Detecting molecules on distant objects in our Solar System has been complex due to Earth’s atmospheric and instrumental limitations. However, the James Webb Space Telescope’s near-infrared observations have revealed new molecular details on trans-Neptunian objects (TNOs).
Studies by researchers at the University of Central Florida offer new insights into the formation and evolution of the outer solar system. They show how TNOs evolve into centaurs as they move toward the giant planets.
Noemí Pinilla-Alonso, the study’s lead author, said, “With this new research, a more complete, complete picture of diversity is presented, and the puzzle pieces are starting to come together.”
“For the first time, we have identified the molecules responsible for the remarkable diversity of spectra, colors, and albedo observed in trans-Neptunian objects. These molecules — like water ice, carbon dioxide, methanol, and complex organics — directly connect the spectral features of TNOs and their chemical compositions.”
Using the James Webb Space Telescope (JWST), researchers classified trans-Neptunian objects (TNOs) into three compositional groups shaped by ice retention lines from the early solar system. At different distances from the Sun, these lines mark regions where temperatures were low enough for ice to form, linking the early conditions of planetesimal formation to their current compositions.
Rosario Brunetto, the paper’s second author and a Centre National de la Recherche Scientifique researcher at the Institute d’Astrophysique Spatiale (Université Paris-Saclay), says the results are the first clear connection between the formation of planetesimals in the protoplanetary disk and their later evolution. The work sheds light on how today’s observed spectral and dynamical distributions emerged in a planetary system shaped by complex dynamical evolution, he says.
“The compositional groups of TNOs are not evenly distributed among objects with similar orbits,” Brunetto says. “For instance, cold classical, which formed in the outermost regions of the protoplanetary disk, belongs exclusively to a class dominated by methanol and complex organics. In contrast, TNOs on orbits linked to the Oort cloud, originating closer to the giant planets, are all part of the spectral group characterized by water ice and silicates.”
Brittany Harvison, a UCF physics doctoral student who worked on the project while studying under Pinilla-Alonso, says the three groups defined by their surface compositions exhibit qualities hinting at the protoplanetary disk’s compositional structure.
“This supports our understanding of the available material that helped form outer solar system bodies such as the gas giants and their moons or Pluto and the other inhabitants of the trans-Neptunian region.”
A study in *Nature Astronomy* found that centaurs, TNOs that shift orbits after close encounters with Neptune, show unique spectral signatures and dusty regolith mantles.
As they warm up near the Sun, they develop distinct surface features compared to TNOs, including two surface types, Bowl and Cliff, which are low in volatile ice. The study also identified a new surface class in centaurs, resembling those on ice-poor objects like cometary nuclei and active asteroids.
Javier Licandro, senior researcher at the Instituto de Astrofisica de Canarias and lead author of the centaur study, explains that the spectral diversity observed in centaurs is broader than expected. This indicates that current models of their thermal and chemical evolution may need adjustments. The variety of organic signatures and the extent of irradiation effects were also surprising.
The detected diversity in water, dust, and complex organics suggests that centaurs have varied origins within the TNO population and are at different stages of evolution. This highlights that centaurs are not a uniform group but dynamic and transitional objects.
Licandro says, “The effects of thermal evolution observed in the surface composition of centaurs are key to establishing the relationship between TNOs and other small bodies populations, such as the irregular satellites of the giant planets and their Trojan asteroids.”
Study co-author Charles Schambeau, a planetary scientist with UCF’s Florida Space Institute (FSI) who specializes in studying centaurs and comets, emphasized the importance of the observations and that some centaurs can be classified into the same categories as the DiSCo-observed TNOs.
“This is profound because when a TNO transitions into a centaur, it experiences a warmer environment where surface ices and materials change. In some cases, the surface changes are minimal, allowing individual centaurs to be linked to their parent TNO population. The TNO versus centaur spectral types are different, but similar enough to be linked.”
The studies are part of the DiSCo project, led by Pinilla-Alonso, which aims to uncover the molecular composition of trans-Neptunian objects (TNOs). Pinilla-Alonso, now a professor at the Universidad de Oviedo, conducted this research using the James Webb Space Telescope (JWST), which provided unprecedented views of TNO and centaur surfaces.
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The researchers analyzed the spectra of 54 TNOs, identifying specific molecules and categorizing them into three groups based on their surface compositions: “Bowl,” “Double-dip,” and “Cliff,” named for the shapes of their light absorption patterns.
They found that:
- Bowl-type TNOs comprised 25% of the sample and were characterized by strong water ice absorptions and a dusty surface. They showed clear signs of crystalline water ice and had low reflectivity, indicating the presence of dark, refractory materials.
- Double-dip TNOs accounted for 43% of the sample and showed strong carbon dioxide (CO2) bands and some signs of complex organics.
- Cliff-type TNOs made up 32% of the sample. They were the reddest in color and showed strong signs of complex organics, methanol, and nitrogen-bearing molecules.
In the centaur study, researchers analyzed the spectra of five centaurs and found diverse surface compositions. Thereus and 2003 WL7 were classified as Bowl-type, while 2002 KY14 was Cliff-type. Okyrhoe and 2010 KR59 formed a new “Shallow-type” group with comet-like dust and little volatile ice.
Pinilla-Alonso noted that previous DiSCo research revealed carbon oxides on TNO surfaces. This study shows that water ice is less common than thought, with CO₂ and carbon monoxide more widely present.
Harvison notes that the findings are just the beginning, and there is much more to explore about how these compositional groups formed. NASA funded the research through the Space Telescope Science Institute.
Journal Reference:
- 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 (2024). DOI: 10.1038/s41550-024-02433-2