Scientists discover ring around dwarf planet but can’t explain it

big picture: In addition to the iconic rings of Saturn, several other planets also have rings. Scientists still don’t fully understand their properties or how they form, and a recent discovery seriously casts doubt on what they thought they knew.

The dwarf planet Quaoar (pronounced “Quaoar”) lies far beyond Neptune in a region of solar system orbiting objects known as the Kuiper belt. At around 690 miles (1,100 km) in diameter, Quaoar is the seventh largest Kuiper belt object — about half the size of the Kuiper’s nearby known neighbor Pluto.

Astronomers lately confirmed the existence of a ring around Quaoar thanks to the telescope of the European Space Agency (ESA), characterizing the satellite of the exoplanet (Cheops). Watching Quaoar pass in front of distant stars, observers noticed that something else around the dwarf world was also obscuring those stars. They confirmed it was the ring by combining Cheops findings with data from ground-based telescopes taken between 2018 and 2021.

However, this discovery puzzled astronomers. Thanks to tidal forces, celestial objects with strong enough gravity, such as planets, will tear apart any smaller objects held together only by gravity that gets too close. We call the distance at which this happens the Roche limit. Quaoar’s ring orbits nearly seven and a half times the planet’s radius, far beyond the distance science dictates for moon formation.

Science has noticed that rings tend to form within the Roche limit, while the moons orbit around the boundary. We have seen this with other planets in our solar system, including Saturn and the dwarf planets Chariklo and Haumea. Quaoar’s ring lies beyond Rosh, which raises the question: why didn’t it merge into a second moon along with the planet’s only known moon, Weywot?

Currently, the ESA suggests that extreme cold at Quaoar’s orbital distance – about 44 times the distance between Earth and the Sun – prevents the chunks of ice that make up the ring from sticking together. ESA recognizes that more information is needed and that we need to rethink our understanding of ring systems and the Roche limit.

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