Abstract
<p>Atmospheric compositions can provide powerful diagnostics of formation and migration histories of planetary systems. In this talk, I will present the results of our latest survey of atmospheric compositions focused on atmospheric abundances of H<sub>2</sub>O, Na, and K. We employ a sample of 19 exoplanets spanning from cool mini-Neptunes to hot Jupiters, with equilibrium temperatures between ~300 and 2700 K. We employ the latest transmission spectra, new H<sub>2</sub> broadened opacities of Na and K, and homogeneous Bayesian retrievals. We confirm detections of H<sub>2</sub>O in 14 planets and detections of Na and K in 6 planets each. Among our sample, we find a mass-metallicity trend of increasing H<sub>2</sub>O abundances with decreasing mass, spanning generally substellar values for gas giants and stellar/superstellar for Neptunes and mini-Neptunes. However, the overall trend in H<sub>2</sub>O abundances, is significantly lower than the mass-metallicity relation for carbon in the solar system giant planets and similar predictions for exoplanets. On the other hand, the Na and K abundances for the gas giants are stellar or superstellar, consistent with each other, and generally consistent with the solar system metallicity trend. The H<sub>2</sub>O abundances in hot gas giants are likely due to low oxygen abundances relative to other elements rather than low overall metallicities, and provide new constraints on their formation mechanisms. Our results show that the differing trends in the abundances of species argue against the use of chemical equilibrium models with metallicity as one free parameter in atmospheric retrievals, as different elements can be differently enhanced.</p>
Highlights
Exoplanet science has entered an era of comparative studies of planet populations
Our study reveals three key trends in the atmospheric compositions of our exoplanet sample
From miniNeptunes to hot Jupiters, H2O abundances are generally consistent with or depleted compared to equilibrium expectations based on stellar abundances, and lower than the solar system metallicity trend
Summary
Several studies have used empirical metrics for comparative characterization of giant exoplanetary atmospheres based on their transmission spectra (e.g., Heng 2016; Sing et al 2016; Stevenson 2016; Fu et al 2017). Comparative studies are being carried out using full atmospheric retrievals, primarily constraining H2O abundances and/or cloud properties from transmission spectra (e.g., Madhusudhan et al 2014b; Barstow et al 2017; Pinhas et al 2019). Previous ensemble studies have focused on H2O and found low abundances compared to solar system expectations (e.g., Madhusudhan et al 2014b; Barstow et al 2017; Pinhas et al 2019). It has been unclear if the low H2O abundances. We conduct a homogeneous survey of Na, K, and H2O abundances for a broad sample of transiting exoplanets, and investigate their compositional diversity
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