Constraining planet structure and composition from stellar chemistry: trends in different stellar populations

Abstract

Context. The chemical composition of stars that have orbiting planets provides important clues about the frequency, architecture, and composition of exoplanet systems. Aims. We explore the possibility that stars from different galactic populations that have different intrinsic abundance ratios may produce planets with a different overall composition. Methods. We compiled abundances for Fe, O, C, Mg, and Si in a large sample of solar neighbourhood stars that belong to different galactic populations. We then used a simple stoichiometric model to predict the expected iron-to-silicate mass fraction and water mass fraction of the planet building blocks, as well as the summed mass percentage of all heavy elements in the disc. Results. Assuming that overall the chemical composition of the planet building blocks will be reflected in the composition of the formed planets, we show that according to our model, discs around stars from different galactic populations, as well as around stars from different regions in the Galaxy, are expected to form rocky planets with significantly different iron-to-silicate mass fractions. The available water mass fraction also changes significantly from one galactic population to another. Conclusions. The results may be used to set constraints for models of planet formation and chemical composition. Furthermore, the results may have impact on our understanding of the frequency of planets in the Galaxy, as well as on the existence of conditions for habitability.

The Chemical Homogeneity of Sun-like Stars in the Solar Neighborhood

“…Combined spectra for a total of 79 targets were created in this manner. An additional three stars (HIP 19911, HIP 67620, and HIP 103983) were included in the initial solar twin sample, but were later dropped due to spectral contamination by a nearby companion (dos Santos et al 2017). The combined spectra of the remaining stars varies in quality depending on the brightness of the target and the number of HARPS spectra observed, 1 Data were used from ESO programme IDs 072.C-0488, 074.C-0364, 075.C-0202, 075.C-0332, 076.C-0155, 077.C-0364, 088.C-0323, 089.C-0415, 089.C-0732, 090.C-0421, 091.C-0034, 091.C-0936, 092.C-0721, 093.C-0409, 095.C-0551, 096.C-0499, 097.C-0090, 097.C-0571, 097.…”

“…The excluded stars are: HIP 19911, HIP 14501, HIP 28066, HIP 30476, HIP 33094, HIP 65708, HIP 73241, HIP 74432, HIP 108158, HIP 109821, and HIP 115577. One additional star, HIP 64150, was excluded because its abundances of several s-process elements are highly anomalous, potentially as a result of past accretion from its close binary companion (dos Santos et al 2017;Spina et al 2018).…”

Exoplanet Clouds

“…It appears more feasible to address the relation between the host star's element abundances and the element abundance of the planet's core (e.g. Santos et al 2017). The challenges hereby, however, lay with the precise measurement The cumulative cloud material volume fractions for a set of 1D DriftPhoenix model atmospheres for varying carbon-to-oxygen ratio (C/O) and sulphur abundance.…”

Exoplanet interiors and habitability

“…The relative fractions of the different elements in the mantle depend in a complicated way on the composition of the protoplanetary nebula and on the formation processes of the planet, which together determine the bulk chemical composition of the planet, but also on the core formation process, which partitions elements between the mantle and the core. For the bulk Earth, the relative abundance of the chemical elements in refractory componentselements or minerals that have a high equilibrium condensation temperature above about 1300 K and condense first from a cooling protoplanetary nebulais nearly equal to that of the Sun, suggesting that the refractory composition of a central star might be used as a first estimate of the bulk chemical composition of a rocky exoplanet [87][88][89][90][91]. However, even among the terrestrial planets of the Solar System, large differences in bulk compositions exist.…”