Influence of sub- and super-solar metallicities on the composition of solid planetary building blocks

Abstract

The composition of the protoplanetary disc is thought to be linked to the composition of the host star, where a higher overall metallicity of the host star provides more building blocks for planets. However, most of the planet formation simulations only link the stellar iron abundance [Fe/H] to planet formation and the iron abundance in itself is used as a proxy to scale all elements. On the other hand, large surveys of stellar abundances show that this is not true. We use here stellar abundances from the GALAH surveys to determine the average detailed abundances of Fe, Si, Mg, O, and C for a broad range of host star metallicities with [Fe/H] spanning from -0.4 to +0.4. Using an equilibrium chemical model that features the most important rock forming molecules as well as volatile contributions of H 2 O, CO 2 , CH 4 and CO, we calculate the chemical composition of solid planetary building blocks around stars with different metallicities. Solid building blocks that are formed entirely interior to the water ice line (T>150K) only show an increase in Mg 2 SiO 4 and a decrease in MgSiO 3 for increasing host star metallicity, related to the increase of Mg/Si for higher [Fe/H]. Solid planetary building blocks forming exterior to the water ice line (T<150K), on the other hand, show dramatic changes in their composition. In particular the water ice content decreases from around ∼50% at [Fe/H]=-0.4 to ∼6% at [Fe/H]=0.4 in our chemical model. This is mainly caused by the increasing C/O ratio with increasing [Fe/H], which binds most of the oxygen in gaseous CO and CO 2 , resulting in a small water ice fraction. Planet formation simulations coupled with the chemical model confirm these results by showing that the water ice content of super-Earths decreases with increasing host star metallicity due to the increased C/O ratio. This decrease of the water ice fraction has important consequences for planet formation, planetary composition and the eventual habitability of planetary systems formed around these high metallicity stars.

Characterization of the Nucleus, Morphology, and Activity of Interstellar Comet 2I/Borisov by Optical and Near-infrared GROWTH, Apache Point, IRTF, ZTF, and Keck Observations

“…where m V is the apparent magnitude; H abs is the absolute magnitude; r h is the heliocentric distance in au; Δ is the observercentric distance in au; Φ(α) is a function describing the brightening of the comet, which we approximate with Φ(α)= −0.04α (Jewitt 1991); and α is the phase angle of the comet measured in degrees, appropriate for comets at smaller phase angles than ∼20° (Bertini et al 2017). We translate the H abs magnitude computed from Equation (1) into an effective cross section, C, in units of km 2 within a 10,000 km aperture using from Jewitt et al (2016), where p v is the albedo of the comet, assumed to be 0.10, typical for comet dust (Jewitt & Meech 1986;Kolokolova et al 2004).…”

Parameterisations of interior properties of rocky planets

“…It has been suggested that close-in exoplanets may contain higher Al and Ca abundances than observed in the Solar System (Dorn et al 2019) and that stars with very high carbon abundances may lead to planets with completely different chemistries (Kuchner & Seager 2005;Unterborn et al 2014;Nisr et al 2017;Madhusudhan et al 2012). However, it is not unlikely to assume that the majority of rocky exoplanets formed in close proximity to their star (as was the case for the planets in the inner Solar System) would show mantle minerals quite similar to Earth with variations in Mg-Fe-Si concentrations depending on the stellar evolution history (Bitsch & Battistini 2020). Dorn et al (2015) and other recent publications have also shown that the ratio of these elements is related to the abundances of Mg, Si and Fe in the star and can in some cases be observed in the stellar spectrum.…”

Influence of grain size and composition on the contraction rates of planetary envelopes and on planetary migration

“…We list these species, as well as their condensation temperature and their volume mixing ratios v Y in Table 1. More details on the chemical model can be found in Bitsch et al (2018a) and Bitsch & Battistini (2020).…”

“…We note that the Mg abundance is always larger than the Si abundance. We follow here the different mixing ratios from Bitsch & Battistini (2020). As water ice is the most abundant molecule in our model, the change of opacity is largest at the water ice line.…”

“…Buder et al 2018), which can be explained through different elemental production sites, for example low-mass stars and supernovae (Burbidge et al 1957). Bitsch & Battistini (2020) derived the water content of formed super-Earths depending on the underlying stellar abundance and found that stars with super-solar [Fe/H] should host waterpoor super-Earths, while stars with sub-solar [Fe/H] should host water worlds, on average. This result is based on the assumption that the host star metallicities are a proxy for the chemical composition of the planet forming disc.…”