The James Webb Space Telescope (JWST) has detected that the centaur 2060 Chiron is a distinctive object among the celestial bodies of the outer Solar System. This entity, first spotted in 1977, spans a diameter of approximately 218 kilometers and orbits between Jupiter and Neptune. Centaurs are believed to originate from the frozen realm beyond Neptune, migrating inward due to gravitational disturbances induced by this icy giant. As they near the Sun, solar warmth may cause certain ices to sublimate, forming a gaseous halo or coma around the centaur, akin to the behavior of comets.
Charles Shambeau from the University of Central Florida describes Chiron as a “uniqueness” among other centaurs and even among trans-Neptunian objects (TNOs), which possess their own captivating histories. “It has periods when it behaves like a comet, it has rings around it, and possibly a debris field of fine dust or rocky material orbiting around it,” Shambeau remarked.
Recent JWST observations of Chiron, led by Shambeau and Noemi Pinilla-Alonso from the University of Oviedo in Spain, have unveiled that Chiron’s surface ice composition is markedly different from any other centaur observed before. Although none of the ices, in isolation, are especially peculiar, their combination on Chiron has come as a surprise.
The James Webb Telescope discovered carbon monoxide and carbon dioxide ice on its surface and carbon dioxide and methane in Chiron’s thin coma. The presence and abundance of methane align with the material sublimating from the ice in the most sun-warmed region of the surface. Although its temperature never rises above -220 degrees Fahrenheit (-140 degrees Celsius), this is sufficient to cause these ices to sublimate.
Moreover, solar radiation interacting with these ices initiates chemical reactions, generating organic byproducts such as acetylene, ethane, propane, and various carbon oxides, all identified by JWST as ice on Chiron’s surface.
“Discovering which gases comprise the coma and their different relationships to the surface ices aids our understanding of their physical and chemical properties, such as the thickness and porosity of the ice layer, its composition, and how radiation affects it,” stated Pinilla-Alonso.
Centaurs and trans-Neptunian objects are thought to have remained unchanged since their formation 4.5 billion years ago during the early Solar System. They offer insights into how and from what the Solar System was created, the formation of specific objects in the protoplanetary disk around the young Sun, and whether these objects have migrated since then. Active centaurs like Chiron are especially valuable because they reveal much more than inert objects.
“They undergo transformations triggered by solar heating and provide a unique opportunity to learn about surface and subsurface layers. Chiron’s uniqueness is that we can observe not only the surface, where most of the ice is located, but also the coma, where we see gases originating from the surface or just below it,” said Pinilla-Alonso.
Chiron’s 50-year elliptical orbit around the Sun reached its aphelion—the farthest point from the Sun at 18.87 astronomical units (2.8 billion kilometers) in 2021. It will next reach its perihelion in 2047, coming within 8.5 astronomical units (1.27 billion kilometers) from the Sun, just inside Saturn’s orbit. As Chiron approaches the Sun over the next 20 years, it will grow brighter and more active, enabling more precise observations of its ice abundance and nature, organic chemistry, and how solar radiation and sunlight may affect its frozen surface.
Chiron and its fellow centaurs are in a transitional phase—over the next million years, its fate will be determined. Either the family of Jupiter will scatter it inward where it would become a Jupiter-family comet with an orbital period of less than 20 years, or it will be ejected back into the Kuiper Belt.