JWST finds water, CO2 on HAT-P-1 b

Scientists behind a new JWST exoplanet study said data on HAT-P-1 b strengthen the case that the hot Jupiter’s atmosphere contains water vapor and carbon dioxide. The result comes from transmission spectroscopy with JWST’s NIRSpec G395H instrument, combined with earlier Hubble Space Telescope observations to build a spectrum from 0.3 to 5.3 microns. (arxiv.org) The paper was posted on arXiv on August 28 and was circulated widely on September 1 in science-focused social posts. The authors say the dataset also shows tentative evidence for hydrogen sulfide, though that part of the result is less secure. ### What exactly did JWST see in HAT-P-1 b’s atmosphere? The paper reports “strong evidence” for H2O and CO2 in HAT-P-1 b’s atmosphere from transmission spectra taken as the planet crossed in front of its star. In that method, astronomers measure which wavelengths of starlight are absorbed by gases in the planet’s atmosphere during transit, then compare the resulting spectral features with atmospheric models. The team said JWST covered 2.7 to 5.3 microns with NIRSpec G395H and paired that with archival HST STIS and WFC3 data to extend the analysis across visible and near-infrared wavelengths. (arxiv.org) ### Why are water and carbon dioxide the headline molecules here? The authors gave the strongest statistical support to carbon dioxide and water. Their retrievals found Bayes factors of log10B=52.3 for CO2 and log10B=8.9 for H2O, values the paper describes as strong evidence, while hydrogen sulfide came in at log10B=1.4 and was labeled tentative. The study says those combined H2O and CO2 constraints favor an atmosphere near chemical equilibrium rather than one dominated by stronger disequilibrium effects. ### What kind of planet is HAT-P-1 b? (arxiv.org) HAT-P-1 b is a hot Jupiter, meaning a gas giant orbiting very close to its host star. That makes it a good target for transmission spectroscopy because its large atmosphere can leave comparatively strong spectral fingerprints during transit. The authors describe the planet as part of JWST’s Exoplanet Grand Tour Survey, which is using Webb to compare atmospheres across a set of well-studied worlds. ### Did the study say anything beyond “water and CO2 are present”? The paper reports an atmospheric metallicity of about 10 times solar and a 3-sigma upper limit on the carbon-to-oxygen ratio of less than 0.52. (arxiv.org) The authors said that combination points to an oxygen-rich atmosphere. They also wrote that they found no significant evidence for clouds muting the spectrum, saying the molecular structure persists across the observed wavelength range. ### How firm is this result, and what remains uncertain? The August 28 arXiv paper says three separate reduction pipelines — Eureka!, FIREFLy and Tswift — produced mutually consistent transmission spectra across the JWST bandpass. (arxiv.org) That consistency strengthens the core water and carbon-dioxide result. The hydrogen-sulfide signal remains the main caveat: the title itself says “possibly hydrogen sulfide,” and the abstract says confirmation would require further work. ### Why is JWST central to this kind of measurement? NASA says JWST is designed for infrared observations and studies everything from the early universe to the formation and evolution of planetary systems. (arxiv.org) For exoplanet atmospheres, Webb’s infrared coverage lets researchers probe molecular bands that were harder to isolate with earlier facilities alone. In this study, the longer-wavelength JWST data were what allowed the team to connect older Hubble measurements to the stronger molecular features now reported for HAT-P-1 b. The paper is listed on arXiv as accepted to The Astronomical Journal, with final revision pending. (arxiv.org) Any next step will center on follow-up observations and modeling aimed at testing the possible hydrogen-sulfide feature and refining abundance estimates for HAT-P-1 b’s atmosphere. (science.nasa.gov)