Transiting the Sun II: The impact of stellar activity on Lyman-$α$ transits

Abstract

High-energy observations of the Sun provide an opportunity to test the limits of our ability to accurately measure properties of transiting exoplanets in the presence of stellar activity. Here we insert transits of a hot Jupiter into continuous disk integrated data of the Sun in Lyman-alpha (Ly$α$) from NASA's SDO/EVE instrument to assess the impact of stellar activity on the measured planet-to-star radius ratio $(\textrm{R}_\textrm{p}/\textrm{R}_\star)$. In 75% of our simulated light curves we measure the correct radius ratio; however, incorrect values can be measured if there is significant short term variability in the light curve. The maximum measured value of $(\textrm{R}_\textrm{p}/\textrm{R}_\star)$ is $50\%$ larger than the input value, which is much smaller than the large Ly$α$ transit depths that have been reported in the literature, suggesting that for stars with activity levels comparable to the Sun, stellar activity alone cannot account for these deep transits. We ran simulations without a transit and found that stellar activity cannot mimic the Ly$α$ transit of 55 Cancari b, strengthening the conclusion that this planet has a partially transiting exopshere. We were able to compare our simulations to more active stars by artificially increasing the variability in the Solar Ly$α$ light curve. In the higher variability data, the largest value of $(\textrm{R}_\textrm{p}/\textrm{R}_\star)$ we measured is < 3x the input value which again is not large enough to reproduce the Ly$α$ transit depth reported for the more active stars HD 189733 and GJ 436, supporting the interpretation that these planets have extended atmospheres and possible cometary tails.