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Minor Planet Chiron Doesn't Have Rings After All – It's Something Weirder

We only get to check on this strange phenomenon every few years, and each time it has looked different.

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Stephen Luntz

author

Stephen Luntz

Freelance Writer

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

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This vision of Chiron turns out to be doubly wrong - not only is the minor planet not spherical, but instead of rings it has a shifting disk

This vision of Chiron turns out to be doubly wrong - not only is the minor planet not spherical, but instead of rings it has a shifting disk.

Image Credit: Celestia Team - Celestia, GPL, 

What astronomers in 2011 took for rings around the minor planet Chiron is actually something much stranger, new research reveals. The true nature of the material orbiting the icy world remains unknown, but it appears to be an ever-shifting disk of dust and gas.

The initial discovery of Chiron, an object around 210 kilometers (126 miles) across, marked a significant development in our understanding of the outer solar system. With an orbit that varies from just inside Saturn to near Uranus, Chiron was the first of a new class of objects with similar locations, known as centaurs. It Is a testimony to how much astronomy has advanced that, since Chiron’s discovery in 1977, hundreds of centaurs have been found – including one of the strangest objects in the Solar System.

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No spacecraft has visited Chiron, or any centaur (suspected retired centaur Phoebe aside), and it’s too distant for Earth-based telescopes to see it clearly. However, we learned something dramatic about it after it passed in front of (occulted) an unnamed star in 2011. This revealed the presence of a ring system around Chiron – or at least we thought it did. More recent passages indicate we misinterpreted the evidence.

The case for rings around Chiron came from the fact that, just before and after Chiron’s passage in front of the star, it was seen to dim and brighten again. The process happened four times: twice before and twice after the main event. This led astronomers to conclude the icy world was surrounded by two rings, each capable of partially obstructing light from more distant stars.

The discovery was a surprise, since Chiron’s gravity is so small we don’t know how it could hold onto a ring system. However, in addition to all four of the Solar System’s gas giants having rings, one had been found around Chariklo (the largest centaur) not long before, so this was not completely out of the blue. Apparently, putting a ring on it is all the rage out past Jupiter. Since then, other candidates suggest this happens a lot

Evidence that things weren’t so simple came from an occultation of a different star in 2018. Although this also showed dips in brightness before and after Chiron passed in front of the star, the drops were only about half as large as seen in 2011, and there were three pre-occultation dips, not two. 

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“The locations and amounts of material that were detected around Chiron are different enough from previous observations to suggest that there is not a stable ring system but rather surrounding material that is currently evolving,” said study author Dr Amanda Sickafoose of the Planetary Science Institute in a statement

Chiron's 2018 occultation was mostly visible from the ocean, so it was fortunate one large telescope narrowly made it into the area where it could be seen
Chiron's 2018 occultation was mostly visible from the ocean, so it was fortunate one large telescope narrowly made it into the area where it could be seen
Image Credit: A. Sickafoose/PSI.

Chiron occulted another star on December 15, 2022. Although the star in this case was brighter, observations of the event had lower resolution. What was seen is indicative of a broad lumpy disk, with concentrations inconsistent with those of either the 2011 or 2018 event.

“There is material orbiting around Chiron that is evolving on relatively short timescales,” Sickafoose continued. 

Overall, it appears there was less material to block out starlight around Chiron in 2018 than in 2011, but more by 2022. This may be the result of an outburst in February 2021 that saw Chiron brighten noticeably, possibly because it threw off material that eventually bolstered its disk.

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Like many other centaurs, Chiron has shown signs of comet-like activity. Even the closest part of its orbit is too far from the Sun for it to get very warm, but some of its more volatile gasses do escape, and how this relates to the disk is currently unclear.

Whether Chariklo and other objects with apparent ring systems also have something more complex is also a question that needs further investigation, as is what is causing the changes observed between 2011 and 2018.

On the other hand, the 2018 observations were able to rule out any substantial atmosphere, such as might have been created by large-scale escaping gasses.

Small objects like Chiron only block out light reaching a small portion of the Earth. In the case of the 2018 occultation, most of that was ocean – but fortunately, the event was visible to the 1.9-meter telescope at the South African Astronomical Observatory, as well as a smaller one at the same site.

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Standard declaration. Chiron should not be confused with Pluto’s moon Charon, but often is.

The study of the 2018 occultation is published open access in The Planetary Science Journal


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spaceSpace and PhysicsspaceAstronomy
  • tag
  • occultation,

  • Astronomy,

  • Chiron,

  • Centaurs,

  • minor planet rings

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