Improbable discovery of a ring around the transneptunian object Quaoar

The successive and unexpected discoveries of rings in 2013 around the centaur object - Chariklo -, then in 2017 around the transneptunian object - Haumea -, led to an astonishing observation : far from being reserved for the four giant planets, rings would be commonplace among the small objects of the outer solar system. This idea was, obviously, a winning bet of the European Research Council (ERC) "Lucky Star" project led between 2015 and 2021 by Bruno Sicardy, professor at Sorbonne University and astrophysicist at the Observatoire de Paris - PSL.

Thus, in turn, Quaoar has been targeted. A large transneptunian object half the size of Pluto and orbiting at an average of 43 astronomical units from the Sun, Quaoar has given rise to four campaigns of stellar occultation observations that took place between 2018 and 2021, mobilizing a cohort of ground and space-based telescopes :

• the ATOM robotic telescope in Namibia (HESS project) ;
• the Gran Telescopio Canarias, 10.40 m in diameter, located in La Palma
• the space telescope CHEOPS of the ESA ;
• and stations of Australian citizen astronomers.

The purposes were multiple : to observe the passage of Quaoar in front of or near stars in the plane of the sky, to collect information on the body itself and to highlight matter around the object, in the form of rings, jets or dust envelopes.

Characteristics of the Quaoar ring

Taken together, the observations have highlighted the existence of a ring surrounding Quaoar. This ring is inhomogeneous [1] and has a radius of 4 100 km. The dense part of the ring - which has a limited azimuthal extension (parallel to the ring) - blocks more than 50% of the stellar flux that crosses it. It is included in a more tenuous and continuous component which hardly blocks the stellar flux (less than 1%).

But the unique and astonishing property of the ring is its large distance to the central body : estimated at about 7.4 times the radius of Quaoar. This ring is thus well beyond the so-called "Roche limit" of the central body.

However, after the limit calculated by the French mathematician and astronomer, Edouard Roche, in 1850, a collisional ring is not supposed to be formed, the particles having to quickly accrete in a satellite and thus disappear. Indeed, according to Ed. Roche’s calculations, accretion is prevented in the case of a collisional ring below this limit. On the other hand, if the disk is located beyond this limit it should, in a few weeks, aggregate into a satellite.

And in fact, the main rings of the four giant planets, of Chariklo and Haumea are all well within or near the Roche limits of their respective bodies.

Numerical simulations to better understand

Improbable therefore, this discovery of the Quaoar ring has led to further numerical studies presented by the team. Thus, the parameters of the Quaoar ring have been used to carry out, at the University of Oulu in Finland, simulations of colliding particles subjected to their mutual attractions.

Until now, the collision laws, used for example to describe the rings of Saturn, resulted in a rapid accretion. However, alternative collision laws, more elastic and obtained in laboratory at low temperature, show in fact the opposite. The post-impact velocities between the particles remain sufficiently high for them to overcome their mutual attractions. This inhibits their tendency to accrete. If the Roche criterion seems robust enough to explain how a satellite is broken by tidal forces to form a ring, the reverse process - namely, the accretion of particles from a ring to a satellite - involves more complex mechanisms that have been neglected so far.

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Read also the information
on the website of the Instituto de Astrofísica de Canarias (IAC) [in Spanish] :
"El objeto transneptuniano quaoar"

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However, there are still some major questions to be answered, including the following : how is the Quaoar ring confined in both the radial (perpendicular to the ring) and azimuthal (parallel to the ring) directions ? It is interesting to note the particular configuration in which the ring is found, namely that a particle of the ring accomplishes only one revolution when Quaoar accomplishes three rotations. Coincidentally or not, this configuration (called "resonance") is also observed for the rings of Chariklo and Haumea. This suggests that this resonance could play a key role in the very existence of the ring and its radial location.

Preliminary collisional simulations (also performed at the University of Oulu) actually show that radial confinement does occur at this resonance, a result that now needs to be corroborated by theoretical studies. The azimuthal confinement of the dense part, or arc, remains more mysterious. It could come from perturbations of Weywot, the satellite of Quaoar, or perhaps from a satellite that has not yet been discovered.  
This hypothesis, even if it is still debated, has been evoked to explain the stability of Neptune’s arcs. But it could just as easily result from a completely different mechanism, for example from an interrupted process of accretion of the ring into a satellite.

In any case, the observation is without appeal. The Quaoar ring shows a behavior towards the Roche limit that has never been observed before. As such, it questions the very principle of the accretion mechanisms that allow (or prevent) the formation of satellites from a collisional disk orbiting a planetary body.

Reference

The article is published under the title : "A dense ring of the trans-Neptunian object Quaoar outside its Roche Limit" in the journal Nature dated February 8, 2023.
DOI  : 10.1038/s41586-022-05629-6

This research was partially funded by the European Research Council project "Lucky Star", led by Bruno Sicardy (ERC Advanced Grant n°669416).

It is the result of scientific activities carried out, among others, in France, at the Observatoire de Paris - PSL at the Laboratoire d’études spatiales et d’instrumentation en astrophysique (Observatoire de Paris - PSL / CNRS / Sorbonne Université, Université Paris Cité ) and at the Institut de mécanique céleste et de calcul des éphémérides (Observatoire de Paris - PSL / CNRS / Sorbonne Université / Université de Lille), in Brazil (Federal University of Rio de Janeiro and National Observatory of Rio de Janeiro, Federal University of Technology in Curitiba), in Spain (Institute of Astrophysics of Andalusia, Granada), in Finland (University of Oulu), at the ESA (CHEOPS mission) and by citizen astronomers in Australia

The occultation campaigns have greatly benefited from the ESA Gaia mission, which has provided very accurate star positions, thus ensuring reliable predictions of events.

International
Other resources related to this discovery  :

• The press release from ESA/CHEOPS ;
• The press release of the Federal University of Rio de Janeiro (Brazil) ;
• The press release from the University of Oulu (Finland) ;
• The press release of the Institute of Astrophysics of Andalusia (Spain) ;
• The video of the Institute of Astrophysics of Andalusia (Spain).

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