  Transits of hot Jupiters in X-rays and the ultraviolet have been shown to be
both deeper and more variable than the corresponding optical transits. This
variability has been attributed to hot Jupiters having extended atmospheres at
these wavelengths. Using resolved images of the Sun from NASA's Solar Dynamics
Observatory spanning 3.5 years of Solar Cycle 24 we simulate transit light
curves of a hot Jupiter to investigate the impact of Solar like activity on our
ability to reliably recover properties of the planet's atmosphere in soft
X-rays (94 {\AA}), the UV (131-1700 {\AA}), and the optical (4500 {\AA}). We
find that for stars with similar activity levels to the Sun, the impact of
stellar activity results in the derived radius of the planet in soft X-ray/EUV
to be underestimated by up-to 25% or overestimated by up-to 50% depending on
whether the planet occults active regions. We also find that in up-to 70% of
the X-ray light curves the planet transits over bright star spots. In the far
ultraviolet (1600 & 1700 {\AA}), we find the mean recovered value of the
planet-to-star radius ratio to be over-estimated by up-to 20%. For optical
transits we are able to consistently recover the correct planetary radius. We
also address the implications of our results for transits of WASP-12b and HD
189733b at short wavelengths.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics