Astronomers observe centaur 450P transforming into comet 3 billion miles from Earth
First-time observations of a distant icy body developing comet-like activity provide a missing evolutionary link between inert centaurs and active comets.
What happened
Astronomers led by Charles Schambeau at the University of Central Florida have observed object 450P/LONEOS, a centaur orbiting beyond Jupiter, developing a coma—a cloud of gas and dust—for the first time. Using the Gemini North telescope and the James Webb Space Telescope, they detected carbon dioxide gas, crystalline ice particles, and dust in the forming coma between 2019 and 2024 as 450P reached perihelion in August 2024. Orbital analysis reveals that 450P came within 2.9 million miles (4.6 million kilometers) of Saturn in 1992, an encounter that significantly shortened its orbit and brought its closest approach to the sun (perihelion) to 5.4 astronomical units, slightly beyond Jupiter's orbit of 5.2 astronomical units. This gravitational interaction with Saturn increased solar heating on 450P's nucleus, causing subsurface volatile ices and trapped gases to release and produce the observed coma.
Context
This observation marks the first documented case of a centaur transitioning toward becoming a Jupiter-family comet—a transformation long theorized but never before witnessed. Centaurs are inactive bodies that originated in the Kuiper Belt and now orbit between Jupiter and Neptune; their unstable orbits mean giant planets can push them closer to the sun or eject them from the solar system. The discovery of carbon dioxide as the primary driver of 450P's activity (rather than water vapor) indicates that at its current distance, the nucleus remains too cold for normal water-ice sublimation. The presence of crystalline ice alongside the carbon dioxide suggests thermal processing is occurring, with amorphous ice transforming into structured crystalline ice as the object warms—a process that releases trapped gases and drags material from the surface. Studying objects like 450P provides insight into how primitive material from the outer solar system evolves as it moves inward and how comet nuclei physically change during this transition.