In the outer reaches of the Solar System, beyond the ice giant Neptune, are a series of objects called Centaurs, and transneptunian objects (TNO’s). The Instituto de Astrofísica de Canarias (IAC) is one of the most advanced international centers for the study of these objects, and co-leads one of the key studies which show what the coldest objects in the Solar Sytem are made of, and how thermal change is produced in their interiors.

In two articles recently published in the same volumen of Nature Astronomy, spectra obtained with the James Webb Space Telescope (JWST) have shed new light on the composition and evolution of the populations of small bodies in the outer Solar System.

These studies are within the framework of the programme DiSCo (Discovering the Surface Composition of Trans-Neptunian Objects), led by Dr. Noemí Pinilla Alonso, currently distinguished Researcher at the Institute of Space Science and Technology of Asturias, and the University of Oviedo. 

Within this program the IAC researchers Javier Licandro and Vania Lorenzi have shown how the transneptunian objects (TNO’s) and the Centaurs can serve as windows to the past history of the Solar System, preserving clues to its formation and evolution. In these studies spectra of over fifty TNO’s and Centaurs observed with the JWST instrument NIRSpec are presented.

In one of the articles, with Javier Licandro as first author, the composition of the Centaurs is studied. These objects are in fact TNO’s which have fallen inwards from the region beyond Neptune, and are presently orbiting between Jupiter and Neptune.

The observations with the JWST have permited an analysis in unprecedented detail of the transformations in the surface composition of these objects, picking out how the sublimation of ices, and other thermal processes have shaped their surfaces as they have come nearer to the Sun. This study shows that the Centaurs are not only an intermediate link between the TNO’s and the comets, but also provide key information on the thermal evolution of icy bodies in the Solar System.

“We have identified Centaurs that have a surface composition similar to two of the three types of TNOs, some rich in water and amorphous silicates and others rich in methanol and complex organics (Bowl type and Cliff type respectively), but that show the imprint of a layer of dusty regolith intermixed with ice,” Pinilla Alonso states. 

“Curiously we have identified a new class of surfice, not found before among the TNO’s, which are similar to active comets and asteroids” states Javier Licandro, and adds that “the diversity detected in the populations of Centaurs, in terms of the presence of water, dust, and organic complexes, suggests a variety of origins for the transneptunian population and different stages of evolution, which brings out the fact that the Centaurs are not a homogeneous population, but more a dynamic and transitional group of objects".

The second article, led by Noemí Pinilla Alonso, to which the IAC researchers have also contributed, presents the first systematic classification of the TNO’s base don spectral data obtained with the JWST.

This analysis shows three main groups of TNO’s, defined by their surface composition, and specifically by the relative proportions of water ices, carbon dioxide, methanol, and complex organic molecules. These groups reflect the “lines of retention of ices” in the protoplanetary disc, zones where the physical conditions allowed the accumulation of different types of ices during the first phases of the Solar System.

The work shows that those TNO’s whose surfaces are covered with abundant water ice formed in the zone nearest to the Sun, while those which have a greater proportion of methanol and organic complexes formed further away.

“For the first time we have identified the specific molecules responsible for the considerably diversity observed in the transneptunian objects” clamis Noemí Pinilla Alonso, the first author of the study. “These molecules, such as water ice, carbon dioxide, methanol, and complex organic compounds give us a direct connection between the spectral characteristics of the TNO and their chemical compositions” adds the researcher.

A key contribution from the IAC

The IAC has played a major role in the study of the Centaurs, leading the research into the spectra of 5 Centaurs observed in the framework of DiSCo and which shows how these bodies undergo important changes in their surface composition due to thermal processes. These processes sublimate volatile elements such as CO2 and water and entrain dust grains that are deposited on the surface, dust that closely resembles the dust detected on the surface of comets. 

According to Javier Licandro “The effects of the thermal evolution we observed in the surface composition of the Centaurs is key to establish the relation between the TNO’s and other populations, such as irregular satellites of the giant planets, their Trojan asteroids, and the comets”

Further, IAC researchers have contributed actively to the analysis of the diversity of the composition of the TNO’s, led by Noemí Pinilla, contributing their experience in the observations of spectra, and of data analysis. ”This research not only brings us closer to understanding how the Solar System was formed, but it also brings out the value of the JWST as a unique tool for exploring remove and cold regions of our cosmic neighbourhood” points out Javier Licandro.

Another step forward in Solar System exploration

According to Licandro, “both works open new lines of research on the connections between the different bodies of the outer Solar System, from the TNOs to the Centaurs and comets, including the Trojan objects of Jupiter and the irregular satellites of the giant planets”. They also show how JWST observations are revolutionizing our understanding of the earliest stages of planetary formation by revealing processes that occurred more than 4.5 million years ago.

The DiSCo program, led by an international team, will continue to supply key data about the surface compositions and evolutionary paths of the transneptunian objects, and of other icy objects in the outer Solar System.

IAC Contact

Javier Licandro: jlicandro [at] iac.es (jlicandro[at]iac[dot]es)