H2O abundances in the atmospheres of three hot Jupiters

TL;DR

This study measures water vapor in three hot Jupiter atmospheres using HST data, providing insights into their composition and formation, with the most precise water measurement in an exoplanet to date.

Contribution

First precise measurement of H2O in HD 209458b, revealing sub-solar water abundance and advancing understanding of hot Jupiter atmospheres and formation conditions.

Findings

HD 189733b and HD 209458b have nearly solar or sub-solar H2O abundances.

WASP-12b's H2O abundance is not well constrained by current data.

HD 209458b shows the most precise sub-solar H2O measurement in an exoplanet to date.

Abstract

The core accretion theory for giant planet formation predicts enrichment of elemental abundances in planetary envelopes caused by runaway accretion of planetesimals, which is consistent with measured super-solar abundances of C, N, P, S, Xe, and Ar in Jupiter's atmosphere. However, the abundance of O which is expected to be the most dominant constituent of planetesimals is unknown for solar system giant planets, owing to the condensation of water in their ultra-cold atmospheres, thereby posing a key unknown in solar system formation. On the other hand, hundreds of extrasolar hot Jupiters are known with very high temperatures (>~1000 K) making them excellent targets to measure H2O abundances and, hence, oxygen in their atmospheres. We constrain the atmospheric H2O abundances in three hot Jupiters (HD 189733b, HD 209458b, and WASP-12b), spanning a wide temperature range (1200-2500 K), using their near-infrared transmission spectra obtained using the HST WFC3 instrument. We report conclusive measurements of H2O in HD 189733b and HD 209458b, while that in WASP-12b is not well constrained by present data. The data allow nearly solar as well as significantly sub-solar abundances in HD 189733b and WASP-12b. However, for HD 209458b, we report the most precise H2O measurement in an exoplanet to date that suggests a ~20-135 sub-solar H2O abundance. We discuss the implications of our results on the formation conditions of hot Jupiters and on the likelihood of clouds in their atmospheres. Our results highlight the critical importance of high-precision spectra of hot Jupiters for deriving their H2O abundances.