Dwarf planet Quaoar has a ring!

Scientists have labeled nine objects in our solar system as dwarf planets, or petite-sized planets that have not cleared their orbits. Pluto is one, and Ceres in the asteroid belt is another. Quaoar is also a dwarf planet. Astronomers discovered it out beyond Neptune’s orbit in 2002. Then, in 2007, they discovered that Quaoar has a moon, which they named Weywot. On February 8, 2023, ESA announced a surprise discovery: Quaoar has a ring! And, it’s rather far away from the dwarf planet. According to theory, at the ring’s distance, it should instead have formed into another moon. Why didn’t it?

The peer-reviewed journal Nature published the study on Quaoar and its ring on February 8, 2023.

Now on sale! The 2023 EarthSky lunar calendar. A unique and beautiful poster-sized calendar showing phases of the moon every night of the year. Treat yourself!

A surprising ring for dwarf planet Quaoar

Quaoar is about 690 miles (1,110 km) in diameter. Its moon, Weywot, is about 110 miles (170 km) in diameter and lies 9,000 miles (14,500 km) away from Quaoar. The newly discovered ring orbits at a distance of about 2,500 miles (4,100 km). That’s far enough away to be outside the dwarf planet’s Roche limit. Inside the Roche limit, tidal forces prevent material from coalescing into a moon. But scientists expected that material outside that limit would be a moon, not a ring.

So, as the paper explained:

… this limit does not always determine where ring material can survive.

The ring’s discovery

Using both ground-based telescopes and the space-based telescope CHaracterising ExOPlanet Satellite (Cheops), astronomers observed Quaoar between 2018 and 2021. As Quaoar passed in front of distant stars, they watched the background star’s light wink out. But the light also had small dips in brightness before and after Quaoar, indicating a ring of material blocking the background star. Events like these are called occultations.

Interestingly, this is not how astronomers found the moon Weywot. The Hubble Space Telescope was able to get a clear enough view of Quaoar that Weywot showed up in one of its images.

How can the ring exist?

So, if the ring is outside the Roche limit, how can it exist? Other solar-system bodies that have ring systems, including Saturn and the dwarf planets Chariklo and Haumea, have rings inside their Roche limits. One of the paper’s authors, Giovanni Bruno of INAF’s Astrophysical Observatory, said:

What is so intriguing about this discovery around Quaoar is that the ring of material is much farther out than the Roche limit. 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.

One of the early speculations for how this ring can be so close to Quaoar, relates to the cold temperatures out in this region of space. Astronomers think the cold may keep the icy particles from sticking together.

In the meantime, scientists will continue theorizing how Quaoar has a distant ring while the search for ring systems around other distant solar system objects continues.

Bottom line: Scientists discovered that the dwarf planet Quaoar has a ring! It’s far enough away from Quaoar that scientists would have expected it to form into a moon.

Source: A dense ring of the trans-Neptunian object Quaoar outside its Roche limit

Via ESA