The Webb Captures Spectra of Trans-Neptunian Objects, and Reveals a History of Our Solar System

Trans-Neptunian Objects (TNOs) are small planetoids that orbit the Sun beyond Neptune and Pluto. Their dark and icy character contains the remnant of the early solar system, and as such, they have the potential to reveal its history. But since they are small, distant, and dim, TNOs are very difficult to study. We know that different groups of TNOs have unique histories based on their surface colors and orbits. A new study has looked at their spectra, and it reveals a rich diversity unseen before now.

The team used observations from the James Webb Space Telescope (JWST) to capture the spectra of 54 TNOs. They found the planetesimals could be grouped into three categories based on the overall shape of their spectra. Double-dip TNOs have a strong presence of carbon dioxide ice and are the most common of the survey objects. Cliff-type TNOs are reddish and are rich in nitrogen molecules and complex organics. Finally, bowl-type TNOs have dark and dusty surfaces rich in water ice.

The authors argue that these categories formed because of different “ice lines” that existed during the early period of the solar system. That is, beyond a certain distance, temperatures are cold enough for water ice to form. Further out, it becomes cold enough for carbon dioxide ice to form, and so forth. The different categories of TNOs therefore formed at different distances from the Sun, likely before the great migration of the large planets.

This idea is supported by the fact that there is a correlation between the spectral category of TNOs and their orbital types. For example, cold classical TNOs with orbits at the outer edge of the planetary disk are mostly cliff-type TNOs.

The team was also able to connect TNOs to another type of planetoid known as centaurs, which orbit the Sun between Jupiter and Saturn. While the spectra of centaurs differ significantly from those of TNOs, there are enough similar features to identify many centaurs as part of a particular TNO type. The centaur Thereus matches the bowl-type category, for example. On the other hand, some centaurs, such as Okyrhoe don’t fall into any TNO category. This supports the idea that many centaur planetoids were TNOs that migrated inward over time, while others are likely comets that became centaurs after a close approach with Jupiter or Saturn.

In the future, the team would like to gather even more detailed spectra of TNOs. This could tell us the specific histories of each TNO category and how they connect to the early evolution of our solar system.

Reference: Pinilla-Alonso, Noemí, et al. “A JWST/DiSCo-TNOs portrait of the primordial Solar System through its trans-Neptunian objects.” Nature Astronomy (2024): 1-15.

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