Late gas released in the young Kuiper belt could have significantly contributed to the carbon enrichment of the atmospheres of Neptune and Uranus

¹ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92190 Meudon, France
² Université Côte d’Azur, Laboratoire Lagrange, OCA, CNRS UMR 7293, Nice, France

^★ Corresponding author: paul.huet@obspm.fr

Received: 21 March 2025
Accepted: 2 June 2025

Abstract

Context. Exo-Kuiper belts have been observed for decades, but the recent detection of gas in some of them may change our view of the Solar System’s youth. Late gas produced by the sublimation of CO (or CO₂) ices after the dissipation of the primordial gas could be the norm in young planetesimal belts. Hence, a gas-rich Kuiper belt could have been present in the Solar System. The high C/H ratios observed on Uranus and Neptune could be a clue to the existence of such late gas that could have been accreted onto young icy giants.

Aims. The aim of this paper is to estimate the carbon enrichment of the atmospheres of Uranus and Neptune caused by the accretion of the gas released from a putative gas-rich Kuiper belt. We want to test whether a young, massive Kuiper belt such as that usually assumed by state-of-the-art models can explain the current C/H values of ~50–80 times the protosolar abundance for Uranus and Neptune.

Methods. We developed a model that can follow the gas released in the Kuiper belt, as well as its viscous evolution and its capture onto planets. We calculated the final C/H ratio and compared it to observations. We studied the influence of several important parameters such as the initial mass of the belt, the viscosity of the disc, and the accretion efficiency.

Results. We find that the assumption of a primordial Kuiper belt with a mass of tens of Earth masses leads to significant CO gas accretion onto the giants, which can lead to high C/H ratios, especially for Uranus and Neptune. We find that an initial Kuiper belt of ~50 M_⊕ could entirely account for the present-day C/H enrichment in the atmospheres of Uranus and Neptune. However, given the fact that S/H is also significantly enriched in the deep atmospheres of these planets, but still less enriched than C/H, a more likely scenario is that these planets first accreted an envelope enriched in C/H and S/H in similar amounts, and that the sublimation of CO from the Kuiper belt led to an additional enrichment in C/H of perhaps 30 times the protosolar value in Neptune, and 20 times in Uranus. For the same model, the additional enrichments in C/H are 2 and 0.2 in Saturn and Jupiter, respectively.

Conclusions. Our model shows that a relatively massive gas-rich Kuiper belt could have existed in the Solar System’s youth, which significantly enriched the atmospheres of Uranus and Neptune with carbon. Late gas accretion and its effect on the metallicities of the outer giant planets could be a universal scenario that also occurs in extrasolar systems. Observations of sub-Jupiter exoplanets could provide very useful information to better constrain this scenario, with an enrichment in carbon and oxygen (for sufficiently war planets) compared to other elements that should be inversely proportional to their envelope mass.