Six and a half billion kilometers from the Sun, in the deep Kuiper belt, orbits Quaoar (pronounced kwa-whar), a dwarf planet around which Cheops space telescope has detected a ring of debris well beyond the ‘Roche limit‘.
The term ‘Roche limit‘ refers to the boundary within which tidal forces prevent the matter from accumulating densely enough to form a true satellite. In other words, within this zone, any object orbiting a planet, for example, manages to remain gravitationally stable. Once this limit is crossed, the object of lesser mass is torn apart by the object of greater mass.
The discovery of the ring
The Quaoar ring was first tracked during several observing sessions between 2018 and 2021, using not only ground-based telescopes but also ESA’s Cheops space telescope. The team behind this project used the so-called ‘occultation method’ to observe Quaoar’s transit in front of some very distant stars.
During the occultation by the small planet, which lasted less than a minute (and mysteriously happened twice), the astronomers were able to calculate how much the light was dimmed, but more importantly, how long it lasted, how it was dimmed, and whether the object had an atmosphere. By combining all this data, it was possible to determine Quaoar’s size, mass, and shape with extreme precision, and also to establish that the small planet does not have an atmosphere.
Until a few days ago, it was thought that the presence of a ring system was exclusive to planets, but thanks to the work of ESA’s Cheops space telescope, this belief has fallen! In fact, the ring is only 4,107 kilometers away from the planet (7.4 times the radius of Quaoar), and apparently, a ring of debris could easily survive such close proximity to the celestial object, thanks to ‘elastic collisions’, without settling on a natural moon. In other words, the Quaoar ring is very close to the 1/3 spin-orbit resonance.
“What is so fascinating about this discovery around Quaoar is that the ring of material extends much further than the Roche limit.Giovanni Bruno, INAF – Catania Astrophysical Observatory
As a result of our observations, the classical idea that dense rings only survive within the Roche limit of a planetary body has to be completely revised.”
The Lucky Star project will continue to work on this issue and will return periodically to observe Quaoar and other TNOs (Trans-Neptunian Objects).
TNOs, Quaoar, and his little moon
The acronym TNO refers to the category of celestial bodies that lie beyond the orbit of Neptune: hence the name ‘trans-Neptunian object’. Beyond the orbit of the icy giant lies a zone populated exclusively by small icy bodies such as asteroids and dwarf planets: the Kuiper belt.
Of the various known objects in this zone, Pluto, Eris, Haumea, and Makemake are officially recognized as dwarf planets, while Gonggong, Albion, Varuna, Sedna, Orcus, and Quaoar are Cubewanos and Plutinos.
Quaoar is the seventh largest TNO. It was first discovered on 4 June 2002 by Michael Brown and Chad Trujillo during a series of observations from the Palomar Observatory, although the first official image of the small planet dates back to 25 May 1954.
Quaoar has a nearly circular orbit of 40 AU and orbits in a zone between Pluto’s perihelion and aphelion, the largest TNO ever recorded. Contrary to popular belief, Quaoar is an object composed of a mixture of rock and ice that is not on the surface.
In 2004, astronomers studying Quaoar were surprised to find traces of crystalline ice on its surface, a sign that Quaoar had apparently been the victim of a micro-meteor shower or cryovolcanic event in the last ten million years, causing its surface temperature to rise to -163 degrees Celsius.
Quaoar is not alone but has a small moon 160 kilometers in diameter: Weywot. This moon was first discovered at the end of February 2007, but due to its enormous distance from the Sun and its small size, there is not much more data available. The only thing that is known for sure is that the moon was formed by an accumulation of fragments from Quaoar.