Mass-Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of H2O, Na, and K


Mass-metallicity relation for planets with chemical detections above 2σ significance (see Table 1). The H2O/H, Na/H, and K/H abundances are shown in blue, yellow, and orange, respectively. All the abundances are normalized to expectations based on their host stars. The metallicity estimates for the solar system giant planets using their methane (CH4) abundances (Atreya et al. 2016) are shown in coral. The coral shaded regions show the 1σ and 2σ metallicity trend for the solar system planets resulting from a linear fit. The corresponding fit to the exoplanet H2O abundances, excluding WASP-39b (see Section 5.2), and the confidence intervals are shown in the blue line and sky blue shaded regions.

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 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.

Luis Welbanks, Nikku Madhusudhan, Nicole F. Allard, Ivan Hubeny, Fernand Spiegelman, Thierry Leininger
(Submitted on 10 Dec 2019)
Comments: Accepted for publication in The Astrophysical Journal Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:1912.04904 [astro-ph.EP] (or arXiv:1912.04904v1 [astro-ph.EP] for this version)
Submission history
From: Luis Welbanks
[v1] Tue, 10 Dec 2019 19:00:00 UTC (4,368 KB)
https://arxiv.org/abs/1912.04904
Astrobiology, Astrochemistry

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