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

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

Even before the observation of planets in extrasolar systems, there were numerous observations of analogues to our Kuiper Belt, first detected by their infrared excess (Aumann et al., 1984; Eiroa et al., 2010). ALMA has revolutionized our understanding of such debris discs, and one of the most outstanding discoveries is the observation of CO gas in approximately 30 systems thus far (e.g., Moór et al., 2017; MacGregor et al., 2017; Matrà et al. 2017). Models suggest that this gas is not primordial but is produced within the belt through collisions or sublimation of CO ices (Kral et al., 2017, 2019).

In our Solar System, it is estimated that only a small fraction of the gas could currently be produced within the Kuiper Belt, all of which would be expelled by the stellar wind (Kral et al., 2021). However, the dynamical models of the young solar system suggest the existence, for at least 10 Myr, of a massive planetesimal belt, also referred to as the primordial Kuiper Belt, with masses ranging between 5 and 50 M_⊕ (Liu et al., 2022; Griveaud et al., 2024). This massive belt would be equivalent to the extrasolar belts observed around young main-sequence stars.

Hence, our young Solar System could have hosted a secondary gas-rich disc that could have significantly altered the planets in the system. In particular, large amounts of CO gas could have been accreted onto Uranus and Neptune given that Kral et al. (2020) estimate that the accretion efficiency of gas onto planets in the debris disc is highly efficient.

Observations of Neptune and Uranus suggest that their atmospheres are enriched in carbon, with a C/H ratio approximately 80 times the protosolar value (see Guillot et al. 2023 and references therein). This could be explained by the formation process of the planets, but a significant fraction may also originate from late gas accretion. The amount of accreted carbon depends on the properties of the belt, such as its mass and lifetime, and thus on the dynamical history of the system. We therefore simulate the evolution of a gas disc for different belt origin and evolution corresponding to the Nice model with an initial compact configuration for planets (see Figure 1, Tsiganis et al., 2005; Griveaud et al., 2024) or the "rebound" instability with a more extended configuration for planets (see Figure 1, Liu et al., 2022). We also follow the evolution of the young "modern" Kuiper Belt after the dissipation of the primordial Kuiper Belt, since the Kral et al. (2021) model implies a higher mass production rate at the beginning of the belt's existence and therefore a putative gas disc for a significant timescale.

Figure 1: Schematic presenting a description of our set of simulations. The heavy belt cases are at the top (compact configuration) and middle (extended configuration), and the light cases are at the bottom. J, S, U, and N represent Jupiter, Saturn, Uranus, and Neptune, respectively. The different simulations vary in terms of the locations of planets, accretion efficiency, gas viscosity, and the belt’s mass. For the heavy belt configurations, the time when depletion starts is also a parameter. Each parameter has its own symbol and colour, as shown in the bottom part of the cartoon.

Our results indicate that the amount of carbon accreted onto Uranus and Neptune considering the modern Kuiper Belt is negligible. However, for all other dynamical models, the additional C/H ratio from the primordial Kuiper Belt is super-solar. Using a value of 50 M_⊕ belt based on the latest version of the Nice model (for a low-viscosity protoplanetary disc, Griveaud et al., 2024) leads to the highests C/H ratios, close to the observed C/H ratios for Uranus and Neptune.

To distinguish between the accretion of late gas and the planetary formation process (pebble/planetesimal accretion) from the observations, we use the S/H ratios. During planet formation, both carbon and sulphur are accreted (Mousis et al., 2024). However, since late gas is only produced by CO/CO_₂ ices, only carbon is accreted during this stage. The S/H ratio is thus a tracer of early accretion of pebbles or planetesimals. By comparing the observed C/H and S/H ratios, we obtain a rough estimate of the amount of late gas accreted into the atmospheres of Uranus and Neptune. This estimate is of the same order of magnitude as our simulation with the 50 M_⊕ belt (see Figure 2).

Figure 2: [C/H] for the fiducial simulation as a function of time. The filled areas correspond to predictions for Uranus (purple) and Neptune (blue). The uncertainties are shown via the extension of the filled areas and are due to uncertainties in the respective atmospheric masses. The solid and dashed black lines represent the maximum estimation of the gas disc contribution obtained from observations for Uranus and Neptune.

Since their atmospheres are much larger than those of Uranus and Neptune, we demonstrate that Jupiter and Saturn have experienced much less carbon enrichment due to late gas accretion. However, for our fiducial simulation, we still get significant enrichment in carbon due to late gas accretion.

We conclude our study by considering how this mechanism may be a universal process in extrasolar systems and how it could be detected in exoplanet atmospheres.

 

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