1,2,1,4,7,3,8,9,8,9,13,- 1CNRS, Université Paris-Saclay, IAS, Orsay, France (rosario.brunetto@gmail.com)
- 2Instituto de Ciencias y Tecnologías Especiales de Asturias, Oviedo, Spain
- 3Space Telescope Science Institute, Baltimore, MD, USA
- 4Northern Arizona University, Flagstaff, AZ, USA
- 5Lowell Observatory, Flagstaff, AZ, USA
- 6LGL-TPE, CNRS, UCBL, ENSL, Villeurbanne, France
- 7Université Grenoble Alpes, CNRS, IPAG, Grenoble, France
- 8Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain
- 9Universidad de La Laguna, Departamento de Astrofísica, La Laguna, Tenerife, Spain
- 10University of Auckland, New Zealand
- 11Florida Space Institute, University of Central Florida, Orlando, FL, USA
- 12Planetary Science Institute, 1700 East Fort Lowell, Tucson, 85719, AZ, USA
- 13Department of Physics, University of Michigan, Ann Arbor, MI, USA
- 14Fundación Galileo Galilei-INAF, La Palma, Santa Cruz de Tenerife, Spain
- 15Collège de France, CNRS, PSL Univ., Sorbonne Univ., Paris, France
- 16Southwest Research Institute, Boulder, CO, USA
- *A full list of authors appears at the end of the abstract
Trans-Neptunian objects (TNOs), Centaurs, and Neptune Trojans are relatively pristine remnants of the original Solar System planetesimals and therefore can inform us on planetary formation and evolution processes in the cold regions of our protoplanetary disk. Since 2022, using the James Webb Space Telescope's NIRSpec instrument, different observational programs have obtained near-infrared spectra (0.7-5.0 µm) of over one hundred medium-sized bodies (20-1000 km) from these populations, assembling the largest and most compositionally diverse spectral dataset of icy outer Solar System objects to date.
Our analysis of this large sample of objects confirms the existence of compositional categories identified by DiSCo-TNOs [1,2], mainly defined by absorption features of H2O, CO2, CH3OH, CO and OCN- ices, and other complex organic materials. The different dynamical classes (Resonant, Scattering, Detached, Centaurs, etc.) each show a mixture of compositional categories. In contrast, the Cold Classical TNOs, thought to still be on their formational low-inclination, low-eccentricity orbits in the outer disk, nearly all exhibit a single spectral category dominated by organics and N-bearing materials (DiSCo Cliff2 TNOs) [3,4], consistent with their distinct dynamical and collisional history.
Our observations highlight an outstanding trend seen in the CO2 ice abundance across the population. While all targets show some level of CO2, CO2-rich objects are almost absent at perihelion distances smaller than ~31 AU, and CO2 band areas tend to correlate with the true-anomaly-averaged solar flux. The boundary between CO2-rich and CO2-poor objects in perihelion-diameter space follows a diagonal trend, pointing to a coupled thermal and gravitational control of CO2 retention: bodies that are either too small or too warm tend to lose their CO2 to space, while larger, colder objects can retain a segregated CO2 layer. Furthermore, we performed laboratory ion irradiation experiments and we observed that a thin (~5-20 µm) and irradiated CO2 layer over CH3OH/organic substrates can reproduce the spectral features of CO2-type TNOs. In contrast, objects where CO2 is trapped within H2O or CH3OH matrices show a more gradual depletion with no sharp transition.
Taken together, the observed surface compositions reflect evolutionary processing superimposed on primordial compositional categories. Sharp transitions between spectral categories point to early sculpting by ancient icelines, such as a pre-accretional CO iceline or post-accretional CO2 and CH3OH retention icelines, probably established before planetary migration. Organics-type TNOs likely represent a cold primordial population that retains the closest link to the composition of materials in the outer regions of the protoplanetary disk.
These results provide new information on the distribution of ices in the outer Solar System, the processes governing volatile retention and loss on icy bodies, and the chemical gradients established during the earliest stages of Solar System formation.
1. Pinilla-Alonso, N., et al., Nat Astron 9, 230–244 (2025)
2. Brunetto, R., et al., APJL, 982:L8 (2025)
3. McClure, L.T., et al., AJ, 171:54 (2026)
4. Cryan, S., et al., APJ, 993:188 (2025)
Rosario Brunetto1, Noemi Pinilla-Alonso2, John Stansberry3, Sasha Cryan1, William Grundy4,5, Aurélie Guilbert-Lepoutre6, Joshua Emery4, Elsa Hénault7, Bryan Holler3, Javier Licandro8,9, Larissa Markwardt10, Léa Bouvet1, Brittany Harvison11, Tania Le Pivert-Jolivet8,9, Eva Lilly12, Hsing-Wen Lin13, Vania Lorenzi14, Lucas McClure4, Alessandro Morbidelli15, David Nesvorny16, Yvonne Pendleton11, Mario de Prá17, Silvia Protopapa16, Charles Schambeau11, Marie Thabault1, Ian Wong3
How to cite: Brunetto, R., Pinilla-Alonso, N., Stansberry, J., Cryan, S., Grundy, W., Guilbert-Lepoutre, A., Emery, J., Hénault, E., Holler, B., Licandro, J., Markwardt, L., Bouvet, L., Harvison, B., Le Pivert-Jolivet, T., Lilly, E., Lin, H.-W., Lorenzi, V., McClure, L., Morbidelli, A., and Nesvorny, D. and the DiSCo-TNOs & GTO-KBOs teams: Detection, Composition and Distribution of Ices on Outer Solar System Planetesimals by JWST, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-200, https://doi.org/10.5194/epsc2026-200, 2026.
