While the European Space Agency’s ExOPlanet Satellite (Cheops) mission was on hiatus from exploring planets around other stars, it observed a dwarf planet in our solar system, making a crucial contribution to the discovery of a dense ring of material around it. The dwarf planet is known as Quaoar. The existence of a ring almost seven and a half times the radius of Quaoar reveals a mystery to astronomers: Why didn’t this material turn into a small moon?

Artist’s impression of the dwarf planet Kvaoar and its rings. Kwaoar’s friend Weivot is shown on the left. Kvaoar’s ring was discovered during a series of observations from 2018 to 2021. Using an array of ground-based telescopes and ESA’s Cheops Space Telescope, astronomers watched Quaoar pass through a series of distant stars, briefly blocking its dimming light as it passed. Credit: ESA, CC BY-SA 3.0 IGO

How to observe distant objects in the solar system?

The ring was discovered as a result of a series of observations made from 2018 to 2021. Using a collection of ground-based telescopes and the Cheops Space Telescope, astronomers watched Quaoar pass a series of distant stars, briefly blocking their light as they passed.

Such an event is known as occlusion. Observing how the light from the hidden star falls provides information about the size and shape of the hidden object and can reveal whether the intervening object has an atmosphere. In this case, smaller dips before and after the main eclipse revealed the presence of material in orbit around Kvaoar.

Quaoar is one of a series of small distant worlds known as Trans-Neptunian Objects (TNOs). About 3,000 of them are known, as the name suggests, TNOs are located in the outer reaches of the Solar System, beyond the orbit of the planet Neptune. The largest of the TNOs are Pluto and Eris. With an estimated radius of 555 km, Kvaoar ranks seventh on the size list and orbits a small satellite called Veivot with a radius of about 80 km.

Studying these dwarf planets is difficult due to their small size and extreme remoteness. Quaoar itself orbits the Sun at a distance about 44 times greater than the Sun-Earth distance. For this reason, clogs are particularly valuable tools. However, until recently it was difficult to predict exactly when and where they would occur.

For an eclipse to occur, the alignment between the eclipsed object (TNO here), the star, and the observing telescope must be extremely precise. In the past, it was nearly impossible to meet strict accuracy requirements to be sure you were observing an event. To achieve this goal, the European Research Council Lucky Star project, coordinated by Bruno Sicardi, Sorbonne University and Paris Observatory – PSL (LESIA), was created to predict future TNO eclipses and coordinate observations of these events with professionals and amateurs alike. Resource to observatories all over the world