This Strange Comet-Asteroid Hybrid Is Like Nothing Else in The Solar System

In the distant space out beyond the orbit of Jupiter lurks a strange object.

Its name is Chiron, a type of outer Solar System body known as a centaur. But even among its fellow centaurs, Chiron is special – and new observations from JWST reveal how truly Chiron is like nothing else we've ever seen.

"It's an oddball when compared to the majority of other centaurs," says physicist Charles Schambeau of the University of Central Florida.

"It has periods where it behaves like a comet, it has rings of material around it, and potentially a debris field of small dust or rocky material orbiting around it. So, many questions arise about Chiron's properties that allow these unique behaviors."

The Solar System is brimming with rocky and icy remnants from its formation, some 4.5 billion years ago. Centaurs, which hang out primarily between Jupiter and Neptune, with orbits that cross the paths of at least one of the giant planets, are an interesting set.

Chiron, for instance, has properties similar to those of asteroids, but also sometimes has comet-like activity. Oh, and it has a ring around it, like a little miniature planetary ring.

Because Chiron is thought to be something of a time capsule of the formation of the Solar System, scientists are keen to learn more about this strange rock, but its distance makes it difficult to resolve.

Led by planetary scientist Noemí Pinilla-Alonso of the University of Oviedo in Spain, a team of researchers turned JWST's powerful eye to see if it could observe details that heretofore have eluded us.

"All the small bodies in the Solar System talk to us about how it was back in time, which is a period of time we can't really observe anymore," Pinilla-Alonso says. "But active centaurs tell us much more. They are undergoing transformation driven by solar heating and they provide a unique opportunity to learn about the surface and subsurface layers."

Chiron sometimes ejects gas and dust, like comets do. The researchers used JWST to try to see what that gas and dust is made of – not just what's in the space around the centaur in what is known as a coma (like the atmosphere of a comet), but what might be inside it.

"What is unique about Chiron is that we can observe both the surface, where most of the ices can be found, and the coma, where we see gases that are originating from the surface or just below it," Pinilla-Alonso says.

"Discovering which gases are part of the coma and their different relationships with the ices on the surface help us learn the physical and chemical properties, such as the thickness and the porosity of the ice layer, its composition, and how irradiation is affecting it."

They took observations in near-infrared, and teased apart the spectrum looking for absorption and emission lines that result from the absorption and re-emission of light at specific wavelengths by specific atoms and molecules.

Similar observations in the past revealed the presence of cyanide and carbon monoxide.

Pinilla-Alonso and her colleagues found carbon dioxide, carbon monoxide, ethane, propane, acetylene, methane, and water ice. The detection of methane is particularly notable, the researchers say, with results that suggest that Chiron's coma is full of it. There's a joke in there somewhere about centaur farts, but we're too mature to make it.

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Scientists think that centaurs were once members of an even more distant population of Solar System icy rocks, the trans-Neptunian objects, or TNOs, that orbit the Sun out past the orbit of Neptune.

Because they are so far from the Sun, TNOs are thought to be relatively pristine remnants of the formation of the Solar System, encapsulating the early composition of the cloud of dust from which the Solar System was born.

As they come closer to the Sun, the ice in the centaurs can sublimate, releasing some of that material. The researchers believe that the methane, carbon dioxide, and water ice they observe on Chiron could be some of the pristine materials it inherited from the birth of the Solar System.

Other compounds, such as the ethane, propane, and acetylene, could have formed on the centaur's surface as the result of reduction and oxidation – that is, rusting.

"Based on our new JWST data, I'm not so sure we have a standard centaur," Pinilla-Alonso says.

"Every active centaur that we are observing with JWST shows some peculiarity. But they cannot be all outliers. There must be something that explains why they appear to all behave differently or something that is common between them all that we cannot yet see."

The researchers plan to continue to observe Chiron as it moves closer to us, to see if there's anything about the way it changes over time and seasons that gives more clues about the natures of these strange, icy, erupting rocks.

The research has been published in Astronomy & Astrophysics.