Detection of H2O and evidence for TiO/VO in an ultra hot exoplanet atmosphere

TL;DR

This study presents Hubble observations of the ultra hot exoplanet WASP-121b, detecting water and evidence for TiO/VO, suggesting complex atmospheric chemistry and thermal inversion effects.

Contribution

First detection of TiO, VO, and possibly iron hydride in the transmission spectrum of an ultra hot exoplanet, indicating diverse atmospheric constituents.

Findings

Detection of water absorption at 5.4 sigma confidence.

Evidence for TiO and VO absorption in the atmosphere.

Indications of iron hydride presence affecting atmospheric properties.

Abstract

We present a primary transit observation for the ultra hot (Teq~2400K) gas giant expolanet WASP-121b, made using the Hubble Space Telescope Wide Field Camera 3 in spectroscopic mode across the 1.12-1.64 micron wavelength range. The 1.4 micron water absorption band is detected at high confidence (5.4 sigma) in the planetary atmosphere. We also reanalyze ground-based photometric lightcurves 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 micron 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.