The Comet-Asteroid Chiron Has Rings — And They're Changing

Chiron is weird. When found in 1977, it was the farthest minor planet known at the time. Its unusual orbit between Saturn and Uranus made it the first of a new class of solar system objects: the Centaurs. Around 1,000 Centaurs are known today, but Chiron remains special: In 2011, astronomers made observations that were later claimed as evidence of a possible ring system. Now, with two new recently published studies, it has become clear that this system has undergone some dramatic changes. 

José Ortiz (Institute of Astrophysics of Andalucía, Spain) and his colleagues discovered Chiron’s rings, inspired by the discovery of rings around 10199 Chariklo. In 2015, they reanalyzed data gathered during stellar occultations in 1993, 1994 and 2011, finding a possible double ring circling Chiron at approximately 324 km (201 miles). (Those results appeared in Astronomy & Astrophysics.) The structures explained Chiron’s brightness variations as well as variable amounts of water ice found in multiple spectra.

More recently, Amanda Sickafoose (Planetary Science Institute) and colleagues analyzed data from a 2018 stellar occultation, when Chiron blocked out a background star. They published results in the Planetary Science Journal. A second team, led by Ortiz, published another occultation, this one having occurred in December 2022, in Astronomy & Astrophysics.

Both investigations clearly show that Chiron’s ring system isn’t stable like those found around other Centaurs, such as the two rings circling Chariklo. “The structure in Chiron has evolved very significantly from 2011 to 2022,” Ortiz and his team conclude. 

Half Asteroid, Half Comet

Chiron has long been known to be an oddball. It is one of the few small solar system bodies with a double personality. Originally classified as an asteroid (2060 Chiron), it also shows behavior more akin to a comet. In addition to the variable water in its spectra, it also produces jet-like dust structures, found during the 1993-1994 occultations. That’s why Chiron also bears a comet’s designation, 95P/Chiron.

Its dual nature is reflected in the naming of its family: In ancient Greek mythology, centaurs were mystical creatures with the upper body of a human and the lower body of a horse. Chiron was the “wisest and noblest” among them.

In astronomy, the origin of these strange objects is still debated. Frequent interactions with the large outer planets destabilize Centaur orbits, so ultimately, their fate is to end up as a Jupiter-family comet or be ejected from the solar system. To still have so many Centaurs present today, an outside source must constantly replenish them. Theory suggests that source is the Kuiper Belt: It’s likely that many Centaurs are actually comets, with complex environments of dust and water ice. But confirming these ideas with observations isn’t easy.

Chiron's Stellar Occultations

Because they are far away and small, directly imaging Centaurs is a challenge for today’s telescopes. Instead, astronomers carefully observe and analyze stellar occultations to measure Centaurs’ size and probe their environments.

Most recently, Chiron has occulted faint stars in 2018 and 2022. Sickafoose and her colleagues used the 1.9-meter and 1-meter telescopes at the South African Astronomical Observatory to record the 2018 event. While the smaller telescope only captured the occultation by the main body, the larger one saw the star dim four additional times.

Those drops in starlight occurred around the positions of the two rings that Ortiz’s team had proposed. Surprisingly, though, the background star didn’t dim as much as expected from the previous observations. The ring material seems to have decreased over a period of seven years, though it’s unclear why. The rings might have lost material to space or to Chiron’s surface.

When Chiron occulted a 12.7-magnitude star in December 2022, both the 1.88-meter telescope at Kottamia Astronomical Observatory in Egypt as well as the 0.45-meter telescope at the Wise Observatory in Israel captured up to six additional drops, consistent with up to three different rings.

The newest observations seen to indicate that the rings have grown in number, from two to three (as seen in the diagram below), as well as in density. Compared to 2011 and 2018, the brightness of the background star dipped more deeply in 2022, as if something had replenished the occulting material.

Cometary Activity?

Ortiz and his team suspect an explanation might come from a brightening event in early 2021. That year, Chiron’s brightness rose about 0.6 magnitude, and it hadn’t returned to pre-outburst levels by the time of the 2022 observations.

Sunlight might have heated icy material below Chiron’s surface; the original brightening would have occurred when this material was ejected into space. Unlike a purebred comet though, the ejection didn’t go far. The material remained close to the main body, never appearing like a comet to observers on Earth.

Or Mass Collision?

Alternatively, Chiron might have crossed a swarm of debris from a disintegrated centaur or comet. Those impacts could have released dust or ice particles, causing the brightening. Archival evidence supporting this scenario: A similar event preceded the 2021 brightening almost 50 years ago, which is close to Chiron’s orbital period.

“We don’t yet know the connection between [the density] of surrounding material that we detect in stellar occultations and activity on Chiron,” adds Sickafoose.

So far, Chariklo and Chiron are the only Centaurs with known rings, but rings also surround the trans-Neptunian objects Haumea and Quaoar, which are farther out. Quaoar’s ring seems to contain at least one dense arc as well as additional tenuous material, which suggests that the inhomogenities found in Chiron’s rings might be common to ring-like structures in the outer solar system.

“At this point, small-body rings are an emerging field of study,” concludes Sickafoose. “We need more observations to understand their formation and evolution mechanisms.”