50000 Quaoar

Quaoar has one known moon, named Weywot. It also has a ring. Unexpectedly, the ring is at twice the distance of the Roche limit, which was thought to be the maximum distance of a ring: beyond the Roche limit, the particles in a ring should clump together and form a moon; yet this has not happened to Quaoar's ring. It is thought that the particles do not clump together because of tides created by Weywot. There may also be small shepherd moons on either side of the ring, as there are at Saturn's F Ring.

 Physical characteristics 
 Size and shape 
+ History of diameter estimates for QuaoarYearDiameter (km)MethodRefs2004imaging2007thermal2010thermal/imaging2013thermal2013occultation2023occultation2024thermal/occultation

, measurements of Quaoar's shape from its rotational light curve and stellar occultations show that Quaoar is a triaxial ellipsoid with an average 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 was also the first trans-Neptunian object to be measured directly from Hubble Space Telescope images.

Quaoar's far-infrared thermal emission and brightness in visible light both vary significantly (visible light curve amplitude 0.12–0.16 magnitudes) as Quaoar rotates every 17.68 hours, which most likely indicates Quaoar is elongated along its equator. A 2024 analysis of Quaoar's visible and far-infrared rotational light curve by Csaba Kiss and collaborators determined that the lengths of Quaoar's equatorial axes differ by 19% (a/b = 1.19) and the lengths of Quaoar's polar and shortest equatorial axis differ by 16% (b/c = 1.16), which corresponds to ellipsoid dimensions of . The ellipsoidal shape of Quaoar matches the size and shape measurements from previous stellar occultations, and also explains why the size and shape of Quaoar appeared to change in these occultations.

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, 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 
Quaoar has a mass of , which was determined from Weywot's orbit using Kepler's third law. Measurements of Quaoar's diameter and mass  indicate it has a density between , which suggests its interior is composed of roughly 70% rock and 30% ice with low porosity. 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 Jewitt and Jane Luu in 2004 revealed signs of crystalline water ice and 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, which is supported by the presence of ammonia hydrate on Quaoar's surface. Ammonia hydrate is believed to be cryovolcanically deposited onto Quaoar's surface. 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.

 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.

 Notes 

 References 

 Other websites 
 Quaoar discoverers' webpage
 Orbital simulation from JPL (Java) /Ephemeris

Category:Kuiper belt objects
Category:Dwarf planets