THE KUIPER BELT (WHTM) — Astronomers at the European Space Agency got quite a surprise recently when they observed a dwarf planet in the Kuiper Belt, the doughnut-shaped ring of icy objects which starts beyond the orbit of Neptune. What they found exploded an assumption about how rings form around planets.
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The dwarf planet involved is Quaoar, which was discovered on June 4, 2002, by American astronomers Chad Trujillo and Michael Brown at the Palomar Observatory. It was named after the god of creation of the Native American Tongva people in Southern California. In February 2007, Brown discovered a moon orbiting Quaoar, which was named Weywot, after the son of Quaoar. Quaoar has a radius of 345 miles. Weywot is about 50 miles radius, and orbits about 9,000 miles from Quaoar.
From 2018 to 2021, Astronomers made a series of observations of Quaoar, using ground-based telescopes and ESA’s Cheops (CHaracterising ExOPlanet Satellite). They watched for occultations, moments when the dwarf planet blocks light from a star. Occultations can reveal all kinds of information about the occulting object; size, shape, and even the presence of an atmosphere. But there were also smaller drops in the amount of starlight both before and after the main occultation event, which indicated material orbiting Quaoar.
The astronomers combined the Cheops data with data from ground-based telescopes. The person who led the analysis, Bruno Morgado of the Universidade Federal do Rio de Janeiro said “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.’”
Being able to detect a ring around a planet that’s about 4.06 billion miles from the sun is a remarkable achievement. What’s even more remarkable was that the astronomers were able to determine the ring’s distance from Quaoar-about 2,587 miles. And that’s what’s exploding a long-held assumption. The ring is way outside Quaoar’s Roche limit.
And what, may you ask, is a Roche limit? The term is named after Édouard Roche, a French astronomer who calculated it back in 1848. It’s all about a planet’s gravitational pull and the effect it will have on the material orbiting it. (Different-sized planets will have different-sized Roche limits.) Inside the Roche limit, rings will form, because the planet’s gravity will either keep smaller particles from clumping together or rip larger objects apart. Outside the Roche limit, rings will coalesce into moons, possibly within a few decades. All the rings around all the planets in our Solar System have them inside their Roche limits, including Saturn, Jupiter, Uranus, Neptune, the asteroid Chariklo which orbits the Sun between Saturn and Uranus, and Haumea, another dwarf planet in the Kuiper Belt.
So what makes Quaoar different? For now, we have no answers, just speculation. It could be it’s so cold (flirting with absolute zero) around Quaoar that icy particles can’t stick together. Or perhaps the ring itself is fairly new, and we might see a moon forming there in a few decades. More educated people than I will no doubt make more educated guesses. But for now, the last word goes to Giovanni Bruno of the Italian National Institute for Astrophysics.
“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.”