Dwarf planet Quaoar: The largest ring system in the solar system is a mystery


A research team has discovered a ring on the dwarf planet called Quaoar that, according to current theories, should not even exist. Orbiting in the frigid outer reaches of the solar system, the mini-planet has a Saturn-like ring of dust and debris that defies the rules of physics.

Astronomers have discovered a new ring system in our solar system. It is centered around a newly discovered dwarf planet called Quaoar, about half the size of Pluto, orbiting the Sun beyond Neptune in the Kuiper Belt, a ringed formation of rocky and icy debris in the outer Solar System. The discovery was made by an international team of astronomers using HiPERCAM, an extremely sensitive, high-speed camera developed by scientists at the University of Sheffield. It is attached to the largest optical telescope in the world, the Gran Telescopio Canarias, on La Palma.

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The ring was discovered while observing a series of occultations, essentially eclipses, as Quaoar passed between Earth and several more distant but much brighter stars. During an occultation, the light from the background star temporarily dims. Visible only to very sensitive telescopes, this effect is often used to detect exoplanets orbiting stars in our Milky Way galaxy. When the astronomers analyzed the data recorded during the observation, they found that in addition to the main drop in the brightness of the background stars, they could also detect two smaller dips. Because the drops occurred before and after the main occultation, the researchers hypothesized that Quaoar must be surrounded by a ring.

That alone wouldn’t be so special. The gas giant Saturn is known to have a whole series of rings in which solid particles collect and orbit the celestial body. Jupiter, Neptune and Uranus also have them. Likewise, two minor planets named Chariklo and Haumea. So what exactly makes the Ring of Quaoar so special? While all other rings in the solar system are within or near a mathematically determined distance from their parent bodies, Quaoar’s ring is much further away.

“That the ring of Quaoar is outside of this boundary is very, very strange,” Giovanni Bruno, an astronomer at Italy’s National Institute for Astrophysics (INAF) and one of the study’s authors, is quoted as saying in a statement from the European Space Agency (ESA). Because Quaoar’s ring is at a very unusual distance from his mother’s body. According to the astronomers involved in the study, the ring system around Quaoar is remarkable because it is located at a distance of more than seven planetary radii – twice what was previously thought to be the upper limit – the so-called “Roche limit “, where it was believed that ring systems could survive.

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Until now, scientists believed that material orbiting a planet beyond a certain point would form a moon. If that moon then moves too close to the planet – a line known as the “Roche limit” – it will be torn apart by the planet’s tidal forces. Based on these results, the general notion that rings only survive within the Roche limit must now be completely revised, the astronomers explained.

With a radius of about 3,885 kilometers from the center of Quaoar, its ring is so far from the dwarf planet that its gravity should no longer be able to disperse the ring-forming material. Instead, it should coalesce under its own gravity, forming a moon. In failing to do so, the ring has breached what astronomers call the Roche limit – the first known ring around a celestial body to have done so. Therefore, the discovery has now forced a rethink of ring formation theories.

To explain the quaoar system, Bruno and his colleagues then observed other small ring objects. They found that both the rings of Haumea and those of the asteroid Chariklo lie near regions where the ring particles complete one orbit for every three revolutions of the parent body. The irregular structure of these planetoids – in an ellipsoidal shape – could lead to gravitational perturbations at these sites, preventing the ring material from expanding.

Thus, at Haumea and Chariklo, the so-called “orbital resonances” coincide near their Roche limits. An orbital resonance occurs when two or more celestial bodies are subject to periodically recurring gravitational influences. In contrast, for Quaoar, the resonance occurs far beyond the Roche distance, very close to the discovered ring position. Another resonance caused by the gravity of the Quaoar moon Weywot occurs in the same region.

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The researchers also ran simulations to explain how those events might result in the ring material not assembling into another satellite. “As a result of our observations, the classical notion that dense rings only survive within the Roche limit of a planetary body needs to be thoroughly revised,” says Bruno. Because the first observations and investigations of the planet already throw light on the question of why the ring system holds. According to them, the frigid temperatures on the ringed planet could prevent the ice particles in the ring from sticking together.

The scientists now want to carry out further investigations to find out more. In the meantime, however, until new results are available, astronomers must either reconsider the Roche limit or come up with another explanation for the existence of the ring around Quaoar. The study was published Wednesday in the journal Nature published.

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