During a break from observing planets around other stars, ESA’s characterizing ExOPlanet Satellite (Cheops) mission has observed a dwarf planet in our own solar system and made a key 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 leaves astronomers with a puzzle to solve: Why hasn’t this material merged into a small moon?
How to observe distant objects in the solar system
The ring was discovered through a series of observations that took place between 2018 and 2021. Using a collection of ground-based telescopes and the space-based telescope Cheops, astronomers watched Quaoar pass in front of a series of distant stars, briefly blocking out their light as they passed.
Such an event is known as an occultation. Observing how the light from the occluded stardrops can provide information about the size and shape of the occluding object and whether the intervening object has an atmosphere or not. In this case, smaller blobs before and after the main occlusion revealed the presence of material in orbit around Quaoar.
Quaoar is one of a collection of small, distant worlds known as Trans-Neptunian Objects (TNOs). About 3000 are known. As the name suggests, TNOs are found 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, Quaoar ranks seventh on the size list and is orbited by a small moon named Weywot with a radius of about 80 km.
Studying these dwarf planets is difficult due to their small size and extreme distance. Quaoar itself orbits the Sun at almost 44 times the Sun-Earth distance. Occultations are therefore particularly valuable tools. However, until recently it was difficult to predict exactly when and where they will take place.
For occlusion to take place, the alignment between the occluding object (here the TNO), the star, and the observing telescope must be extremely precise. In the past it was almost impossible to meet the strict accuracy requirements to be sure to see an event. However, in pursuit of this goal, the European Research Council’s Lucky Star project, coordinated by Bruno Sicardy, Sorbonne University & Paris Observatory – PSL (LESIA), was launched to predict upcoming occultations of TNOs and coordinate the observation of these events professional and amateur observatories around the world.
Precise Alignment
Recently, the number of observed star occultations has increased. This is due in large part to the contribution of data from ESA’s Gaia mission to stellar mapping. The spacecraft has delivered such amazing accuracy in its stellar positions that the Lucky Star team’s predictions have become much more certain.
One of the people involved in the Lucky Star project is Isabella Pagano from the INAF Astrophysical Observatory in Catania, Italy, and a member of the Cheops Board. Isabella was contacted by Kate Isaak, ESA Project Scientist for the Cheops mission, who was curious if the space telescope would also be able to detect an occultation.
“I was a bit skeptical about the possibility of doing this with CHEOPS,” Isabella admits, “but we investigated the feasibility.”
The main problem was that the satellite’s trajectory can be easily altered due to drag in the upper parts of the Earth’s atmosphere. This is due to the unpredictable solar activity that can hit our planet and bloat its atmosphere.
When the team first attempted to observe an occultation of Cheops involving Pluto, the prediction was not entirely accurate and no occultation could be observed.
However, on the second try, the alignment was more favorable when they observed Quaoar. They discovered for the first time a stellar occultation by a trans-Neptunian object from space.
Put a ring on it
Artist’s impression of Cheops
“The Cheops data is amazing for the signal-to-noise ratio,” says Isabella. The signal-to-noise ratio is a measure of how strong the detected signal is over the random noise in the system. Cheops gives the noise a great signal because the telescope doesn’t see through the distorting effects of Earth’s lower atmosphere.
This clarity proved crucial for detecting Quaoar’s ring system, as researchers were able to rule out the possibility that the light drops were caused by a side effect in Earth’s atmosphere. By combining multiple secondary detections made with telescopes on Earth, it was possible to be sure they were caused by a ring system surrounding Quaoar.
Bruno Morgado, Universidade Federal do Rio de Janeiro, Brazil, led the analysis. He combined the Cheops data with that from major professional observatories around the world and amateur scientists, all of whom had observed Quaoar occulting various stars over the past several years. “Putting it all together, we saw dips in brightness that weren’t caused by Quaoar, but indicated the presence of material in a circular orbit around it. The moment we saw that, we were like, ‘Okay, we see a ring around Quaoar.’”
When it comes to ring systems, the giant planet Saturn holds the crown. Known as a ringed planet, Saturn features a collection of dust and small moons orbiting the planet’s equator. Although it is an impressive observation point, the mass of the ring system is quite small. If collected, it would represent between one-third and one-half the mass of Saturn’s moon Mimas, or about half the mass of Earth’s Antarctic Ice Shelf.
Quaoar’s ring is much smaller than Saturn’s, but no less fascinating. It is not the only known ring system around a dwarf or minor planet. Two others – around Chariklo and Haumea – were spotted by ground-based observations. However, what makes Quaoar’s ring unique is where it can be found relative to Quaoar himself.
The Roche limit
Any celestial object with an appreciable gravitational field has a boundary within which an approaching celestial object will be blown to bits. This is known as the Roche limit. Dense ring systems are expected to exist within the Roche limit, which is the case for Saturn, Chariklo, and Haumea.
“So the fascinating thing about this discovery around Quaoar is that the ring of material is much further out than the Roche limit,” says Giovanni Bruno, INAF Astrophysical Observatory in Catania, Italy.
This is a mystery, because according to conventional thinking, rings beyond the Roche limit will merge into a small moon within a few decades. “As a result of our observations, the classical notion that dense rings only survive within the Roche limit of a planetary body needs to be thoroughly revised,” says Giovanni.
Initial results suggest that the cold temperatures at Quaoar may play a role in preventing the ice particles from sticking together, but more research is 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 mission’s more typical exoplanet science,” says Kate.
While theorists work on how the Quaoar rings can survive, the Lucky Star project will continue to study Quaoar and other TNOs as well, while eclipsing distant stars to measure their physical properties and see how many others also have ring systems .
And Cheops will return to its original mission to study nearby exoplanets.
Note to editors
“A dense ring around the transneptunian object (50000) Quaoar well outside its Roche limit” by BE Morgado et al., is published in Nature. DOI: 10.1038/s41586-022-05629-6
More about Cheops
Cheops is an ESA mission developed in partnership with Switzerland, with a dedicated consortium led by the University of Bern and with key contributions from Austria, Belgium, France, Germany, Hungary, Italy, Portugal, Spain, Sweden and the United Kingdom.
ESA is the Cheops mission architect, responsible for the procurement and testing of the satellite, the launch and early operational phases, the commissioning in orbit and the guest observer program, through which scientists worldwide can apply to observe with Cheops. The consortium of 11 ESA member states led by Switzerland provided essential elements of the mission. The prime contractor for the design and construction of the spacecraft is Airbus Defense and Space in Madrid, Spain.
The Mission Consortium Cheops operates the Mission Operations Center at INTA in Torrejón de Ardoz near Madrid, Spain and the Science Operations Center at the University of Geneva, Switzerland.
For more information, visit: https://www.esa.int/Cheops
For more information please contact:
ESA Media Relations
Email: media@esa.int
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