JWST Uncovers a Carbon-Rich Planet-Forming Disk Around a Young Star

The new study by M. Volz et al. (2025) uses the James Webb Space Telescope (JWST) to explore two young, planet-forming systems: GM Aur and RX J1615.3–3255 (J1615). Both are “transitional disks”, protoplanetary disks with large inner gaps where planets may be forming. Though the two stars are similar in mass and age, JWST reveals that their chemical makeups are strikingly different: J1615 is rich in carbon-bearing molecules, while GM Aur is not.

Introduction: Why Study Transitional Disks?

The inner regions of protoplanetary disks, within about ten times the Earth–Sun distance, are where rocky and gaseous planets form. The mix of molecules in this region shapes the composition of future planets. Earlier telescopes like Spitzer detected water and carbon-based molecules but could not observe the faintest chemical species. JWST’s much higher sensitivity now allows astronomers to detect molecules like ¹³CO₂ and C₂H₂ that were previously hidden. This capability helps reveal how chemical diversity arises in young planetary systems.

Observations: A Tale of Two Disks

Volz and collaborators observed both targets with JWST’s Mid-Infrared Instrument (MIRI) and combined the data with ground-based optical spectra from the Chiron spectrograph. The two disks orbit stars of similar mass (about one solar mass) and size, but their chemical fingerprints differ greatly. J1615 displays strong spectral features from H₂O, HCN, C₂H₂, CO₂, ¹³CO₂, OH, and ¹³C¹²CH₂, while GM Aur shows only H₂O and OH emission lines. Optical data revealed that J1615 is accreting gas onto its star about ten times more slowly than GM Aur.

Analysis: Modeling the Molecular Emission

To understand these differences, the team used slab models, simulations that estimate how hot gas emits light, to measure the temperature and abundance of molecules. J1615’s carbon-rich molecules arise from relatively warm gas (about 200–500 K) close to the star, within one astronomical unit (AU). GM Aur, in contrast, shows weaker emission and cooler, more distant gas. The water vapor in both disks is relatively faint, suggesting that some inner regions have been depleted of water, but J1615 retains more warm water near its star than GM Aur does. These findings point to distinct disk environments that influence which molecules survive or are destroyed.

Dust and Accretion: What Shapes the Chemistry

The team also modeled each disk’s spectral energy distribution (SED), the light from dust warmed by the star, to probe dust composition. J1615 contains larger, more crystalline silicate grains than GM Aur, implying its dust has undergone more growth and processing. Its lower accretion rate means less turbulent mixing and weaker heating of the inner disk. Together, these factors may allow carbon-rich gas to remain stable instead of being converted into water or other oxygen-rich compounds.

Discussion: A Carbon-Rich Transitional Disk

The combination of weaker accretion and more processed dust seems to make J1615’s disk an ideal environment for carbon-bearing molecules to thrive. In comparison, GM Aur’s higher accretion rate likely enhances the delivery of oxygen-rich material that destroys carbon species. This study adds J1615 to a small but growing list of carbon-rich disks around Sun-like stars, showing that such chemistry is not limited to low-mass systems. The diversity revealed by JWST highlights that even nearly identical young stars can host very different chemical worlds.

Conclusion

M. Volz et al. demonstrate how JWST’s infrared vision is transforming our understanding of planet-forming environments. J1615’s carbon-rich chemistry stands out among transitional disks, suggesting that small changes in accretion rate and dust structure can dramatically shift a disk’s molecular makeup. As JWST continues to survey more systems, astronomers will better understand how the ingredients for planets, especially those with carbon-based atmospheres, emerge and evolve around young stars.

Source: Volz