Constraining the surface composition of trans-Neptunian binaries with NIRCAM instrument

Introduction. The trans-Neptunian objects (TNOs) are icy remnants of planetary formation that orbit the Sun beyond Neptune. The study of their physical and compositional properties can be used to constrain the evolutionary process that took place in the early stages of the solar system. In particular, the trans-Neptunian binaries (TNBs) are used in several dynamical studies to constrain these processes (e.g. Nesvorný et al., 2011). The characterization of the TNBs is therefore of particular interest in the field from both the dynamical and compositional perspectives.    

Before the James Webb Space Telescope (JWST) era, the knowledge of the compositions of small and medium-sized TNOs was incomplete. Now, we are aware of 4 main compositional groups in the trans-Neptunian region based on the spectral behavior of carbon dioxide, water ice, methanol ice, carbon monoxide, and tholins (e.g., de Prá et al., 2025; Pinilla-Alonso et al., 2025). Another finding from NIRSpec observations comes from the observation of Mors-Somnus, a TNB in the 3:2 mean motion resonance with Neptune (the plutino TNOs) with components of almost equal size (around 100 km) and a wide orbital separation, with a color similar to the cold classical TNOs (Shepard et al., 2012). The observations of Mors-Somnus from NIRSpec revealed not only the colors of Mors and Somnus are similar in the visible range, but the spectral features are similar in both up to 5 microns (Souza-Feliciano et al., 2024). This result strengthened the outcomes from dynamical simulations regarding the transportation mechanisms in the trans-Neptunian region (Hahn & Malhotra, 2005) and shed light on the origin of this pair and its connection with the cold classical and plutino TNOs (Nesvorný & Vokrouhlický, 2019).   

Goals and sample selection. Aiming to constrain the surface composition of TNBs of similar and different sizes (tight, medium, and wide separation) and shed light on the formation mechanisms of TNBs, we selected a sample of 10 TNBs of cold, hot, resonant, detached, and scattered dynamical groups in the trans-Neptunian region. Due to the faintness and tight separation of some pairs, we chose to use three sets of NIRCam filters as part of program 6064 (PI: Souza-Feliciano). By resolving the components of each system in these multiple filters, we can study their surface compositions up to 4 microns and 1) determine the level of similarity between the components, 2) evaluate if they agree with the current formation scenarios proposed in the literature, and 3) identify which TNBs cannot be explained by current formation scenarios.    

Status and processing. Nine of the ten pairs in our sample have already been observed. Calibration was performed using the most up-to-date reference files provided by the Calibration References Data System (CRDS) and the latest version of the JWST calibration pipeline. This step is necessary to ensure the images are flat-fielded, bias-corrected, and flux-calibrated. After a background estimation, the flux of each source was extracted using a circular aperture and corrected by empirical encircled energy measured with data from the Cycle 1 Absolute Flux calibration program (Gordon et al., 2022).    

Preliminary results. Nine of the ten TNB systems in our sample were resolved by NIRCAM observations. The cold classical TNB pairs observed in our sample (2006 BR284 and Teharonhiawako) show hints of similar surface composition, and differences are noticed for some TNBs that belong to other dynamical groups. The implications of the compositional similarities and differences between TNB components in the context of formation mechanisms in the early outer solar system will be discussed. 

References 

De Prá, M.N., Hénault, E., Pinilla-Alonso, N. et al. Widespread CO2 and CO ices in the trans Neptunian population revealed by JWST/DiSCo-TNOs. Nat Astron 9, 252–261 (2025).  

Gordon, K. D., Bohlin, R., Sloan, G. C. et al. The James Webb Space Telescope Absolute Flux Calibration. I. Program Design and Calibrator Stars. The Astronomical Journal, 163, 6 (2022). 

Hahn, J. M., & Malhotra, R. 2005, AJ, 130, 2392.  

Nesvorný, D., Vokrouhlický, D., Bottke, W.F. et al. Observed Binary Fraction Sets Limits on the Extent of Collisional Grinding in the Kuiper Belt. The Astronomical Journal, Volume 141, 5 (2011). 

Nesvorný, D., & Vokrouhlický, D. 2019, Icarus, 331, 49.  

Pinilla-Alonso, N., Brunetto, R., De Prá, M.N. et al. A JWST/DiSCo-TNOs portrait of the primordial Solar System through its trans-Neptunian objects. Nat Astron 9, 230–244 (2025). 

Sheppard, S. S., Ragozzine, D., & Trujillo, C. 2012, AJ, 143, 58. 

Souza-Feliciano , A. C., Holler, B. J., Pinilla-Alonso, N. et al. Spectroscopy of the binary TNO Mors–Somnus with the JWST and its relationship to the cold classical and plutino subpopulations observed in the DiSCo-TNO project. A&A 681 L17 (2024).