A ring around a star that shouldn’t have it there

Until 2013, only rings were known around Jupiter, Saturn, Uranus and Neptune, that is, the giant planets of the Solar System. This changed with the discoveries of the rings of Chariclo (centaur-like object) in 2013 and of the dwarf planet Haumea in 2017. Now, astronomers have found a ring on the trans-Neptunian object Quaoar, which lies beyond the limit de Roche and thus challenges the theory of how far bits of dust and ice must accumulate to form rings.

The research team includes scientists from the Instituto de Astrofísica de Andalucía (IAA), dependent on the Higher Council for Scientific Research (CSIC) in Spain.

This study, the result of an international collaboration within the framework of the European ERC Advanced Grant Lucky Star project, describes Quaoar as a large trans-Neptunian object, approximately half the size of Pluto and orbiting 43 astronomical units from the Sun (or 43 times the size of Pluto). distance between the Earth and the Sun). A unique and surprising property of the ring is its large radius which, at 4,100 kilometers, corresponds to about 7.4 Quaoar radii.

This is well beyond the Roche limit, the distance at which, according to the theory developed by Edouard Roche around 1850, tidal forces from the central body prevent particles from aggregating on a satellite. According to this theory, a collision ring inside this boundary cannot accumulate, while outside it particles are expected to aggregate and form a satellite on time scales of just weeks. And so far this was what had been observed: All the dense rings of the four giant planets, as well as the rings of Chariclo and Haumea, lie within or near the Roche limit of their respective bodies. Instead, Quaoar’s ring occupies an orbit where a satellite should have formed.

“When we first saw the possible existence of a ring outside the Roche limit in the excellent data obtained with the Gran Telescopio Canarias, we realized that it could take many years to conclusively prove this circumstance, but we finally got it in few years thanks to the international effort”, points out José Luis Ortiz, a researcher at the IAA who is participating in the work.

Artist’s recreation of the Quaoar and its ring. (Image: IAA)

The discovery of the Quaoar ring triggered numerous numerical studies, and local simulations of self-gravitation were developed. While the collision laws classically used to describe Saturn’s rings resulted in rapid accumulations, which would favor the formation of a satellite in that region, the more elastic collision laws obtained in the laboratory at low temperatures showed the opposite: the Post-impact velocities between particles remain high enough to escape each other’s attractions and eventually overcome their tendency to accumulate. Thus, while the Roche criterion seems robust to explain how tidal forces disrupt the formation of a satellite to form a ring; in the opposite process, the accumulation of particles in a satellite, implies more complex mechanisms that until now have been overlooked.

However, unknowns still persist in relation to this small object. “Interestingly, the ring is at a distance from Quaoar where it takes the particles that make it up to go around Quaoar just three times as long as it takes Quaoar to go around itself. It is a a phenomenon that we have already observed before in the dwarf planet Haumea and we believe that it also occurs in Cariclo, so there seems to be a common pattern in the formation of dense rings”, concludes Ortiz.

The discovery of rings in such small and distant objects in the Solar System is done using the stellar occultation method, which consists of observing objects that transit in front of the background stars.

The discoveries at Cariclo and Haumea implied that rings must be common among small objects in the outermost part of the solar system, and their search was one of the main objectives of the Lucky Star project, led by Bruno Sicardy of the Paris Observatory in France.

The discovery of the Quaoar ring arose from the combination of stellar occultations observed between 2018 and 2021 from a robotic telescope in Namibia (HESS project), the Gran Telescopio Canarias (La Palma), the CHEOPS space telescope (European Space Agency, ESA) and Australian amateur stations in the Brisbane region.

The study is titled “A dense ring of the trans-Neptunian object Quaoar outside its Roche Limit”. And it has been published in the academic journal Nature. (Source: Silbia López de Lacalle / IAA / CSIC)