The Hubble Space Telescope PanCET Program: An Extended Transmission Spectrum of the Warm Neptune HAT-P-26b

We present a new and extended transmission spectrum of the warm Neptune HAT-P-26b obtained with the Hubble Space Telescope (HST) Space Telescope Imagining Spectrograph (STIS) G430L grating as part of the Panchromatic Comparative Exoplanet Treasury (PanCET) program. Our spectra cover a range of 0.3-0.5 microns, and we are combining it with previously published data from the 0.5-1.6 micron region taken with HST STIS G750L grating and HST Wide Field Camera 3 (WFC3), as well as with data at 3.6 and 4.5 microns from Spitzer Infrared Array Camera (IRAC). With our new wavelength coverage, we can better constrain the clouds in the atmosphere of the planet and similarly the metallicity. This work will be useful for upcoming JWST observations, HAT-P-26b is a cycle 1 target and our results will provide a better characterization of the cloud deck level in the shorter wavelength range which will ease the modeling process for JWST. From (Wakeford et al. 2017) we know that HAT-P-26b has a lower bulk density compared to other Neptune-sized planets and it has a measured metallicity that is below what is expected from the mass-metallicity relationship observed in the Solar System giant planets. This planet is one of the few Neptune-mass planets to have a precise metallicity measured. This makes HAT-P-26b an important target because the atmospheric composition of close-in Neptune-sized planets opens a window to distinguish planet formation theories and evolution such as core-accretion, planetesimal accretion or other scenarios, these are unknowns regarding the formation of this class of planets.