Detecting exoplanet transits with the next generation of X-ray telescopes

TL;DR

This paper evaluates the potential of future X-ray telescopes like Athena and AXIS to detect exoplanet transits, focusing on the improved detection capabilities and the influence of stellar and planetary factors.

Contribution

It assesses the detection prospects of exoplanet transits with next-generation X-ray observatories, identifying promising targets and factors affecting detectability.

Findings

At least 5 exoplanets could be detected at >4σ significance with future telescopes.

Enlarged X-ray radius due to atmospheric escape increases detection probability.

Host star coronal temperature significantly impacts transit detection feasibility.

Abstract

Detecting exoplanet transits at X-ray wavelengths would provide a window into the effects of high energy irradiation on the upper atmospheres of planets. However, stars are relatively dim in the X-ray, making exoplanet transit detections difficult with current X-ray telescopes. To date, only one exoplanet (HD~189733~b) has an X-ray transit detection. In this study, we investigate the capability of future X-ray observatories to detect more exoplanet transits, focusing on both the NewAthena-WFI instrument and the proposed Advanced X-ray Imaging Satellite (AXIS), which provide more light-collecting power than current instruments. We examined all the transiting exoplanet systems in the NASA Exoplanet Archive and gathered X-ray flux measurements or estimates for each host star. We then predicted the stellar count rates for both AXIS and NewAthena and simulated light curves, using null-hypothesis testing to identify the top 15 transiting planets ranked by potential detection significance. We also evaluate transit detection probabilities when the apparent X-ray radius is enlarged due to atmospheric escape, finding that $\geq 5$ of these planetary systems may be detectable on the $>4\sigma$ level in this scenario. Finally, we note that the assumed host star coronal temperature, which affects the shape of an X-ray transit, can also significantly affect our ability to detect the planet.