Tiny dwarf planet Quaoar has a mysterious ring

The European Space Agency’s CHEOPS telescope usually searches for planets outside our solar system, but recently it made a discovery closer to home: a large ring around the dwarf planet Quaoar which has researchers intrigued.

An artist’s impression of the dwarf planet Quaoar and its ring. Quaoar’s moon Weywot is shown on the left. Quaoar’s ring was discovered through a series of observations that took place between 2018 to 2021. Using a collection of ground-based telescopes, and ESA’s 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. ESA; Acknowledgement: Work performed by ATG under contract for ESA

The ring was spotted around the dwarf planet Quaoar, located in the Kuiper belt around 44 times farther from the sun than the Earth is. The planet itself is small, at just 690 miles across, but the ring around it is much larger — at seven and a half times its radius.

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Because of the planet’s small size and far distance from the sun, it is hard to observe even using a powerful space-based telescope like CHEOPS. To observe the dwarf planet, researchers had to wait until it passed in front of distant background stars and blocked out their light in events called occultations. But these events are rare and hard to predict.

“I was a little skeptical about the possibility to do this with CHEOPS,” said one of the CHEOPS researchers, Isabella Pagano, in a statement. “But we investigated the feasibility.”

It took several attempts, but the team was able to observe an occultation and were very pleased with the results. “The Cheops data are amazing for signal to noise,” Pagano said.

These results enabled them to see the dwarf planet and its ring. “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. The moment we saw that we said, ‘Okay, we are seeing a ring around Quaoar,’” said lead researcher Bruno Morgado.

There’s something strange about this ring though. Quaoar has a small moon called Weywot, and astronomers would expect that the material in the ring would also coalesce into a moon. When the material gets close to a massive body like a planet, it is pulled apart by gravity once it passes a point called the Roche limit to form a ring. But Quaoar’s ring is well outside the Roche limit.

The team is puzzling over this finding, and theorizes that it could be due to the very cold temperatures of Quaoar which are stopping the particles in the ring from sticking together to form a moon. But for now, the exact reason remains a mystery.

The research is published in the journal Nature.