 July and 1 August 2002
- October 7, 2002Announced[4]Described as approximately half the size of Pluto, the largest body found in the solar system since Pluto itself
- 2013Size refined by occultations[1]Stars passing behind Quaoar give an equivalent radius of 555 km
- August 9, 2022Second ring found[2]A stellar occultation refines the first ring's radius to 4,057 km and reveals a fainter one at 2,520 km
Quaoar is a dwarf-planet candidate in the classical Kuiper belt, about 1,110 km across, with a small moon and, unexpectedly, two rings.[1][2] The International Astronomical Union has not classified it; the five dwarf planets it recognizes are Ceres, Pluto, Haumea, Makemake and Eris.[3]
How it was found
Chad Trujillo and Mike Brown discovered Quaoar in 2002 with the Oschin Schmidt telescope at the Palomar Observatory, as an 18.5-magnitude object; JPL's record gives the date as 4 June 2002.[4][5] They then turned the Hubble Space Telescope on it on 5 July and 1 August 2002 and measured its disk directly, the first time that had been done for a Kuiper belt object: an angular size of 40 milliarcseconds, corresponding to a diameter of about 1,300 km, approximately half the size of Pluto and larger than anything found in the solar system since Pluto itself.[4] The result was announced on 7 October 2002.[4] Quaoar is the creation force in the mythology of the Tongva, the indigenous people of the Los Angeles basin, who dances and sings the sky father Weywot into existence; the moon carries his name.[5]
Orbit
Unlike Pluto's, Quaoar's orbit is very nearly circular.[4] JPL's catalogue gives a path from 41.6 to 44.7 astronomical units, with a semi-major axis of 43.2 astronomical units, tilted 8 degrees to the plane of the planets; one orbit takes about 285 years, and it rotates once every 8.8 hours or so.[5]
Size and interior
Later measurements shrank Hubble's first estimate. When Quaoar passed in front of stars, observers timing the star's disappearance from several sites measured an equivalent radius of 555 km, give or take 2.5, a surface reflecting about 11 per cent of sunlight, and a density of 1.99 grams per cubic centimeter, give or take 0.46, with no detectable atmosphere down to a pressure of about 20 nanobar.[1] The Herschel Space Observatory's thermal measurements agree, at 1,070 km, give or take 38, and its surface is known to carry both water ice and methane ice.[6] Later occultations give an outline with a semi-major axis of 579.5 km and an area-equivalent radius of 543 km.[2]
The moon and the rings
Weywot is about 81 km across, give or take 11, if it reflects light as Quaoar does.[6] The rings were a surprise. Stellar occultations observed between 2018 and 2021 revealed an uneven ring, and an occultation on 9 August 2022, watched from Gemini North and the Canada-France-Hawaii Telescope among others, refined its radius to 4,057 km, give or take 6, and found a second, fainter ring at about 2,520 km.[2] Both lie outside Quaoar's classical Roche limit, the distance within which a body's gravity would be expected to keep ring material from gathering into a moon; farther out, the material should have done so, and why it has not is an open question.[2] The main ring is about 60 km wide with a dense core only about 5 km across; the second is about 10 km wide and far more tenuous.[2] Both rings, and Weywot's orbit, lie in Quaoar's equatorial plane.[2]
What is not yet known
No spacecraft has visited Quaoar. Why its rings survive where theory says they should not is the question its observers are working on, and no atmosphere has yet been detected.[2][1]
Sources
- [1]The Astrophysical Journal, The size, shape, albedo, density, and atmospheric limit of transneptunian object (50000) Quaoar from multi-chord stellar occultations — doi.org, retrieved November 5, 2026
- [2]arXiv (Astronomy & Astrophysics 673, L4, 2023), The two rings of (50000) Quaoar — doi.org, retrieved November 5, 2026
- [3]NASA, Pluto & Dwarf Planets — science.nasa.gov, retrieved November 5, 2026
- [4]NASA, Hubble Spots an Icy World Far Beyond Pluto — science.nasa.gov, retrieved November 5, 2026
- [5]NASA/JPL Small-Body Database, 50000 Quaoar (2002 LM60) — ssd.jpl.nasa.gov, retrieved November 5, 2026
- [6]arXiv (Astronomy & Astrophysics, 2013), TNOs are Cool: A survey of the trans-Neptunian region. VIII. Combined Herschel PACS and SPIRE observations of 9 bright targets at 70–500 micron — doi.org, retrieved November 5, 2026
Related
- What is a dwarf planet?The definition that made Pluto a dwarf planet in 2006, the five the IAU recognises, why the list grows so slowly, and the candidates waiting to join it.
- The Kuiper belt and beyondThe ring of icy worlds past Neptune, the scattered disk and the Oort cloud beyond it, how many objects are known, and how new ones are found.
- HaumeaA dwarf planet shaped like a football by its four-hour spin, with two moons and a ring found in 2017, orbiting about 43 au from the Sun once every 285 years.
- GonggongA dwarf-planet candidate about 1,230 km across on a tilted, elongated orbit far beyond Pluto, with a small moon, Xiangliu, found in Hubble images.
- OrcusA dwarf-planet candidate about 917 km across on an orbit much like Pluto's, in the same resonance with Neptune, with a large moon, Vanth, about half its size.
- SednaA dwarf-planet candidate about 995 km across on one of the most distant orbits known, never closer to the Sun than 76 au and about 12,700 years around.
- 2026 ED138A dwarf-planet candidate found by the Rubin Observatory beyond Neptune and announced on 27 October 2026, an icy world that may be 600 to 1,500 km across.