This dwarf planet has a ring instead of a moon, and scientists don’t know why

By Jackie Wattles, CNN

Recent telescope data revealed that a small planet in the far reaches of our solar system has a dense ring round it. And scientists are baffled as to why.

The planet, Quaoar, is one of roughly 3,000 small planets that orbit the sun beyond Neptune, and at 690 miles (1,110-kilometers) wide, it’s about the seventh largest, with Pluto and Eris ranking as the biggest.

Observations of Quaoar made between 2018 and 2021 revealed that the planet has a ring sitting farther away from it than scientists previously believed to be possible, according to a news release from the European Space Agency, which used ground-based telescopes and a new space-based telescope called Cheops to collect the data.

Based on conventional thinking, all the material that makes up Quaoar’s dense ring should have condensed and formed a small moon. But it didn’t.

“Early results suggest that the frigid temperatures at Quaoar may play a role in preventing the icy particles from sticking together but more investigations are needed,” according to the news release.

Beyond the Roche limit

Before these new observations of Quaoar, scientists largely believed that it was impossible for planets to form rings beyond a certain distance. It’s a generally accepted rule of celestial mechanics that material in orbit around a planet will form a spherical object — or a moon — if it orbits at a far enough range away from the planet. But that moon will be ripped apart if it moves closer than what’s called the “Roche limit,” a point at which the planet’s tidal forces would be stronger than the gravity holding the moon together.

All the rings around Saturn, for example, lie inside of the planet’s Roche limit. What’s puzzling about Quaoar, however, is that its ring lies well beyond the planet’s Roche limit, in an area where the material should form a moon.

“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,” said Giovanni Bruno of INAF’s Astrophysical Observatory of Catania, Italy, in a statement.

How to study a small planet

Collecting the data that revealed Quaoar’s puzzling ring was in itself a cause for celebration, according to ESA. Because of the planet’s small size and distance from Earth, researchers wanted to observe it using an “occultation” — a means of observing a planet by waiting for it to be essentially backlit by a star, illuminating its silhouette.

That can be an extremely difficult process, according to ESA, because the telescope, planet and star must all be in perfect alignment. This observation was made possible by the space agency’s recent efforts to provide an unprecedentedly detailed map of the stars.

ESA also used Cheops, which was launched in 2019. Cheops typically studies exoplanets, or bodies that lie outside of Earth’s solar system. But for this instance, it set its sights on the nearer target of Quaoar, which orbits the sun even farther than Neptune — about 44 times farther than Earth’s orbit.

“I was a little skeptical about the possibility to do this with CHEOPS,” said Isabella Pagano, the director of the INAF’s Astrophysical Observatory of Catania, in a statement.

But it worked. And Cheops’ observation marked the first ever of its kind — an occulation of one of the most distant planets in our solar system by a space-based telescope, according to ESA.

Researchers then compared data collected by Cheops with observations by Earth-based telescopes, leading to their surprising revelation.

“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,'” said Bruno Morgado, a professor at The Federal University of Rio de Janeiro in Brazil, who led the analysis, in a statement.

Theoreticians — scientists who are experts in various theories — are now at work attempting to surmise how Quaoar’s ring survived, according to ESA.

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