Rings in the Solar System aren’t exactly rare. Half of the planets have them, and others may have them in the past. Some asteroids have rings, as does the dwarf planet Haumea. Even the Sun has some kind of rings.
Now, astronomers have discovered an entirely new ring system. Only this one left them scratching their heads as it is unlike anything else in the Solar System.
Quaoar, a small dwarf planet that lies in the Kuiper Belt beyond Pluto, is also surrounded by a dense ring – a ring circling at such a great distance that it should still be joined together like a moon.
The discovery means that scientists may need to revise our understanding of how moons and rings form and are affected by gravitational interaction with their larger companion.
Quaoar, measuring just 1,110 kilometers (690 miles) in diameter, was discovered in 2002 and over the years has grown into a rather interesting little ball of rock. It shows signs of ice volcanism and even has a small moon called Weywot, just 170 kilometers across.
But in 2021, astronomers noticed something else. They were watching Quaoar, a dark shadow in the outer reaches of the Solar System, move to obscure a distant star, a type of observation called occultation. Observations from a ground-based telescope in Australia suggested the dwarf planet may be harboring a ring.
Led by astronomer Bruno Morgado of the Federal University of Rio de Janeiro in Brazil, a team of astronomers set out to see if they could find more evidence. But you can’t just, you know, point a telescope at a small dwarf planet in the deep darkness of the Kuiper Belt and expect to see any detail, let alone see the rings themselves.
Instead, the researchers had to rely on data collected by multiple ground-based telescopes between 2019 and 2020, looking for more occultations, as well as observations about occultation that he and his team collected in 2021 using European Space Agency (ESA) characterization. . ExOPlanet Satellite (Cheops). Cheops, based on space, was needed to show that a ring’s earthly hints were not the effect of atmospheric distortion.
Ultimately, the combined data provided more than just a hint.
“When we put it all together, we saw dips in brightness that were not caused by Quaoar, but that pointed to the presence of material in a circular orbit around it,” says Morgado. “The moment we saw that, we said, ‘Okay, we’re seeing a ring around Quaoar’.”
The data allowed the researchers to characterize the ring, and this is where it got really weird. The ring is orbiting the dwarf planet at a distance of 4,100 kilometers from Quaoar’s center, or approximately 7.4 Quaoar radii. (Weywot, for the record, is much farther away, at a distance of 24 radii). But the Roche de Quaoar boundary is only 1,780 kilometers from the center of Quaoar.
The Roche limit is a critical distance from a body at which tidal forces – that is, gravity – will pull it apart into a messy pile of debris, as the gravity of the larger body exceeds the gravity needed to hold the smaller body together.
Once a sizable object crosses Roche’s boundary, you can reasonably expect it to be reduced to rubble that soon turns into a ring. Outside Roche’s boundary, what you should find are intact moons.
Of course, debris can exist beyond the Roche boundary, but it should still clump together over a relatively short period of time, just a few decades, and merge into a moon of sorts. No other body in the Solar System has rings outside the Roche boundary.
“What’s so intriguing about this discovery around Quaoar is that the ring of material is much further away than the Roche limit,” says astronomer Giovanni Bruno of Italy’s National Institute of Astrophysics.
“As a result of our observations, the classical notion that dense rings survive only within the Roche limit of a planetary body must be revised completely.”
More research will need to be done to find out why the ring didn’t turn into a moon, but there are several possible explanations. One is that the debris that makes up the ring is, for some reason, more likely to bounce off than stick together. Another possibility is that Weywot, or even an as-yet-undetected moon Quaoar, is providing gravitational perturbations that keep the ring collision velocity high enough to avoid clumps.
But the discovery also suggests that there may be more such rings out there, orbiting smaller objects in the Solar System, which have yet to be found. Maybe, in one of them, we can also find some answers.
The research was published in Nature.