The James Webb Space Telescope (JWST) has identified Centaur 2060 Chiron as a distinctive entity among the outer Solar System’s celestial bodies. Initially spotted in 1977, this object measures approximately 218 kilometers in diameter and follows an orbit situated between Jupiter and Neptune. Centaurs are generally thought to originate from an icy region beyond Neptune, eventually migrating inward due to gravitational disruptions caused by the ice giant. As they approach the Sun, solar heat can lead to the sublimation of certain ices, forming a gaseous halo or coma around the centaur, akin to comets.
Charles Shambo from the University of Central Florida characterizes Chiron as a «singularity» among other centaurs and even among trans-Neptunian objects (TNOs), which possess their own captivating histories. «It exhibits phases where it behaves like a comet, possesses rings, and potentially a debris field of fine dust or rocky material revolving around it,» explained Shambo.
Recent JWST observations of Chiron, led by Shambo and Noemí Pinilla-Alonso from the University of Oviedo in Spain, revealed that Chiron’s surface ice composition is remarkably different from any other centaur previously observed. While none of the constituent ices are particularly rare, their unique combination on Chiron was a revelation.
The James Webb Telescope has identified carbon monoxide and carbon dioxide ice on the surface, along with carbon dioxide and methane within Chiron’s thin coma. The presence and abundance of methane are consistent with material sublimating from ice on surface areas exposed to the most intense heat. Although its temperature never exceeds -220 degrees Fahrenheit (-140 degrees Celsius) due to its distance from the Sun, this is sufficient to trigger the sublimation of these ices.
Furthermore, solar radiation on these ices induces chemical reactions that generate organic byproducts like acetylene, ethane, and propane, along with various carbon oxides—all of which JWST detected as surface ice on Chiron.
«Uncovering which gases are part of the coma and their different relationships with surface ices helps us understand physical and chemical characteristics such as the ice layer’s thickness, porosity, composition, and radiation effects,» stated Pinilla-Alonso.
It is believed that centaurs and trans-Neptunian objects have remained unchanged since their inception 4.5 billion years ago at the dawn of the Solar System. They provide insights into how the Solar System was structured, detailing the formation locations of certain objects within the protoplanetary disk surrounding the young Sun and whether these objects have migrated from their original positions. Active centaurs, like Chiron, hold particular significance as they reveal far more than inert objects.
«Undergoing transformation due to solar warming, they offer a unique chance to explore both surface and subsurface layers. Chiron’s distinctiveness lies in our ability to observe both the surface, where most of the ices reside, and the coma, where gases originate from the surface or just beneath it,» Pinilla-Alonso elaborated.
Chiron’s 50-year elliptical orbit around the Sun reached its aphelion—the furthest point from the Sun, at 18.87 astronomical units (2.8 billion kilometers)—in 2021. It is next poised to reach perihelion in 2047, coming within 8.5 astronomical units (1.27 billion kilometers) of the Sun, just inside Saturn’s orbit. As Chiron moves closer to the Sun over the next two decades, it will brighten and become more active, allowing for more precise observation of the abundance and nature of its ices, organic chemistry, and the impact of solar irradiation on its icy surface.
Chiron and its fellow centaurs are in a transitional phase—over the next million years, its fate will be determined. Either Jupiter’s gravitational influence will disperse it into the solar system, transforming it into a Jupiter-family comet with an orbital period of less than 20 years, or it will be ejected back to the Kuiper Belt.