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

Reza Ashtari and co-authors reported on August 28 that James Webb Space Telescope observations of the exoplanet HAT-P-1 b found strong evidence for water vapor and carbon dioxide in the planet’s atmosphere. The team used JWST’s Near-Infrared Spectrograph, or NIRSpec, to observe the hot Jupiter across wavelengths from 2.7 to 5.3 microns, then combined those data with earlier Hubble Space Telescope observations to build a broader 0.3-to-5.3 micron picture of the atmosphere. (arxiv.org) The paper was posted on arXiv on August 28 and says it has been accepted by *The Astronomical Journal*. The result is the underlying study behind a September 1 social-media post that listed HAT-P-1 b among recent exoplanet findings. In the paper, the authors said the JWST spectrum revealed strong evidence for H2O and CO2 and tentative evidence for hydrogen sulfide. ### Which planet did Webb observe? HAT-P-1 b is a gas-giant exoplanet listed in Exo.MAST with a radius of about 1.319 Jupiter radii and a mass of about 0.525 Jupiter masses. The planet orbits a Sun-like star in the constellation Lacerta every 4.4652998 days at a distance of about 0.056 astronomical units, with an equilibrium temperature listed near 1,325 kelvin. (arxiv.org) The Exo.MAST entry places the system about 158.98 parsecs away and lists the host star at visual magnitude 9.8. Those characteristics have made HAT-P-1 b a recurring target for atmospheric studies with both Hubble and now JWST. (arxiv.org) ### What exactly did the new JWST data show? The August 28 paper said atmospheric retrievals found strong evidence for water and carbon dioxide, with Bayes factors of log10B=8.9 for H2O and log10B=52.3 for CO2. The authors said those signals came from transmission spectroscopy, which measures starlight filtered through the planet’s atmosphere during transit. (exo.mast.stsci.edu) The same analysis said there was tentative evidence for hydrogen sulfide, with a Bayes factor of log10B=1.4. The authors also reported that three separate reduction pipelines — Eureka!, FIREFLy and Tswift — produced mutually consistent transmission spectra across the JWST bandpass. (exo.mast.stsci.edu) ### Why did the team combine Webb and Hubble data? The study said the JWST observations extended HAT-P-1 b’s atmospheric coverage to longer wavelengths than earlier Hubble studies could reach. By combining the new JWST spectrum with archival HST STIS and WFC3 observations, the authors said they could analyze the atmosphere across a much broader wavelength range. (arxiv.org) That broader view helped the team estimate the planet’s atmospheric metallicity at about 10 times solar and place a 3-sigma upper limit of less than 0.52 on the carbon-to-oxygen ratio. (arxiv.org) The paper said those constraints favored an atmosphere near chemical equilibrium and pointed to an oxygen-rich composition. ### Did the paper say anything else about the atmosphere? The authors said they found no significant evidence for clouds muting the spectrum. Instead, the paper said the persistence of molecular structure across the observed wavelengths argued against strong cloud suppression. (arxiv.org) The paper also said the possible hydrogen-sulfide signal, if confirmed, would suggest limited photochemical processing at the pressures probed. That point was presented by the authors as tentative, not definitive. (arxiv.org) ### Where can readers find the next step? The August 28 preprint is posted on arXiv under identifier 2608.28538, and the paper says a final revision is pending after acceptance at *The Astronomical Journal*. The Exo.MAST page for HAT-P-1 b also lists basic system parameters and JWST target information for the planet. (arxiv.org)