Best glimpse ever into icy planetesimals of the early solar system

The findings, published today in Nature Astronomy, reveal the distribution of ices in the early solar system and how TNOs evolve when they travel inward into the region of the giant planets between Jupiter and Saturn, becoming centaurs.

TNOs are small bodies, or 'planetesimals,' orbiting the sun beyond Pluto. They never accreted into planets, and serve as pristine time capsules, preserving crucial evidence of the molecular processes and planetary migrations that shaped the solar system billions of years ago. These solar system objects are like icy asteroids and have orbits comparable to or larger than Neptune's orbit.

Prior to the new UCF-led study, TNOs were known to be a diverse population based on their orbital properties and surface colors, but the molecular composition of these objects remained poorly understood. For decades, this lack of detailed knowledge hindered interpretation of their color and dynamical diversity. Now, the new results unlock the long-standing question of the interpretation of color diversity by providing compositional information.

"With this new research, a more-complete picture of the diversity is presented and the pieces of the puzzle are starting to come together," says Noemí Pinilla-Alonso, the study's lead author.

"For the very first time, we have identified the specific molecules responsible for the remarkable diversity of spectra, colors and albedo observed in trans-Neptunian objects," Pinilla-Alonso says. "These molecules -- like water ice, carbon dioxide, methanol and complex organics -- give us a direct connection between the spectral features of TNOs and their chemical compositions."

Using the James Webb Space Telescope (JWST), the researchers found that TNOs can be categorized into three distinct compositional groups, shaped by ice retention lines that existed in the era when the solar system formed billions of years ago.

These lines are identified as regions where temperatures were cold enough for specific ices to form and survive within the protoplanetary disk. These regions, defined by their distance from the sun, mark key points in the early solar system's temperature gradient and offer a direct link between the formation conditions of planetesimals and their present-day 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 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 that's 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 classicals, which formed in the outermost regions of the protoplanetary disk, belong exclusively to a class dominated by methanol and complex organics. In contrast, TNOs on orbits linked to the Oort cloud, which originated 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," she says.

In a complementary study of centaurs published in the same volume of Nature Astronomy, the researchers found unique spectral signatures, different from TNOs, that reveal the presence of dusty regolith mantles on their surfaces.

This finding about centaurs, which are TNOs that have shifted their orbits into the region of the giant planets after a close gravitational encounter with Neptune, helps illuminate how TNOs become centaurs as they warm up when getting closer to the sun and sometimes develop comet-like tails.

Their work revealed that all observed centaur surfaces showed special characteristics when compared with the surfaces of TNOs, suggesting modifications occurred as a consequence of their journey into the inner solar system.

Among the three classes of TNO surface types, two -- Bowl and Cliff -- were observed in the centaur population, both of which are poor in volatile ices, Pinilla-Alonso says.

However, in centaurs, these surfaces show a distinguishing feature: they are covered by a layer of dusty regolith intermixed with the ice, she says.

"Intriguingly, we identify a new surface class, nonexistent among TNOs, resembling ice poor surfaces in the inner solar system, cometary nuclei and active asteroids," she says.

Javier Licandro, senior researcher at the Instituto de Astrofisica de Canarias (IAC, Tenerife, Spain) and lead author of the centaur's work says the spectral diversity observed in centaurs is broader than expected, suggesting that existing models of their thermal and chemical evolution may need refinement.

For instance, the variety of organic signatures and the degree of irradiation effects observed were not fully anticipated, Licandro says.

"The diversity detected in the centaurs populations in terms of water, dust, and complex organics suggests varied origins in the TNO population and different evolutionary stages, highlighting that centaurs are not a homogenous group but rather 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 pretty profound because when a TNO transitions into a centaur, it experiences a warmer environment where surface ices and materials are changed," Schambeau says. "Apparently, though, 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."

How the Research Was Performed

The studies are part of the Discovering the Surface Composition of the trans-Neptunian Objects, (DiSCo) project, led by Pinilla-Alonso, to uncover the molecular composition of TNOs. Pinilla-Alonso is now a distinguished professor with the Institute of Space Science and Technology in Asturias at the Universidad de Oviedo and performed the work as a planetary scientist with FSI.

For the studies, the researchers used the JWST, launched almost three years ago, that provided unprecedented views of the molecular diversity of the surfaces of the TNOs and centaurs through near-infrared observations, overcoming the limitations of terrestrial observations and other available instruments.

For the TNOs study, the researchers measured the spectra of 54 TNOs using the JWST, capturing detailed light patterns of these objects. By analyzing these high-sensitivity spectra, the researchers could identify specific molecules on their surface. Using clustering techniques, the TNOs were categorized into three distinct groups based on their surface compositions. The groups were nicknamed "Bowl," "Double-dip" and "Cliff" due to the shapes of their light absorption patterns.

They found that:

• Bowl-type TNOs made up 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 and had strong signs of complex organics, methanol, and nitrogen-bearing molecules, and were the reddest in color.

For the centaurs study, the researchers observed and analyzed the reflectance spectra of five centaurs (52872 Okyrhoe, 3253226 Thereus, 136204, 250112 and 310071). This allowed them to identify the surface compositions of the centaurs, revealing considerable diversity among the observed sample.

They found that Thereus and 2003 WL7 belong to the Bowl-type, while 2002 KY14 belongs to the Cliff-type. The remaining two centaurs, Okyrhoe and 2010 KR59, did not fit into any existing spectral classes and were categorized as "Shallow-type" due to their unique spectra. This newly defined group is characterized by a high concentration of primitive, comet-like dust and little to no volatile ices.

Previous Research and Next Steps

Pinilla-Alonso says that previous DiSCo research revealed the presence of carbon oxides widespread on the surfaces of TNOs, which was a significant discovery.

"Now, we build on that finding by offering a more comprehensive understanding of TNO surfaces" she says. "One of the big realizations is that water ice, previously thought to be the most abundant surface ice, is not as prevalent as we once assumed. Instead, carbon dioxide (CO₂) -- a gas at Earth's temperature -- and other carbon oxides, such as the super volatile carbon monoxide (CO), are found in a larger number of bodies."

The new study's findings are only the beginning, Harvison says.

"Now that we have general information about the identified compositional groups, we have much more to explore and discover," she says. "As a community, we can start exploring the specifics of what produced the groups as we see them today."

The research was supported by NASA through a grant from the Space Telescope Science Institute.