Size and shape Analysis of
stellar occultation observations from 2011–2024 has shown that Quaoar is a slightly elongated
triaxial ellipsoid with dimensions , corresponding to a
volume equivalent diameter of . Quaoar's diameter is roughly half that of Pluto and is slightly smaller than Pluto's moon
Charon. At the time of its discovery in 2002, Quaoar was the largest object found in the
Solar System since the discovery of Pluto. Quaoar's elongated shape contradicts theoretical expectations that it should be in
hydrostatic equilibrium, because of its large size and slow rotation. According to Michael Brown, rocky bodies around in diameter should relax into hydrostatic equilibrium, whereas icy bodies relax into hydrostatic equilibrium somewhere between and . Slowly-rotating objects in hydrostatic equilibrium are expected to be
oblate spheroids (
Maclaurin spheroids), whereas rapidly-rotating objects in hydrostatic equilibrium, such as
Haumea which rotates in nearly 4 hours, are expected to be flattened and elongated ellipsoids (
Jacobi ellipsoids). To explain Quaoar's non-equilibrium shape, Csaba Kiss and collaborators hypothesized that Quaoar originally had a rapid rotation and was in hydrostatic equilibrium, but its shape became "frozen in" and did not change as Quaoar spun down due to
tidal forces from its moon Weywot. This would resemble the situation of Saturn's moon
Iapetus, which is too oblate for its current rotation rate.
Mass and density and the
Moon Quaoar has a
mass of , which was determined from the orbit of its moon Weywot. Measurements of Quaoar's diameter and mass indicate it has a
density of , which suggests it has a
differentiated and compacted interior consisting of a rocky core surrounded by an icy shell. Quaoar's density was previously thought to be much higher, between , because early measurements inaccurately suggested that Quaoar had a smaller diameter and a higher mass. These early high-density estimates for Quaoar led researchers to hypothesize that the object might be a rocky
planetary core exposed by a large
impact event, but these hypotheses have since become obsolete as newer estimates indicate a lower density for Quaoar.
Surface Quaoar has a dark surface that reflects about 12% of the visible light it receives from the Sun. This may indicate that fresh ice has disappeared from Quaoar's surface. The surface is moderately red, meaning that Quaoar reflects longer (redder) wavelengths of light more than shorter (bluer) wavelengths. Many Kuiper belt objects such as
20000 Varuna and
28978 Ixion share a similar moderately red color. Spectroscopic observations by
David C. Jewitt and
Jane Luu in 2004 revealed crystalline water
ice and tentative hints of
ammonia hydrate on Quaoar's surface. These substances are expected to gradually break down due to solar and cosmic radiation, and crystalline water ice can only form in warm temperatures of at least , so the presence of crystalline water ice on Quaoar's surface indicates that it was heated to this temperature sometime in the last ten million years. For context, Quaoar's present-day surface temperature is less than . Jewitt and Luu proposed two hypotheses for Quaoar's heating, which are
impact events and
radiogenic heating. The latter hypothesis allows for the possibility of
cryovolcanism on Quaoar. A 2006 study by Hauke Hussmann and collaborators suggested that radiogenic heating alone may not be capable of sustaining an
internal ocean of liquid water at Quaoar's mantle–core boundary. More precise observations of Quaoar's near
infrared spectrum in 2007 indicated the presence of small quantities (5%) of solid
methane and
ethane. Given its
boiling point of , methane is a volatile ice at average surface temperatures of Quaoar, unlike water ice or ethane. Both models and observations suggest that only a few larger bodies (
Pluto, and ) can retain the volatile ices whereas the dominant population of small
trans-Neptunian objects lost them. Quaoar, with only small amounts of methane, appears to be in an intermediary category. In 2022, low-resolution near-infrared (0.7–5 μm)
spectroscopic observations by the
James Webb Space Telescope (JWST) revealed the presence of
carbon dioxide ice, complex organics, and significant amounts of
ethane ice on Quaoar's surface. Other possible chemical compounds include
hydrogen cyanide and
carbon monoxide. JWST also took medium-resolution near-infrared spectra of Quaoar and found evidence of small amounts of
methane on Quaoar's surface. However, both JWST's low- and medium-resolution spectra of Quaoar did not show conclusive signs of ammonia hydrates. High-resolution imaging by the Hubble Space Telescope has shown that Quaoar's equator is relatively brighter than its poles. One possible hypothesis proposed by Benjamin Proudfoot and colleagues suggests that infalling ring material may brighten Quaoar's equator. Another hypothesis proposed by the same researchers suggests that Quaoar's poles are darkened due to radiation processing of ethane that has condensed there, in a similar fashion to Pluto's moon Charon.
Possible atmosphere The presence of methane and other
volatiles on Quaoar's surface suggest that it may support a tenuous
atmosphere produced from the
sublimation of volatiles. With a measured mean temperature of approximately , the upper limit of Quaoar's
atmospheric pressure is expected to be in the range of a few
microbars. Due to Quaoar's small size and mass, the possibility of Quaoar having an atmosphere of
nitrogen and
carbon monoxide has been ruled out, since the gases would escape from Quaoar. The possibility of a methane atmosphere, with the upper limit being less than 1 microbar, was considered until 2013, when Quaoar
occulted a 15.8-magnitude star and revealed no sign of a substantial atmosphere, placing an upper limit to at least 20 nanobars, under the assumption that Quaoar's mean temperature is and that its atmosphere consists of mostly methane. The upper limit of atmosphere pressure was tightened to 10 nanobars after another stellar occultation in 2019. == Satellites ==