# X-ray Studies of Exoplanets: A 2020 Decadal Survey White Paper

## TL;DR

This white paper discusses how high-quality X-ray observations over the next 10-15 years can significantly advance our understanding of exoplanet atmospheres, their formation, and their interactions with stellar environments.

## Contribution

It highlights the potential of X-ray imaging and spectroscopy to study exoplanet environments and characterizes their atmospheres in ways not possible with other methods.

## Key findings

- X-ray data can reveal high-energy flux impacting exoplanets.
- X-ray observations enable direct characterization of exoplanet upper atmospheres.
- X-ray studies can inform about stellar activity and its effects on exoplanets.

## Abstract

Over the last two decades, the discovery of exoplanets has fundamentally changed our perception of the universe and humanity's place within it. Recent work indicates that a solar system's X-ray and high energy particle environment is of fundamental importance to the formation and development of the atmospheres of close-in planets such as hot Jupiters, and Earth-like planets around M stars. X-ray imaging and spectroscopy provide powerful and unique windows into the high energy flux that an exoplanet experiences, and X-ray photons also serve as proxies for potentially transfigurative coronal mass ejections. Finally, if the host star is a bright enough X-ray source, transit measurements akin to those in the optical and infrared are possible and allow for direct characterization of the upper atmospheres of exoplanets. In this brief white paper, we discuss contributions to the study of exoplanets and their environs which can be made by X-ray data of increasingly high quality that are achievable in the next 10--15 years.