HST PanCET program: a complete near-UV to infrared transmission spectrum for the Hot Jupiter WASP-79b

Alexander D. Rathcke*, Ryan J. MacDonald, Joanna K. Barstow, Jayesh M. Goyal, Mercedes Lopez-Morales, Joao M. Mendonca, Jorge Sanz-Forcada, Gregory W. Henry, David K. Sing, Munazza K. Alam, Nikole K. Lewis, Katy L. Chubb, Jake Taylor, Nikolay Nikolov, Lars A. Buchhave

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


We present a new optical transmission spectrum of the hot Jupiter WASP-79b. We observed three transits with the STIS instrument mounted on the Hubble Space Telescope (HST), spanning 0.3–1.0 μm. Combining these transits with previous observations, we construct a complete 0.3–5.0 μm transmission spectrum of WASP-79b. Both HST and ground-based observations show decreasing transit depths toward blue wavelengths, contrary to expectations from Rayleigh scattering or hazes. We infer atmospheric and stellar properties from the full near-UV to infrared transmission spectrum of WASP-79b using three independent retrieval codes, all of which yield consistent results. Our retrievals confirm previous detections of H2O (at 4.0σ confidence) while providing moderate evidence of H− bound–free opacity (3.3σ) and strong evidence of stellar contamination from unocculted faculae (4.7σ). The retrieved H2O abundance (∼1%) suggests a superstellar atmospheric metallicity, though stellar or substellar abundances remain consistent with present observations (O/H = 0.3–34× stellar). All three retrieval codes obtain a precise H− abundance constraint: log() ≈ −8.0 ± 0.7. The potential presence of H− suggests that James Webb Space Telescope observations may be sensitive to ionic chemistry in the atmosphere of WASP-79b. The inferred faculae are ∼500 K hotter than the stellar photosphere, covering ∼15% of the stellar surface. Our analysis underscores the importance of observing UV–optical transmission spectra in order to disentangle the influence of unocculted stellar heterogeneities from planetary transmission spectra.

Original languageEnglish
Article number138
Number of pages20
JournalAstronomical Journal
Issue number4
Early online date10 Sept 2021
Publication statusPublished - Oct 2021


  • Gaussian process framework
  • Neptune-mass exoplanet
  • Molecular line lists
  • HD 189733B
  • Giant planets
  • Atmosphere
  • Absorption
  • Spectroscopy
  • Retrieval
  • Hubble


Dive into the research topics of 'HST PanCET program: a complete near-UV to infrared transmission spectrum for the Hot Jupiter WASP-79b'. Together they form a unique fingerprint.

Cite this