How JWST Watched Centaur 450P/LONEOS Begin Acting Like a Comet

A distant icy body may be offering astronomers a rare look at the opening stage of comet-like activity. Observations with Gemini North and NASA’s James Webb Space Telescope (JWST) show that Centaur 450P/LONEOS developed an extended coma as it approached its August 2024 perihelion. Its gas signature was dominated by carbon dioxide, and orbital modeling links the new activity to a close encounter with Saturn in 1992. 320

Gemini and JWST caught an extended coma

The observing campaign followed 450P/LONEOS between 2019 and 2024. Gemini North documented the growth and brightening of a cloud of gas and dust around the object’s solid nucleus as it moved toward perihelion. That cloud is a coma—the defining visible feature of an active comet. 8

JWST added the chemical and spatial evidence. Its NIRSpec integral-field observations resolved elongated dust around 450P and a more symmetric distribution of carbon-dioxide gas. The spectra also showed water-ice absorption features in the coma, but did not show emission features from water vapor or carbon monoxide. 3

Together, those observations distinguish an active object surrounded by released material from an inactive, bare Centaur. The study characterizes the activity as modest, but its timing and composition are particularly informative because 450P remained far from the Sun by inner-solar-system comet standards. 3

A 1992 pass by Saturn changed the heating environment

Researchers traced 450P’s recent dynamical history to a close encounter with Saturn in 1992. That gravitational interaction substantially altered its orbit, decreasing its semimajor axis and placing it on a more Sun-exposed path. At perihelion, the object now reaches roughly Jupiter’s orbital distance. 35

The important consequence was cumulative heating, not an immediate transformation. A body moved inward experiences stronger solar energy over repeated orbits, allowing heat to work gradually into its near-surface layers. The team’s orbital and thermal modeling connects 450P’s current activity with the warmer conditions produced by its post-1992 orbit. 320

This type of inward orbital shift is often called an orbital “jump.” The authors note that close encounters with giant planets can create such changes, potentially setting up the phase transitions and volatile loss that make Centaurs active. 20

Why carbon dioxide is the leading explanation

JWST’s spectra provide the clearest observational constraint: carbon dioxide was detected in the coma, while water-vapor and carbon-monoxide emission features were not. 3

That matters because the energy available at 450P’s distance is not expected to produce water-driven activity comparable to the water sublimation familiar from comets closer to the Sun. CO₂ is more volatile, making it a plausible source of escaping gas that can carry dust away from the surface and build a visible coma. 35

The researchers’ proposed physical mechanism involves water ice, but not as the directly observed gas source:

1. Ancient amorphous water ice in the nucleus can trap gases such as CO₂.
2. As inward migration warms subsurface layers, the amorphous ice can crystallize.
3. Crystallization releases trapped gases and heat, allowing CO₂ to escape through porous material or fractures.
4. Escaping gas can entrain dust, producing the observed coma. 3

This explanation is a thermal-model interpretation of the observations. JWST directly measured the coma’s composition and identified icy material; it cannot directly inspect the nucleus’s interior or watch buried amorphous ice crystallize. 3

Why 450P is an important transition object

Centaurs occupy dynamically unstable orbits among the giant planets and can evolve into other small-body populations after planetary encounters. In 450P, astronomers can connect several stages of that pathway: a Saturn-driven orbital change, increased solar heating, volatile release, and the appearance of a dust-and-gas coma. 320

That does not mean 450P has already become a Jupiter-family comet. Instead, it is a useful candidate for observing an earlier transition: a Centaur retaining properties of an outer-solar-system body while showing newly detectable comet-like activity. Its value lies in the unusually traceable sequence from orbital scattering to physical activation. 320

The research, JWST and Gemini Observations of the Active Centaur 450P/LONEOS: Nucleus and Coma Characterizations, was published in The Planetary Science Journal in June 2026. 2