Volatile Voyagers: How Centaurs Chart the Transition from TNOs to Inner-System Comets

1. Introduction

Centaurs are small bodies on unstable orbits between Jupiter and Neptune that dynamically evolve from the trans-Neptunian scattered disk into Jupiter family comets (JFCs). Their intermittent cometary activity, driven by sublimation of volatiles at 5–10 au, offers unique windows into the primordial volatile inventory of the outer Solar System. Before JWST, direct measurements of CO₂, and CO in Centaur comae were sparse: CO was detected in a few centaurs, CO₂ remained directly undetected.In this presentation we will discuss the comparisons and correlations of centaurs with trans Neptunian objects, and Oort cloud comets.

2. Observations and Data

2.1 Centaurs

JWST program GO 2416 observed five active centaurs: 29P/SW1, C/2008 CL94 (Lemmon), C/2014 OG392 (PanSTARRS), C/2019 LD2 (ATLAS), and 39P/Oterma; in IFU prism mode with dedicated background visits. Each spectrum spans 0.6–5.3 µm at R ∼ 100, enabling detection of emission lines and absorptions of volatiles and ices.

2.2 Trans-Neptunian Objects

Recent JWST observations of TNOs have classified their near-IR reflectance spectra into three classes—Bowl, DoubleDip, and Cliff-type—based on absorption features around 3 µm and spectral slopes (Licandro et al., 2024). These classes correlate with water ice, organics, and silicates, tracing different formation environments in the outer disk.

2.3 Oort Cloud Comets

JWST observations of OCCs have employed NIRSpec and MIRI IFUs to study these comets at a variety of heliocentric distances. Recent sutdies reveal hyperactive water production (>90% active fraction), multiple volatiles (H₂O, ¹²CO, ¹³CO, CO₂, CH₃OH, CH₄, CN, H₂CO, C₂H₆, HCN, NH₂, OH prompt emission), and a dust composition dominated by amorphous carbon, Mg–Fe silicates, and Mg-rich crystalline olivine (Woodward et al., 2025).

3. Results

Several centaurs exhibit clear detections of CO (4.7 µm), CO2 (4.26 µm), and H₂O (2.7 µm) emission lines. 29P/SW1 shows heterogeneous outgassing regions traced by mapped CO jets, indicating localized active areas (Faggi et al., 2024). JWST provided the first detection of CO₂ in 39P/Oterma beyond 5 au, demonstrating sensitivity orders of magnitude beyond previous limits (Harrington Pinto et al., 2023).

4. Discussion and Conclusions

Centaurs occupy an evolutionary niche; they bear surface signatures of TNOs yet they show early stages of volatile loss into JFCs. The matching volatile ratios (CO/H₂O, CO₂/H₂O) across centaur and Oort Cloud comet populations support a continuum of primordial compositions modified by heliocentric distance and thermal history. JWST NIRSpec IFU has, for the first time, provided a direct and sensitive inventory of volatiles in active centaurs, revealing two distinct compositional groups that align with TNO spectral classes and connect to Oort Cloud comet properties. These findings strongly suggest centaurs as transitional bodies bridging outer-solar-system reservoir compositions and the dynamic, compositional diversity of inner solar system comets. Future JWST monitoring over multiple apparitions and expanded target samples will further elucidate the thermal and chemical evolution of small bodies tracing planetary system origins.

 

References:

Faggi, S., Villanueva, G. L., McKay, A., Harrington Pinto, O., Kelley, M. S. P., Bockelee-Morvan, D., Womack, M., ´ Schambeau, C. A., Feaga, L., DiSanti, M. A., Bauer, J. M., Biver, N., Wierzchos, K., and Fernandez, Y. R. (2024). Heterogeneous outgassing regions identified on active centaur 29P/Schwassmann–Wachmann 1. Nature Astronomy, 8(10):1237–1245.

Harrington Pinto, O., Kelley, M. S. P., Villanueva, G. L., Womack, M., Faggi, S., McKay, A., DiSanti, M. A., Schambeau, C., Fernandez, Y., Bauer, J., Feaga, L., and Wierzchos, K. (2023). First Detection of CO2 Emission in a Centaur: JWST NIRSpec Observations of 39P/Oterma. , 4(11):208.

Licandro, J., Pinilla-Alonso, N., Holler, B., Wong, I., de Pra, M., Melita, M., Souza Feliciano, A. C., Brunetto, R., Guilbert-Lepoutre, A., Henault, E., Lorenzi, V., Stansberry, ´ J., Schambeau, C., Harvison, B., Pendleton, Y., Cruikshank, D., Mueller, T., Emery, J., McClure, L., and Peixinho, N. (2024). Deciphering TNOs thermal evolution through Centaur surface studies using JWST. In European Planetary Science Congress, pages EPSC2024–984.

Woodward, C. E., Bockelee-Morvan, D., Harker, D. E., Kelley, M. S. P., Roth, N. X., Wooden, D. H., and Milam, S. N. (2025). A JWST Study of the Remarkable Oort Cloud Comet C/2017 K2 (PanSTARRS). arXiv e-prints, page arXiv:2504.19849.