A DiSCo JWST portrait of the primordial Solar System through its trans-Neptunian objects

The detection of molecules on the coldest and outermost objects in our Solar System has long been limited by the terrestrial atmosphere and the sensitivity of the available instrumentation. However, near-infrared observations by the James Webb Space Telescope (JWST) have now provided an unprecedented view of the molecular diversity on the surfaces of trans-Neptunian objects (TNOs). Using the low spectral resolution PRISM mode on the Near-Infrared Spectrograph (NIRSpec) as part of the Cycle 1 Large Program “Discovering the Surface Composition of trans-Neptunian objects" (DiSCo-TNOs), we report the detection of several molecular ices throughout the TNO population, including H ₂ O, CO ₂ , ¹³ CO ₂ , CO, CH ₃ OH, and complex molecules and refractory materials containing aliphatic C–H, C≡N, O–H, and N–H bonds. As a result of the imprint that these molecules leave on the spectra, three main compositional groups consistently emerge from multiple independent cluster analysis efforts. Our results unlock the long-standing question of the interpretation of color diversity providing the much-needed compositional information. The marked separation of the three spectral clusters reveals sharp variations in the surface molecular constituents. The C / O and ( CH + NH )/( C + O ) ratios on the surface of TNOs are the primary indicators of the spectral differences among the three TNO compositional groups that we observe today. We propose that these objects are fossil remnants of icy planetesimals, and that the three compositional groups provide a picture of the ice retention lines in the Solar System that likely occurred in the outer protoplanetary disk, possibly just before a major planetary migration.