Dust evolution during protoplanetary disk buildup enhances CO ice relative to water

Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

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Received: 27 January 2026
Accepted: 25 March 2026

Abstract

Context. Water ice is expected to be the dominant volatile component of bodies formed in the outer Solar System. However, recent observations of comets and trans-Neptunian objects suggest that the relative abundances of ices can vary substantially, with some bodies exhibiting unusually high CO/H₂O ratios.

Aims. We studied the prospects of CO-rich pebbles and planetesimals being produced.

Methods. We used a one-dimensional protoplanetary disk model with dust evolution including coagulation, fragmentation, and radial drift; water and CO ice and vapor evolution; and planetesimal formation via the streaming instability. We compared models with and without the disk formation stage.

Results. CO-rich pebbles can form at the CO snow line due to the cold-finger effect, regardless of whether the disk buildup is included. Models including disk buildup show stronger CO enhancement relative to water in the outer disk. However, CO-rich planetesimals do not form in the smooth disk models.

Conclusions. The formation of CO-rich planetesimals likely requires mechanisms that preserve the CO-enriched ice reservoir, such as pressure traps or gas removal processes. Models dealing with the chemical evolution of protoplanetary disks and its impact on the atmospheric C/O ratio of forming planets should consider the disk buildup stage.