Evolution of Atmospheric Composition in Giant Exoplanets with Diluted Cores

Transmission spectroscopy has reported sub-solar C/O ratios (<0.55) for atmospheres of some transiting gas giants/dwarfs (e.g., Sing et al. 2024; Welbanks et al. 2024). However, static disk chemistry models (e.g., Öberg et al. 2011) predict super-solar C/O ratios for protoplanetary disk gas and, consequently, for planetary envelopes. In this study, we consider two possible origins of such low atmospheric C/O ratios–formation and internal evolutionary origins–and propose how to distinguish them with young exoplanetary systems.
Possible mechanisms for the formation origin include evaporation of radially-drifting pebbles in the protoplanetary disk and of accreting pebbles in the envelope during planet formation to supply heavy elements (e.g., Brouwers et al. 2018). In contrast, a possible mechanism for the internal evolutionary origin is the transport of heavy elements with sub-solar C/O from the planetary interior. Recent studies of Jupiter and Saturn have suggested the presence of diluted cores, in which the heavy-element abundance decreases continuously with radius (e.g., Wahl et al. 2017; Mankovich & Fuller 2021). In gas giants with such internal structures, heavy elements may be transported into the atmosphere through internal mixing with time (Knierim & Helled, 2025), providing another possible origin of low C/O ratios.
Thus, both formation and internal evolution may produce low C/O ratios. Distinguishing between them is important for understanding the diversity of planet formation and evolution; however, this may be challenging because most currently observed planets are older than 1 Gyr when internal mixing might have already been completed. The C/O ratios of young planets may provide an important clue to the origin of low atmospheric C/O.
In this study, we investigate the atmospheric C/O evolution of gas giants with diluted cores as a candidate internal evolutionary origin, focusing on when low C/O can be realized. We use MESA (Paxton et al. 2011–2019; Jermyn et al. 2023) together with the planetary module MESPA (Helled, Müller, & Knierim 2025). The initial entropy and diluted-core profiles are taken from Knierim & Helled (2025), and the initial atmospheric C/O is given from gas-phase values in the model of Öberg et al. (2011). We also include internal heating, in which a fraction of the incident stellar irradiation is transported into the deep envelope (Komacek & Youdin 2017). This is motivated by the fact that some planets appear to show low C/O together with hotter interior states than predicted by standard gas-giant evolution models (Fortney et al. 2020).
As a result, in some of our model setups, sub-solar C/O can be realized by ~10 Myr. This suggests that low C/O inferred for young planets may not uniquely determine its origin. On the other hand, in strongly heated environments, characterized by high equilibrium temperatures and high energy transport efficiencies, the post-formation reduction of atmospheric C/O tends to become less effective. This suggests that strongly irradiated young planets, particularly hot to ultra-hot Jupiters, may be promising observational targets for distinguishing the origin of low atmospheric C/O. In this presentation, we discuss the implications of these results for observations of young planets.