Astronomers watch an icy centaur begin behaving like a comet

Astronomers have documented a rare early stage in the transformation of 450P/LONEOS, an icy body orbiting beyond Jupiter. Once largely inactive, it has developed a brightening cloud of gas and dust around its solid core, a feature known as a coma.

Observations with the Gemini North telescope detected the cloud, which grew brighter between 2019 and 2024. James Webb Space Telescope data identified carbon dioxide and ice particles within it, while no water vapor was detected. The findings indicate that solar heating is releasing carbon dioxide and carrying dust away from the surface, producing the comet-like activity.

The change appears to trace back to 1992, when the object passed about 4.6 million kilometres from Saturn. The planet’s gravity altered its orbit and brought its closest approach to the Sun down to roughly 813 million kilometres. That exposed the frozen body to more sunlight and heat.

Classified as a centaur, 450P travels among the giant planets and currently takes about 22 years to orbit the Sun. It does not yet meet the orbital definition of a Jupiter-family comet, whose periods are under 20 years. But continued activity and another gravitational encounter could eventually complete that transition, giving scientists a valuable view of how dormant ice-rich bodies become active comets.