Astronomers used 14 years of stellar occultations to measure the trans-Neptunian object (50000) Quaoar. They combined 28 new events recorded between 2018 and 2025 with nine earlier published occultations. Fitting all the observations at once gives an oblate ellipsoid with equatorial semi-axes a = b = 566.1 (+2.5/−2.2) km and a polar semi-axis c = 511.2 (+3.6/−3.7) km. That shape corresponds to a volumetric diameter of 1,094.4 ± 4.6 km and a polar flattening (oblateness) of 0.097 ± 0.011.

A stellar occultation happens when a distant star is briefly hidden by a passing object. Observers at many sites time the disappearance and reappearance of the star. Each timing gives a chord across the object. By joining many chords from different events and locations, the team reconstructed Quaoar’s overall size and shape. The paper also reports 36 new precise sky positions (astrometry) for Quaoar gathered during those campaigns.

The team used the newly derived mass of the Quaoar–Weywot system (Weywot is Quaoar’s known moon) to turn the measured size into a bulk density. They find a density of 1.760 ± 0.109 grams per cubic centimetre. Using Quaoar’s known rotation period (8.8394 ± 0.0002 hours) and assuming the body is in Maclaurin hydrostatic equilibrium — a simple rotating, liquid-like shape set by gravity and spin — gives a compatible density of 1.859 ± 0.200 g/cm3. Because these values agree within their errors, the authors say Quaoar could be a Maclaurin object and so qualifies as a candidate dwarf planet under the usual physical criteria.

The study also updates Quaoar’s reflectivity and atmosphere limits. With an assumed absolute magnitude H = 2.79 ± 0.35, they derive a geometric albedo (a measure of surface reflectivity) pV = 0.125 ± 0.038, meaning Quaoar reflects about 12% of the visible light that hits it. From the occultation light curves they set new upper limits on any methane (CH4) atmosphere: at most 0.15 nanobar at the 1-sigma level and 0.65 nanobar at 3-sigma. A nanobar is one billionth of Earth’s atmospheric pressure, so any methane atmosphere would be vanishingly thin or absent.