FINESST: The Ultraviolet Imperative for Assessing the Habitability of Planets

FINESST: The Ultraviolet Imperative for Assessing the Habitability of Planets The Ultraviolet Imperative for Assessing the Habitability of Planets Proposal Summary: Efforts to discover and characterize habitable zone planets have primarily focused on Sun-like stars and M dwarfs. Yet the intermediate K stars provide an appealing compromise between these two extremes that has been relatively unexplored. Compared to solar-type stars, K stars are more abundant, maintain longer main-sequence lifetimes, and their planets are more suitable to observations based on the mass/radius ratios of the planet to star. While M stars offer better solutions in these regards, they have other potentially life-destroying disadvantages compared to K dwarfs. For example, super-flares with ultraviolet (UV) energies over 100 times more powerful than anything produced by our Sun have been seen on Proxima Centauri and on other M stars. These studies suggest that perhaps these highly-energetic flares are not uncommon and may occur on the order of once per day. The UV radiation incident on a planet (bothdissociating both the most important molecules for the development of life and signatures that would be indicative of life, with the potential for complete erosion of the atmosphere. In Richey-Yowell et al. (2019), we measured K star near-UV (NUV) and far-UV (FUV) stellar activity photometrically with GALEX and determined that K stars may show a greater potential for habitability due to their lower (up to 1000 times less) UV flux incident on HZ planets. Unfortunately, the GALEX bandpass leaves out some of the most important emission features (e.g. Lya, MgII) and does not provide information about emission lines at different formation temperatures. To fully characterize K star UV evolution, UV spectroscopy is essential. We propose to utilize a sample of 23 K stars at~5 Gyr that will be observed in the NUV and FUV with the Hubble Space Telescope in the next two years (GO 15955, PI: Richey-Yowell). These measurements will allow us to study the UV, flare, and variability evolution of K stars. The implications of this work will be a critical component in determining the potential habitability of planets orbiting K stars by characterizing the UV spectral range as a function of age. Additionally, we aim to get simultaneous optical spectra and photometry from ground-based observatories through both collaborations and our own efforts to make correlations between these two wavelength regimes. UV-optical relationships will be the only means by which we can estimate the UV flux of exoplanet host stars that were not observed prior to the discontinuation of HSTs UV capabilities. This project directly supports NASAs exoplanet exploration goals. We will address the evolution of space weather around host stars and determine implications for detection of habitable zone planets by determining the UV evolution of K dwarfs, which will refine planetary atmospheric models that require realistic NUV and FUV input values to determine the probability of planet habitability. These values may be used to boost efforts for prioritizing JWST targets for planetary confirmation and characterization and will inform our interpretation of transmission and emission planetary spectra by characterizing the UV environment surrounding these planets.