Detection of H2O and Evidence for TiO/VO in an Ultra-hot Exoplanet Atmosphere

Thomas M. Evans, David K. Sing, Hannah R. Wakeford, Nikolay Nikolov, Gilda E. Ballester, Benjamin Drummond, Tiffany Kataria, Neale P. Gibson, David S. Amundsen, Jessica Spake

Research output: Contribution to journalArticlepeer-review

107 Citations (Scopus)
249 Downloads (Pure)


We present a primary transit observation for the ultra-hot (T eq ~ 2400 K) gas giant expolanet WASP-121b, made using the Hubble Space Telescope Wide Field Camera 3 in spectroscopic mode across the 1.12–1.64 μm wavelength range. The 1.4 μm water absorption band is detected at high confidence (5.4σ) in the planetary atmosphere. We also reanalyze ground-based photometric light curves taken in the B, r', and z' filters. Significantly deeper transits are measured in these optical bandpasses relative to the near-infrared wavelengths. We conclude that scattering by high-altitude haze alone is unlikely to account for this difference and instead interpret it as evidence for titanium oxide and vanadium oxide absorption. Enhanced opacity is also inferred across the 1.12–1.3 μm wavelength range, possibly due to iron hydride absorption. If confirmed, WASP-121b will be the first exoplanet with titanium oxide, vanadium oxide, and iron hydride detected in transmission. The latter are important species in M/L dwarfs and their presence is likely to have a significant effect on the overall physics and chemistry of the atmosphere, including the production of a strong thermal inversion.
Original languageEnglish
Article numberL4
Pages (from-to)1
Number of pages7
JournalThe Astrophysical Journal Letters
Issue number1
Publication statusPublished - 22 Apr 2016


  • planets and satellites: atmospheres
  • stars: individual: WASP-121
  • techniques: photometric
  • techniques: spectroscopic


Dive into the research topics of 'Detection of H2O and Evidence for TiO/VO in an Ultra-hot Exoplanet Atmosphere'. Together they form a unique fingerprint.

Cite this