JWST and Gemini Watch 450P/LONEOS Develop a Comet-Like Coma

450P/LONEOS offers an unusually well-observed case of a Centaur becoming comet-like. Gemini North followed the appearance of a resolved dust coma, and the James Webb Space Telescope (JWST) identified carbon dioxide gas and icy dust within it. Together, the observations connect an orbital disruption, warming, volatile release, and dust loss in a single outer-solar-system object. 33

The important distinction is that 450P has not been observed completing a transformation into a Jupiter-family comet. Rather, it provides evidence for an earlier step: the awakening of cometary activity in a body whose orbit was recently shifted inward.

What Gemini North and JWST saw

The observing campaign covered 2019 to 2024. As 450P moved inward, from 7.83 to 7.24 astronomical units (AU) from the Sun, a coma developed around the nucleus. The study estimates a relatively low dust-production rate of about 4–8 kilograms per second. 33

JWST’s NIRSpec observations then separated the coma’s dust and gas signatures:

• Dust: The spectrum showed water-ice absorption features near 2.0 and 3.0 micrometres. A subtler feature near 3.1 micrometres is consistent with crystalline water ice in some larger grains. 2
• Gas: JWST detected a symmetric distribution of carbon dioxide (CO₂) gas, while finding no emission features from water vapor (H₂O) or carbon monoxide (CO). 33

That combination matters. Water ice was present in the expelled dust, but water vapor was not detected in the coma. A non-detection means the signal was below the observation’s sensitivity threshold; it does not mean 450P contains no water.

Why CO₂ is the likely driver of the coma

At 450P’s distance from the Sun, ordinary water-ice sublimation is relatively weak. The detected CO₂ is therefore a strong candidate for the gas that dragged dust from the surface or near-surface layers and made the coma visible. 33

The observations do not establish every step of that process directly, but they provide a coherent picture: CO₂ gas is present, dust is being released, and that dust contains water ice. This is different from a typical closer-in comet, where water vapor can dominate the activity.

Saturn changed 450P’s thermal future

Orbital calculations trace a crucial event to 1992, when 450P passed within about 0.03 AU of Saturn. Earlier work reported that this encounter abruptly lowered the object’s perihelion distance from roughly 10 AU to roughly 5 AU. 15

That shift placed 450P on a warmer orbit, with perihelion near Jupiter’s orbital region. Repeated trips through this new environment could gradually heat subsurface layers that had remained cold on its former trajectory. The recent study likewise identifies the Saturn encounter as a major alteration to the orbit that preceded the observed activity. 33

The proposed role of ancient amorphous ice

The leading explanation for the delay between the 1992 encounter and the later coma is a subsurface phase change. The researchers’ thermal model is consistent with amorphous water ice crystallizing at about 140–160 K and releasing CO₂ that had been trapped in the ice. 2

Amorphous water ice is a disordered form of ice that can retain volatile gases. As it warms and becomes crystalline, trapped gas can escape; crystallization may also release heat that helps the process propagate. This mechanism has long been proposed as a way to generate activity far from the Sun, where water-ice sublimation alone is inefficient. 315

It remains an interpretation, not a direct detection of amorphous ice inside 450P. The observational result is the CO₂-rich, icy-dust coma; the crystallization model explains how that activity may have been initiated after the orbital change.

Why 450P is a potential missing link

Centaurs occupy dynamically unstable orbits among the giant planets and are often considered possible predecessors of short-period comets. 450P is especially valuable because astronomers can tie several stages together:

1. A close planetary encounter changed its orbit.
2. The new orbit exposed it to more solar heating over repeated passages.
3. A coma appeared as it moved inward.
4. JWST detected CO₂ gas alongside ice-bearing dust. 33

Its current orbit has a period of about 22 years, so it is not yet a Jupiter-family comet. No Centaur has been watched through the full dynamical conversion into that class. 6

Still, 450P is a compelling physical bridge between a cold, volatile-rich outer-solar-system body and an active comet. Continued observations on future orbits can test whether its activity persists, strengthens, or changes composition—key evidence for understanding how dormant icy objects become cometary.