Icy Insights from Comets and Planet-Forming Disks

Are the icy objects that inhabit the outer solar system truly pristine remnants of the early solar system, or have they been altered by the chaotic process of planet formation? Recent research that compares the chemistry of solar system comets and planet-forming systems may provide an answer to this key question.

Time Capsules from the Early Solar System(?)

The Kuiper Belt and Oort Cloud contain unknown numbers of icy planetesimals left over from the formation of the solar system. One of the most pressing questions in planetary science today is whether these objects represent unaltered material from the dusty nebula from which the Sun and the planets formed, or if the process of planet formation and the intervening billions of years have changed their properties. The answer has profound implications for understanding the formation of our own solar system and planetary systems in general.

There’s a good reason that this question poses such a challenge: these far-flung objects are difficult or impossible to observe directly. Luckily, they make occasional journeys into the inner solar system, transformed by warm sunlight into comets sporting twin tails of dust and ions. Now, a team led by Manuela Lippi (National Institute for Astrophysics, Italy) has amassed a sample of these icy interlopers to assess their chemistry and understand whether they’ve changed since their formation.

Comet Calculations

Using infrared and sub-millimeter observations, Lippi and collaborators studied the chemical compositions of 35 solar system comets. They found that the abundances of methanol, formaldehyde, and ammonia didn’t depend on the comet’s dynamical family. (A dynamical family is a group of comets with similar orbital characteristics.) For example, dynamically new comets — those making their first trek into the inner solar system — had similar chemistry to Jupiter-family comets whose orbits remain within the orbit of Jupiter and have visited the inner solar system many times.

The similarity in chemical composition between comets in different dynamical families suggests that comets retain their chemical makeup after they form. If this is the case, comets in the solar system today should be chemically similar to the material surrounding young, planet-forming Sun-like stars.

Throwing Disks into the Mix

As a test of this theory, Lippi’s team compared the typical chemistry of solar system comets to that of 11 planet-forming systems. The systems in the comparison sample ranged in age from 10,000-year-old hot molecular cloud cores to million-year-old protoplanetary disks. Overall, the abundance ratios of solar system comets were similar to those of planet-forming systems. This is the first statistical evidence that planet-forming systems of all ages are chemically similar and that comets are indeed unaltered remnants of the early solar system.

However, a note of caution: when Lippi and coauthors compared their solar system comet sample against measurements of the well-studied comet 67P/Churyumov–Gerasimenko, which hosted the Rosetta spacecraft in 2014–2016, they found substantial differences. If comet 67P stands apart from the average solar system comet, its use as a benchmark could bias studies of planet-forming systems.

Citation

“The Ice Chemistry in Comets and Planet-Forming Disks: Statistical Comparison of CH₃OH, H₂CO, and NH₃ Abundance Ratios,” Manuela Lippi et al 2024 ApJL 970 L5. doi:10.3847/2041-8213/ad5a6d