Quaoar is one of about 3,000 dwarf planets in our Solar System’s Kuiper Belt. Astronomers discovered it in 2002. It’s only half as large as Pluto, about 1,121 km (697 mi) in diameter. Quaoar has a tiny moon named Weywot, and the planet and its moon are very difficult to observe in detail.
Astronomers took advantage of an occultation to study the dwarf planet Quaoar and found that it has something unexpected: a ring where a moon should be.
Occultations are observational gold to astronomers. Occultations occur when an object they want to observe lines up directly in front of a distant star. The star backlights the object bringing out all kinds of details.
In this case, a number of ground-based telescopes watched as tiny Quaoar occulted a distant star. But the ESA’s CHEOPS space telescope also watched. What the telescopes saw surprised astronomers. Outside Quaoar’s Roche Limit, a ring of debris sits where a moon should be.
These findings are in a paper titled “A dense ring of the trans-Neptunian object Quaoar outside its Roche limit.” The paper is published in the journal Nature, and the lead author is Bruno Morgado. Morgado is from the Universidade Federal do Rio de Janeiro, Brazil.
The Roche Limit is the distance from a planet inside of which a moon will be torn apart by the planet’s gravity. Outside the Roche Limit, dust and debris are expected to naturally coalesce into a moon. Saturn’s familiar rings are inside the gas giant’s Roche Limit, and the same is true for all other objects with rings that astronomers can see. But Quaoar’s ring is at a distance of almost seven and a half times the planet’s radius.
The ring wasn’t discovered in a single moment. It took multiple observations from 2018 to 2021 to confirm its existence. During that period, Quaoar occulted several distant stars, and each time it did, astronomers observed how it blocked the light. Each time the light dipped, it revealed more about the dwarf planet. But before and after each main occultation, there was another dip. Astronomers already knew about Quaoar’s tiny moon Weywot, an object only about 80 km (50 miles) in diameter; there was something else there.
The Hubble Space Telescope captured this image of Quaoar and its moon Weywot on 14 February 2006. Image Credit: By Hubble Space Telescope/Michael E. Brown, Public Domain, https://commons.wikimedia.org/w/index.php?curid=78835233
The ring wasn’t found by accident. Astronomers had to know in advance exactly when occultations would occur in order to study Quaoar and the other Trans-Neptunian Objects (TNOs.) And the occultations had to be precise.
Bruno Sicardy is an astronomy professor at the Paris Observatory at the Sorbonne. He led a project called Lucky Star that identified upcoming occultations by Quaoar so multiple telescopes could observe them.
The ESA’s Gaia mission made identifying these occultations easier. Gaia is a star-mapping project of unprecedented scope and precision. It allowed Sicardy to identify even more future occultations and observing opportunities for little Quaoar.
This is where the ESA’s CHEOPS comes in. CHEOPS stands for CHaracterising ExOPlanet Satellite, and it’s a space telescope that studies nearby bright stars that host known exoplanets. Kate Isaak, the ESA’s Project Scientist for the Cheops mission, wondered if CHEOPS could play a role by observing some of Quaoar’s occultations. She reached out to Lucky Star team member Isabella Pagano to see if CHEOPS could help.
Artist’s impression of Cheops, ESA’s Characterising Exoplanet Satellite, in orbit above Earth. In this view, the satellite’s telescope cover is open. Image Credit: ESA / ATG medialab
“I was a little skeptical about the possibility to do this with CHEOPS,” admits Pagano, “But we investigated the feasibility.”
Extreme precision is required to observe Quaoar’s occultations of distant stars, and that created Pagano’s skepticism. CHEOPS’ trajectory is subject to tiny variations caused by atmospheric drag. Changes in the Sun’s activity can affect the density of Earth’s upper atmosphere, which can disrupt CHEOPS. The team first used CHEOPS to observe an occultation by Pluto but were unsuccessful.
Then they tried again with an occultation by Quaoar, and this time it paid off. “The CHEOPS data are amazing for signal-to-noise,” said Pagano. Since CHEOPS is a space telescope, it doesn’t have to contend with Earth’s messy atmosphere and all the noise it introduces into observations. That clarity meant that the dips in starlight couldn’t be attributed to Earth’s atmosphere. Secondary observations with ground-based telescopes confirmed it.
Lead author Morgado combined the CHEOPS data with the ground-based observations. He also included amateur observations of Quaoar as it occulted different stars over the years. The result was a robust data set. “When we put everything together, we saw drops in brightness that were not caused by Quaoar, but that pointed to the presence of material in a circular orbit around it,” said Morgado. “The moment we saw that, we said, ‘Okay, we are seeing a ring around Quaoar.'”
This figure shows some of the observational data from the research. It’s from the HiPERCAM instrument on the Gran Canarias Telescope. The observed flux (black points) and the models (red lines) are plotted against the time relative to the observer closest approach. The blue-shaded regions are enlarged in corresponding underlying panels and show two occultations. Image Credit: Morgado et al. 2023.
The ring isn’t uniform. The authors describe it as ‘clumpy’ in their paper and similar to Saturn’s F-ring. The clumps are likely kilometre-sized moonlets, and they collide with one another and produce strands of tiny particles that re-accrete into larger objects again in a steady-state process.
“In summary, our observations are consistent with a dense, irregular Quaoar’s ring,” the authors write. “The term ‘dense’ means that collisions play a key role in its dynamics. However, in contrast to all other known dense rings, Quaoar’s ring orbits well outside the classical Roche limit.”
Now the team had a puzzle on their hands. Quaoar isn’t the only small object with rings. Ground-based observations found rings around the minor planets Chariklo and Haumea, too. But those rings are inside the planets’ Roche Limit. Quaoar’s isn’t.
This illustration from the study shows the environment around Quaoar. The Roche limit is different depending on bulk densities of particles, and the inner ring is the Roche limit for the density shown. The co-rotation radius is where the orbital period of the particles in the ring match Quaoar’s rotation. The two black lines represent two different ring solutions since the researchers aren’t exactly certain of their distance from the planet. The green and blue show the limits of the mean-motion resonance and the spin-orbit resonance of Weywat Quaoar, respectively. Image Credit: Morgado et al. 2023.
“So, what is so intriguing about this discovery around Quaoar is that the ring of material is much farther out than the Roche limit,” said Giovanni Bruno, INAF’s Astrophysical Observatory of Catania, Italy, who is one of the paper’s authors.
Quaoar’s ring appears to overturn a piece of foundational knowledge. Debris this far away from a planet should coalesce into a moon. “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,” said Giovanni.
It’s too soon to conclude why the rings are surviving so far away from the dwarf planet. The frigid temperature out there in the Kuiper Belt could be the cause because it could prevent the icy grains from adhering to one another. Only more observations can confirm that.
Whatever causes the ring, this unorthodox use of CHEOPS played a pivotal role in discovering it.
Astronomers aren’t done with Quaoar yet. They intend to observe more occultations to see what they can see and to refine their observations. At the same time, they’ll look to formulate a theory on why the planet’s ring survives so far from the planet.
As observational methods and tools improve, astronomers will likely find more of these small rings. Quaoar, Haumea, and Charikly are unlikely to be the only three. “Quaoar’s ring is the third example of a dense ring around a small body found in the Solar System, suggesting that more still await discovery,” the authors write. “Meanwhile, the large distance of this ring from Quaoar means that the classical notion that dense rings survive only inside the Roche limit of a planetary body must be revised.”