High energy (X-ray / UV) observations of transiting exoplanets have revealed the presence of extended atmospheres around a number of systems. At these energies, stellar radiation is absorbed in the upper atmosphere of the planet, making X-ray / UV transits an exciting tool for investigating the composition of exoplanetary atmospheres. However, the effects of stellar activity on transits at these wavelengths is far from understood. In X-rays the stellar disk appears limb-brightened, and active regions appear as extended bright features that evolve on a much shorter timescale than in the optical. This makes measuring the true planet-to-star radius ratio challenging. The Sun offers a unique opportunity to study the impact of stellar activity on high energy transits. Using disk resolved soft X-ray and UV images from NASA's Solar Dynamics Observatory taken over the last solar cycle I will show how both occulted and unocculted active regions can mimic an inflated planetary atmosphere by changing the depth and shape of a transit profile. I will also show how the disk integrated Lyman-α Solar irradiance varies on both short and long timescales and how this variability can also impact our ability to recover the true radius ratio of a transiting exoplanet. Finally, I will present techniques to overcome these challenges in high-energy transits.

Contributed talk at the Splinter Session on "Variability of Solar/Stellar Magnetic Activity" (http://coolstars19.com/splinters/stellar-var/index.html) at the "Cool Stars 19" Workshop