Abstract

Observations of gas in exo-Kuiper belts (mostly CO) suggest it may be common in young planetary systems, potentially reshaping our understanding of the Solar System’s youth. Uranus and Neptune’s high atmospheric C/H ratios (60–80× protosolar) could trace late accretion of carbon-rich gas from a primordial Kuiper belt. We model gas release and viscous evolution in a Solar System-analog belt, quantifying gas capture by the ice giants. Using a disk model with varied initial masses (up to 50 M⊕), viscosities, and accretion efficiencies, we simulate CO release and planetary enrichment. Results show a massive belt (∼50 M⊕, similar to that considered in e.g. the Nice model) can supply sufficient CO via late accretion to explain observed C/H values. While solid accretion during formation contributes to carbon enrichment, we find that an additional late accretion may be needed to explain the very high super solar values, which aligns with gas capture from a young, gaseous Kuiper belt. This mechanism may be universal, influencing metallicity in exoplanetary giants, with observational implications for sub-Jupiter exoplanets. Our findings support a once-gas-rich Kuiper belt as a key driver of ice giant atmospheric evolution.