A dwarf planet has been discovered, with an average diameter of about 1,100 kilometers, surrounded by a system of “impossible” rings.
About 43 astronomical units from the Sun (one astronomical unit corresponds to just under 150 million kilometers), in the remote Kuiper belt, lies Quaoar: a dwarf planet with a slightly ellipsoidal shape, with an average diameter of about 1,100 kilometers.
Not only does it have a moon, Weywot – discovered in 2006, about 170 kilometers across and orbiting at about 13,300 kilometers – but it is also surrounded by a completely unexpected ring system.
“Impossible” rings. In February 2023, thanks to a series of stellar occultations observed by the CHEOPS space telescope, it was confirmed that Quaoar is surrounded by a ring, called Q1R, which orbits far beyond the theoretical Roche limit — where normally matter would aggregate to form a satellite. And this is already something anomalous and difficult to explain.
Subsequently, in the second half of the same year, the Gemini North and CFHT telescopes identified the second innermost ring, Q2R, also beyond the Roche limit. Q1R (outer ring): orbits approximately 4,057 kilometers from Quaoar, has a width varying between 5 kilometers and 300 kilometers and an optical density varying from 0.004 to 0.7.
Optical density (or opacity) is a measure of how much a material blocks or attenuates light passing through it. If the optical density is high, the ring (or object) is very opaque: it lets little light through, so it appears dense and thick. If it is low, however, it is almost transparent: it means that the light from the stars behind it can filter through and that the ring is made up of more sparse or thin material. In the case of this ring, the variation in density from very low values (0.004) to very high values (0.7) means that in some places it is almost invisible, in others it is compact enough to block almost all the light.
The other Q2R ring. The inner ring (orbits at about 2,520 kilometers) is thinner (about 10 kilometers) and very faint (optical density about 0.004). This arrangement raises a question: how do such distant rings, where matter should already have aggregated, remain stable?
Leading hypotheses involve orbital resonances with Quaoar’s rotation and the orbit of its moon, Weywot. An orbital resonance occurs when two celestial bodies (for example, moons, planets, or rings) orbit a central object with periods that have a simple ratio to each other, such as 2:1, 3:2, 7:2, and so on.
In practice, while one body completes a certain number of revolutions, the other always completes a proportional number.
This synchrony causes their gravitational forces to repeat regularly, maintaining or stabilizing their orbits (or, conversely, creating instability and voids as in the case of Saturn’s rings).
Resonances: the key to stability. The outer boundary of Q1R is close to both the 1:3 resonance with Quaoar rotation and the 6:1 resonance with Weywot; Q2R respects a 5:7 resonance with the planet. Dynamic simulations suggest that these resonances would help contain the material and prevent a satellite from forming.
A ring or a hidden moon? Here’s what the new discovery says. During an occultation on June 25, 2025, the star UCAC4 376-136839 was covered not by Quaoar nor its known rings, but by another structure — for 1.23 seconds, about 2.4 minutes before the main event of the occultation of its moon. Researchers have suggested it could be a third ring or a second moon. However, the shape is favorable to the hypothesis of a satellite, given that a hypothetical structured ring would have generated two occultations and not just one.
This second satellite, if confirmed, would orbit approximately 5,757 kilometers from Quaoar, close to the 7:2 resonance with Weywot. It would be extremely dim – about 22 times dimmer than Weywot – and therefore invisible with almost all telescopes except, perhaps, the James Webb Space Telescope (JWST).
The new observations. The James Webb Telescope, thanks to the NIRCam instrument, which observed an occultation in June 2025, confirmed the two known rings and highlighted significant brightness variability in Q2R, suggesting possible asymmetric structures. It also ruled out the presence of a global atmosphere with pressure greater than 1 nbar.
