Abstract
Abstract Atmospheric compositions can provide powerful diagnostics of formation and migration histories of planetary systems. We investigate constraints on atmospheric abundances of H2O, Na, and K, in a sample of transiting exoplanets using the latest transmission spectra and new H2 broadened opacities of Na and K. Our sample of 19 exoplanets spans from cool mini-Neptunes to hot Jupiters, with equilibrium temperatures between ∼300 and 2700 K. Using homogeneous Bayesian retrievals we report atmospheric abundances of Na, K, and H2O, and their detection significances, confirming 6 planets with strong Na detections, 6 with K, and 14 with H2O. We find a mass–metallicity trend of increasing H2O abundances with decreasing mass, spanning generally substellar values for gas giants and stellar/superstellar for Neptunes and mini-Neptunes. However, the overall trend in H2O abundances, from mini-Neptunes to hot Jupiters, 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 H2O 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. 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.
Highlights
Exoplanet science has entered an era of comparative studies of planet populations
The atmospheric constraints for our sample of 19 transiting exoplanets, ranging from hot Jupiters to cool mini-Neptunes are shown in Table 1.6 The detection significance is calculated from the Bayes factor (Benneke & Seager 2013; Buchner et al 2014)
Our study reveals three key trends in the atmospheric compositions of our exoplanet sample
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). 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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