Trans-Neptunian Objects (TNOs) refer to small, primitive bodies in our outer Solar System, orbiting beyond Neptune. Formed quite far from the Sun, these objects are still full of information concerning their formation more than 4 billion years ago, unlike dwarf planets like Pluto which could have undergone major internal evolution. OTNs have also witnessed processes of planetary migration which have, for the majority, redistributed them far from their region of formation. Until now, observations of OTNs were limited, providing a very partial characterization of their chemical composition: only water and methanol ices were detected on a handful of objects.
A major observation program
The James Webb Space Telescope recently provided the first overview of OTNs. Observations of 59 objects obtained with the NIRSpec instrument were analyzed by an international research team involving scientists from the CNRS Terre & Univers (see box). Infrared spectra reveal first-ever detections of CO ice2 and CO on small bodies in the outer Solar System.
Against all expectations, the CO2 is very widespread, present on 95% of objects in varying proportions. While CO is not stable in the trans-Neptunian region, it is nevertheless detected together with CO2 on 47% of objects. This diversity of composition, reflected by variations in abundance but also in the physico-chemical state of the ice, will make it possible to retrace the history of OTNs to find their place of formation in the protoplanetary disk. If the CO2 could have been inherited from the protoplanetary disk, CO would more likely be formed by interaction of the surface with the solar wind and cosmic rays. Remarkably, 13CO2 is also detected which opens the possibility of studying the carbon isotope ratio across the outer solar system.
