Abstract
The composition of the protoplanetary disc is thought to be linked to the composition of the host star, where a higher overall metallicity provides the 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. Here we use 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 compounds as well as volatile contributions of H2O, CO2, CH4, 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 > 150 K) only show an increase in Mg2SiO4 and a decrease in MgSiO3 for increasing host star metallicity, which is related to the increase of [Mg/Si] for higher [Fe/H]. Solid planetary building blocks forming exterior to the water ice line (T < 150 K), 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 CO2, 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.
Highlights
Observations of exoplanets have revealed that close-in superEarths are the most abundant type of planet within 1 AU (Mayor et al 2011; Mulders et al 2018; Zhu & Wu 2018)
Solid building blocks that are formed entirely interior to the water ice line (T > 150 K) only show an increase in Mg2SiO4 and a decrease in MgSiO3 for increasing host star metallicity, which is related to the increase of [Mg/Si] for higher [Fe/H]
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
Summary
Observations of exoplanets have revealed that close-in superEarths are the most abundant type of planet within 1 AU (Mayor et al 2011; Mulders et al 2018; Zhu & Wu 2018) These superEarths typically have masses of a few Earth masses; planets within the same system could be of similar size (Weiss et al 2018) and, presumably of a similar mass, recent analyses have questioned this correlation (Zhu 2019). These super-Earths exist around many different type of stars, even around small M-dwarfs (Gillon et al 2016, 2017). If more building blocks are available, planetary embryos at larger distances can grow faster and form planets through the accretion of pebbles (Bitsch et al 2015b), which could explain these trends and could give rise to the giant planet eccentricity distribution (Buchhave et al 2018)
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