Measuring the first [Fe/H] of an exoplanet

Ultra-hot Jupiters are planets whose day-sides, hotter than 2,200 K, offer the opportunity to study the physics and chemistry of planetary atmospheres in a regime that is not accessible in the solar system or in other exoplanets. At such temperatures, molecules begin to dissociate and atoms become ionized, which gives rise to opacity sources such as H- or metal lines, that are more common in stars than planets. Indeed, recent observations at high resolution (R>100,000) from the ground revealed lines from metals in the atmosphere of the hottest planet of this category: KELT-9b (4,000 K). The most prominent lines belong to neutral and ionized iron. However, this kind of observations suffer from the contamination of Earth atmosphere, resulting in the loss of the planetary continuum. This is what prevented previous authors from measuring [Fe/H] for this exoplanet. With 6 HST orbits, we will observe the secondary eclipse of KELT-9b to: (1) provide the first direct detection of H-, responsible for generating the continuum in UHJ spectra, and constrain the overall metal enrichment of the exoplanet through the strength of this feature; (2) use the measured continuum level to calibrate ground-based high-resolution observations and provide the first measurement of [Fe/H] in an exoplanet.