Abstract for Proposal 096458

X-STELa

Planetary mass loss through photoevaporative winds shapes the current exoplanet population. Transit observations in the Lyman-alpha (Lya) line allow to directly study this atmospheric escape. We have been granted a large HST treasury program, STELa, to probe this planetary mass-loss with Lya transit spectroscopy (625 HST orbits). Stellar X-ray and ultraviolet photons are thought to power the planetary wind and the stellar wind interacts with the outflowing planetary gas shaping the Lya transit. We propose XMM-Newton observations of twelve STELa sources that have bright Lya fluxes and the highest potential to impact our understanding of planetary mass loss (135 ks in total). Combining HST Lya transits with the proposed XMM-Newton data will represent the ultimate test of planetary mass-loss models.

Details on Observing Strategy and Trigger Criteria
The target sample consists of the systems in the STELa survey that (a) have bright Lya emission (Stage 2 targets) and b) harbor planets with the highest or lowest XUV irradiance as detailed in the proposal text. Based on yield simulations, we expect that 7 systems fall into the highly illuminated and 5 into the weakly illuminated category. Because the STELa observations start in Nov. 2024, our targets are not yet known, but will be identified before and during AO 24. We request observations now to minimize the time gap between the HST FUV and the X-ray observations and to use this information to prioritize targets for Stage 2 (Lya transit) observations. The trigger criterion is that the target has a sufficient Lya flux in the STELa reconnaissance observations (so-called Stage 2 targets) and a high or low expected XUV illumination of the planet (details in the proposal text). We do not require any particular reaction time and observations in the next visibility window are sufficient.