Giant Planet Formation Models with a Self-consistent Treatment of the Heavy Elements

Abstract

We present a new numerical framework to model the formation and evolution of giant planets. The code is based on the further development of the stellar evolution toolkit Modules for Experiments in Stellar Astrophysics. The model includes the dissolution of the accreted planetesimals/pebbles, which are assumed to be made of water ice, in the planetary gaseous envelope, and the effect of envelope enrichment on the planetary growth and internal structure is computed self-consistently. We apply our simulations to Jupiter and investigate the impact of different heavy-element and gas accretion rates on its formation history. We show that the assumed runaway gas accretion rate significantly affects the planetary radius and luminosity. It is confirmed that heavy-element enrichment leads to shorter formation timescales due to more efficient gas accretion. We find that with heavy-element enrichment Jupiter’s formation timescale is compatible with typical disks’ lifetimes even when assuming a low heavy-element accretion rate (oligarchic regime). Finally, we provide an approximation for the heavy-element profile in the innermost part of the planet, providing a link between the internal structure and the planetary growth history.

How planets grow by pebble accretion

“…In such an event, the impactor shatters and breaks up into a rapidly expanding cloud. The criterion for such a breakup is given by (e.g., Valletta & Helled 2018)…”

“…Any impactor that travels through an envelope encounters increased gas densities and consequently experiences friction and drag. For planetesimals that are sufficiently large to maintain high velocities, this can lead to significant ablation and cause compressive fractures (Mordasini et al 2015;BVO18;Valletta & Helled 2018). Smaller objects like pebbles are slowed down more efficiently and instead predominantly lose mass through evaporation by thermal radiation in the envelope's deep interior.…”

“…We do not consider frictional heating for two reasons: first, there is great uncertainty on the friction parameter. Including it therefore warrants a parameter study like Valletta & Helled (2018) performed, instead of choosing a single value. Second, friction is expected to be most significant in regions where the ram pressure is high, the same regions that are already affected by fragmentation.…”

“…Such small objects quickly ablate when they encounter gas with temperatures in excess of their evaporation temperature, a condition that is quickly reached in the hot primordial envelopes (Alibert 2017;Brouwers et al 2018, hereafter BVO18). But even km-sized planetesimals are vulnerable and are expected to break up when they impact the envelopes of super-Earth sized planets (Podolak et al 1988;McAuliffe & Christou 2006;Mordasini et al 2015;Pinhas et al 2016;BVO18;Valletta & Helled 2018). As a result, these envelopes become polluted with high-Z vapor and their interiors consequently become significantly more hot and dense (Iaroslavitz & Podolak 2007;Lozovsky et al 2017;Bodenheimer et al 2018).…”

Ariel planetary interiors White Paper

“…Composition gradients in giant planets could be a result of number of physical processes such as: (1) Solids (heavy elements) accretion during the formation process (e.g. [7,11,16,61,89,153,154]). (2) Solubility of materials in metallic hydrogen followed by convective mixing (e.g.…”