An Optical Transmission Spectrum for the Ultra-hot Jupiter WASP-121b Measured with the Hubble Space Telescope

Thomas M. Evans*, David K. Sing, Jayesh M. Goyal, Nikolay Nikolov, Mark S. Marley, Kevin Zahnle, Gregory W. Henry, Joanna K. Barstow, Munazza K. Alam, Jorge Sanz-Forcada, Tiffany Kataria, Nikole K. Lewis, Panayotis Lavvas, Gilda E. Ballester, Lotfi Ben-Jaffel, Sarah D. Blumenthal, Vincent Bourrier, Benjamin Drummond, Antonio García Muñoz, Mercedes López-MoralesPascal Tremblin, David Ehrenreich, Hannah R. Wakeford, Lars A. Buchhave, Alain Lecavelier des Etangs, Éric Hébrard, Michael H. Williamson

*Corresponding author for this work

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We present an atmospheric transmission spectrum for the ultra-hot Jupiter WASP-121b, measured using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Across the 0.47–1 wavelength range, the data imply an atmospheric opacity comparable to—and in some spectroscopic channels exceeding—that previously measured at near-infrared wavelengths (1.15–1.65 ). Wavelength-dependent variations in the opacity rule out a gray cloud deck at a confidence level of 3.7σ and may instead be explained by VO spectral bands. We find a cloud-free model assuming chemical equilibrium for a temperature of 1500 K and a metal enrichment of 10–30× solar matches these data well. Using a free-chemistry retrieval analysis, we estimate a VO abundance of dex. We find no evidence for TiO and place a 3σ upper limit of −7.9 dex on its abundance, suggesting TiO may have condensed from the gas phase at the day–night limb. The opacity rises steeply at the shortest wavelengths, increasing by approximately five pressure scale heights from 0.47 to 0.3 in wavelength. If this feature is caused by Rayleigh scattering due to uniformly distributed aerosols, it would imply an unphysically high temperature of 6810 ± 1530 K. One alternative explanation for the short-wavelength rise is absorption due to SH (mercapto radical), which has been predicted as an important product of non-equilibrium chemistry in hot Jupiter atmospheres. Irrespective of the identity of the NUV absorber, it likely captures a significant amount of incident stellar radiation at low pressures, thus playing a significant role in the overall energy budget, thermal structure, and circulation of the atmosphere.
Original languageEnglish
Article number283
JournalAstrophysical Journal
Issue number6
Number of pages34
Publication statusPublished - 2018


  • Methods: observational
  • Planets and satellites: atmospheres
  • Planets and satellites: gaseous planets

Cite this

Evans, T. M., Sing, D. K., Goyal, J. M., Nikolov, N., Marley, M. S., Zahnle, K., Henry, G. W., Barstow, J. K., Alam, M. K., Sanz-Forcada, J., Kataria, T., Lewis, N. K., Lavvas, P., Ballester, G. E., Ben-Jaffel, L., Blumenthal, S. D., Bourrier, V., Drummond, B., Muñoz, A. G., ... Williamson, M. H. (2018). An Optical Transmission Spectrum for the Ultra-hot Jupiter WASP-121b Measured with the Hubble Space Telescope. Astrophysical Journal, 156(6), [283].