A RINGSYSTEM FOR QUAOAR!

Jupiter, Saturn, Uranus and Neptune: Giant planets are not the only objects of the Solar System surrounded with rings. Some dwarf planets and asteroids also have this feature. Until recently, only two bodies of this type were known, Chariklo and Haumea, but in these days it has been announced the discovery of a system of rings also around Quaoar.

Quaoar is a small 555 km diameter body orbiting the boundaries of the Solar System, within the Kuiper Belt, 44 AU from the Sun. Quaoar is also orbited by a small moon with a radius of 80 km.

The observations that led to the discovery of the ring were conducted between 2018 and 2021 with the Gran Telescopio Canarias in the Canary Islands and with other terrestrial telescopes. On one occasion, the Cheops Space Telescope was also used, usually dealing with exoplanets.

Kuiper Belt objects are among the most complex to study in the entire Solar System because of their great distance from the Sun. One of the most effective ways to study them is through stellar occultations. When the dwarf planet passes in front of a star, it obscures it for a brief moment. By studying the variation of stellar light over this period of time, it is possible to reconstruct the shape and size of the dwarf planet and to probe the presence of an atmosphere and rings around it.

During the three years of observation, astronomers observed a number of stellar occultations from which the presence of a circular system around Quaoar was discovered.

This ring system has a feature that is unique compared to all the others discovered so far within the Solar System: it is located outside the Roche limit.

For every body with a gravitational field, the limit of Roche is defined as the distance below which any external body approaching the planet is destroyed by its gravity. In order not to be destroyed by tidal forces, satellites must be in orbit outside the Roche boundary. Ring systems, composed of small, fragmented bodies, can survive within the Roche boundary.

Before this discovery, astronomers thought that the rings could only exist within this limited distance. According to planetary models, rings outside the Roche boundary should condense to form a small satellite within a few decades. 

However, this has been disproved by what has been observed around Quaoar and it will therefore now be necessary to revise all the models on the formation of ring systems formulated to date.

Credit: ESA.