UVEE: Ultra-Violet Exoplanet Explorer

We are planning a new exoplanet space mission using an UV telescope to measure composition of the major elements in the atmospheres of exoplanets in Ultra-violet light, the Ultra-Violet Exoplanet Explorer (UVEE). Measuring the composition is the next step in exoplanet research. The basic concept we foresee is very similar to the successful CHEOPS mission (CHaracterising ExOPlanet Satellite), a joint mission between ESA and Switzerland. CHEOPS is dedicated to studying bright, nearby stars that are already known to host exoplanets, to make high-precision observations of the planet's size as it passes in front of its host star. Similarly, UVEE will study bright nearby stars with known exoplanets in transit spectroscopically in the UV wavelength range. The UV telescope should have two instruments: 1) an UV spectrometer from 520 to 3000 Å, with a spectral resolution about 10 Å, and 2) a high-resolution spectrometer around the Lyman-alpha line (1216 Å) with a spectral range of ±400 km/s (±16.3 Å) and a resolution of 20 m/s (R ~20'000). The UV spectrometer will provide element abundances for the major element of H, He, C, N, O, Na, S, and Mg. Based on the performance by the Hubble Space Telescope (STIS/COS) and an aperture of 0.5 m of the telescope we estimate that we will be able to observe about 30 exoplanets with a S/N above 2. In addition, in this wavelength range there are molecular absorptions of simple molecules common in atmospheres (CO2, CO, H2S, O3, H2O, several simple hydrocarbons) and a few ion lines (e.g., Ca II and Mg II). Additional benefit is that we will get UV spectra from stars, which will be very valuable once HST is no longer in operation. With the high resolution spectrometer we will investigate the Lyman-alpha line profile to learn more about the interaction of the extended atmosphere of the exoplanets with the stellar wind of their host star. Given the projected size, UVEE would fit as an ESA fast mission or as a NASA SMEX mission. Because of interference with the Geo-corona a large orbit is needed to minimize the UV contamination of the spectra; at the minimum distance of 12 RE the geo-corona is still about 400 R. Ideally, UVEE would be placed at the L2 position as foreseen for the current ESA F missions, which would allow for uninterrupted observations of planetary transits.