In the cold outer reaches of the Solar System, water is considered the main building material for ices. But some comets and distant icy bodies have an unusually high ratio of carbon monoxide (CO) to water, as if a scoop of ice cream suddenly had too much syrup. The authors of a new study set out to find where this imbalance comes from.

They assembled a computer model of the protoplanetary disk: that rotating "pancake" of gas and dust where planets later assemble. In the model, dust grains stick together, fragment, and drift toward the star, while water and CO transition between ice and vapor. They also accounted for the disk's accretion stage — the phase when material is still falling onto it from the surrounding cloud.

However, pebbles are not yet planetesimals — the asteroid-sized seeds of planets. In a smooth disk without features, CO-rich pebbles don't have time to gather into large bodies. To grow, they need "traps"—regions where gas pressure is elevated and dust accumulates, or processes that remove gas, leaving only solid particles.

The model with disk accretion enhanced CO enrichment in the outer region. This implies that the disk's chemical evolution and the future atmospheric composition of planets are tightly linked to how the disk grew. Otherwise, predictions about how much carbon ends up in a newborn planet's atmosphere could be off the mark.