Jupiter’s heavy-element enrichment expected from formation models

Abstract

Aims. The goal of this work is to investigate Jupiter's growth focusing on the amount of heavy elements accreted by the planet, and its comparison with recent structure models of Jupiter. Methods. Our model assumes an initial core growth dominated by pebble accretion, and a second growth phase that is characterized by a moderate accretion of both planetesimals and gas. The third phase is dominated by runaway gas accretion during which the planet becomes detached from the disk. The second and third phases are computed in detail, considering two different prescriptions for the planetesimal accretion and fits from hydrodynamical studies to compute the gas accretion in the detached phase. Results. In order for Jupiter to consist of ∼20-40 M ⊕ of heavy elements as suggested by structure models, we find that Jupiter's formation location is preferably at an orbital distance of 1 a 10 au once the accretion of planetesimals dominates. We find that Jupiter could accrete between ∼1 and ∼15 M ⊕ of heavy elements during runaway gas accretion, depending on the assumed initial surface density of planetesimals and the prescription used to estimate the heavy-element accretion during the final stage of the planetary formation. This would yield an envelope metallicity of ∼0.5 to ∼3 times solar. By computing the solid (heavy-element) accretion during the detached phase, we infer a planetary mass-metallicity (M P -M Z ) relation of M Z ∼ M 2/5 P when a gap in the planetesimal disk is created, and of M Z ∼ M 1/6 P without a planetesimal gap. Conclusions. Our hybrid pebble-planetesimal model can account for Jupiter's bulk and atmospheric enrichment. The high bulk metallicity inferred for many giant exoplanets is difficult to explain from standard formation models. This might suggest a migration history for such highly enriched giant exoplanets and/or giant impacts after the disk's dispersal.

What Can Meteorites Tell Us About the Formation of Jupiter?

“…The combined isotopic and paleomagnetic measurements (between right green and red lines in Figure 1b) indicate that Jupiter grew from 50 M ⊕ to its final mass of ∼318 M ⊕ at a mean growth rate of >518 M ⊕ Ma −1 (Table S1). Both the magnitude of the latter growth rate as well as the rapid increase in growth rate relative to the previous growth phase (by a minimum factor of ∼30 and permissibly by many orders of magnitude) are consistent with typical models of core accretion, which predict runaway gas accretion rates ∼2-4 orders of magnitude faster than during the slow gas accretion phase (Machida et al, 2010;Venturini & Helled, 2020). The timing of the initiation of runaway gas accretion is insensitive to the choice of chronometers for dating CR chondrules and angrites, to uncertainties associated with the age of CAIs (Table S1), and to the factor of ∼2 uncertainty associated with the estimates of the isolation and crossover masses.…”

“…Given that the core accretion model predicts that Jupiter's growth accelerated significantly after it reached ∼50 M ⨁ due to the onset of runaway gas accretion, it is critical to determine the subsequent and final period of Jupiter's growth to ∼318 M ⨁ (Ginzburg & Chiang, 2019; Hubickyj et al., 2005; Lissauer et al., 2009; Venturini & Helled, 2020). At present, models tend to force the accretion rate to zero at ∼3–5 Ma time after CAI‐formation.…”

“…In the second phase, gas slowly accretes along with additional solids until the body reaches the crossover mass, at which point the atmosphere's mass is comparable to that of the core: In the third and last phase, runaway gas accretion at very rapid accretion rates (∼10 2 –10 4 M ⨁ Ma −1 ) forms the final planet over a timescale that may only last the order of ∼0.1 Ma (Lissauer et al., 2009; Machida et al., 2010; Uribe et al., 2013; Venturini & Helled, 2020). A growing giant planet might skip the second core accretion phase if it experiences extensive orbital migration (Alibert et al., 2005; Helled et al., 2014) and/or the opacity of the accreting material is low (Hori & Ikoma, 2010).…”

The nature of the radius valley

“…More work on the topic is needed to elucidate the importance of this mechanism. Another possibility is the accretion of planetesimals in addition to pebbles (Alibert et al 2018;Venturini & Helled 2020). In such a hybrid scenario the heat released by planetesimals delays the accretion of gas once pebble accretion stops at isolation mass (Guilera et al 2020).…”

Most super-Earths formed by dry pebble accretion are less massive than 5 Earth masses

“…We compute the gas accretion by solving the standard planetary internal structure equations, assuming a uniform luminosity that results from the accretion of solids and envelope contraction (see Alibert et al 2013;Venturini et al 2016;Venturini & Helled 2020, for details). The internal structure equations are solved using as boundary conditions the pressure and temperature of the protoplanetary disc at the position of the planetary embryo and defining the planetary radius as a combination of the Hill and Bondi radii, as suggested by 3D hydrodynamical simulations (Lissauer et al 2009):…”