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

Recent studies of ultra-hot Jupiters suggested that their atmospheres could have thermal inversions due to the presence of optical absorbers such as titanium oxide (TiO), vanadium oxide (VO), iron hydride (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-9 b, 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 analyzed the eclipse spectrum obtained using the Hubble Space Telescope's Wide Field Camera 3 and, by utilizing 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 Hubble Space Telescope (HST), Spitzer, and TESS data sets together. Aware of potential biases when combining instruments, we also analyzed 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.