During a break from looking at planets around other stars, ESA's CHaracterising ExOPlanet Satellite (Cheops) mission has observed a dwarf planet in our own Solar System and made a decisive contribution to the discovery of a dense ring of material around it.
The dwarf planet is known as Quaoar. The presence of a ring at a distance of almost seven and a half times the radius of Quaoar, opens up a mystery for astronomers to solve: why has this material not coalesced into a small moon?
The ring was discovered through a series of observations that took place between 2018 to 2021. Using a collection of ground-based telescopes, and the space-based telescope Cheops, astronomers watched as Quaoar crossed in front of a succession of distant stars, briefly blocking out their light as it passed.
Any celestial object with an appreciable gravitational field will have a limit within which an approaching celestial object will be pulled to pieces. This is known as the Roche limit. Dense ring systems are expected to exist inside of the Roche limit, which is the case for Saturn, Chariklo and Haumea. "So, what is so intriguing about this discovery around Quaoar is that the ring of material is much farther out than the Roche limit," says Giovanni Bruno, INAF's Astrophysical Observatory of Catania, Italy.
This is a mystery because according to conventional thinking, rings beyond the Roche limit will coalesce into a small moon within just a few decades. "As a result of our observations, the classical notion that dense rings survive only inside the Roche limit of a planetary body must be thoroughly revised," says Giovanni.
Early results suggest that the frigid temperatures at Quaoar may play a role in preventing the icy particles from sticking together but more investigations are needed.
"The Cheops observations have played a key role in establishing the presence of a ring around Quaoar, in an application of high precision, high cadence photometry that goes beyond the more typical exoplanet science of the mission", says Kate.
While the theoreticians get to work on how the Quaoar rings can survive, the Lucky Star project will continue to look at Quaoar and also other TNOs as they occult distant stars to measure their physical characteristics and see how many others also have ring systems.