The Hubble WFC3 Emission Spectrum of the Extremely-Hot Jupiter, KELT-9b

TL;DR

This study analyzes the emission spectrum of the extremely hot exoplanet KELT-9b using Hubble WFC3 data, revealing the presence of molecular species like TiO, VO, FeH, and H-, challenging the expectation of featureless spectra due to thermal dissociation.

Contribution

It provides the first detailed atmospheric retrieval of KELT-9b's emission spectrum, showing the presence of molecular species in an ultra-hot Jupiter, and highlights the complexity of such atmospheres.

Findings

Detection of TiO, VO, FeH, and H- in KELT-9b's atmosphere

Spectrum inconsistent with a simple blackbody model

Molecular features are present despite high temperatures

Abstract

Recent studies of ultra hot-Jupiters suggested that their atmospheres could have thermal inversions due to the presence of optical absorbers such as TiO, VO, FeH and other metal hydride/oxides. However, it is expected that these molecules would thermally dissociate at extremely high temperatures, thus leading to featureless spectra in the infrared. KELT-9b, the hottest exoplanet discovered so far, is thought to belong to this regime and host an atmosphere dominated by neutral Hydrogen from dissociation and atomic/ionic species. Here, we analysed the eclipse spectrum obtained using the Hubble Space Telescope's Wide Field Camera 3 (WFC3) and, by utilising the atmospheric retrieval code TauREx3, found that the spectrum is consistent with the presence of molecular species and is poorly fitted by a simple blackbody. In particular, we find that a combination of TiO, VO, FeH and H- provides the best-fit when considering HST, Spitzer and TESS datasets together. Aware of potential biases when combining instruments, we also analysed the HST spectrum alone and found that TiO and VO only were needed in this case. These findings paint a more complex picture of the atmospheres of ultra-hot planets than previously thought