The Complete transmission spectrum of WASP-39b with a precise water constraint

TL;DR

This study presents a comprehensive near-infrared transmission spectrum of exoplanet WASP-39b, detecting water features and deriving atmospheric properties, revealing higher metallicity than expected and suggesting diverse planet formation processes.

Contribution

First complete NIR transmission spectrum of WASP-39b with precise water feature measurements, enabling detailed atmospheric characterization and metallicity estimation.

Findings

Detected water absorption features at 0.95, 1.2, and 1.4 microns

Derived atmospheric temperature of approximately 1030 K

Estimated high atmospheric metallicity of about 151 times solar

Abstract

WASP-39b is a hot Saturn-mass exoplanet with a predicted clear atmosphere based on observations in the optical and infrared. Here we complete the transmission spectrum of the atmosphere with observations in the near-infrared (NIR) over three water absorption features with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) G102 (0.8-1.1 microns) and G141 (1.1-1.7 microns) spectroscopic grisms. We measure the predicted high amplitude H2O feature centered at 1.4 microns, and the smaller amplitude features at 0.95 and 1.2 microns, with a maximum water absorption amplitude of 2.4 planetary scale heights. We incorporate these new NIR measurements into previously published observational measurements to complete the transmission spectrum from 0.3-5 microns. From these observed water features, combined with features in the optical and IR, we retrieve a well constrained temperature Teq = 1030(+30,-20) K, and atmospheric metallicity 151 (+48,-46)x solar which is relatively high with respect to the currently established mass-metallicity trends. This new measurement in the Saturn-mass range hints at further diversity in the planet formation process relative to our solar system giants.