Transiting the Sun: The impact of stellar activity on X-ray and ultraviolet transits

TL;DR

This study uses solar observations to simulate how stellar activity affects the measurement of hot Jupiter transits in X-ray, UV, and optical wavelengths, revealing significant biases in radius estimates caused by stellar activity.

Contribution

It introduces a simulation framework based on solar data to assess stellar activity's impact on exoplanet transit measurements across multiple wavelengths.

Findings

X-ray/EUV planetary radius can be underestimated by up to 25% or overestimated by 50%.

In 70% of X-ray transits, planets cross over star spots.

Optical transits reliably recover planetary radii despite stellar activity.

Abstract

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.