Stellar wind induced soft X-ray emission from close-in exoplanets

TL;DR

This paper estimates the soft X-ray emission from close-in exoplanets caused by stellar wind charge exchange, finding hot Jupiters could emit significantly more X-rays than solar system giants, but detection remains challenging.

Contribution

It introduces a simple model for X-ray emission from hot Jupiters via stellar wind charge exchange, highlighting its potential strength and observational challenges.

Findings

Hot Jupiters can emit up to 10^22 erg/s in X-rays due to SWCX.

Emission from hot Jupiters is a million times stronger than Jovian aurora.

Current observational facilities cannot detect these predicted X-ray fluxes.

Abstract

In this paper, we estimate the X-ray emission from close-in exoplanets. We show that the Solar/Stellar Wind Charge Exchange Mechanism (SWCX) which produces soft X-ray emission is very effective for hot Jupiters. In this mechanism, X-ray photons are emitted as a result of the charge exchange between heavy ions in the solar wind and the atmospheric neutral particles. In the Solar System, comets produce X-rays mostly through the SWCX mechanism, but it has also been shown to operate in the heliosphere, in the terrestrial magnetosheath, and on Mars, Venus and Moon. Since the number of emitted photons is proportional to the solar wind mass flux, this mechanism is not very effective for the Solar system giants. Here we present a simple estimate of the X-ray emission intensity that can be produced by close-in extrasolar giant planets due to charge exchange with the heavy ions of the stellar wind. Using the example of HD~209458b, we show that this mechanism alone can be responsible for an X-ray emission of $\approx 10^{22}$~erg~s$^{-1}$, which is $10^6$ times stronger than the emission from the Jovian aurora. We discuss also the possibility to observe the predicted soft X-ray flux of hot Jupiters and show that despite high emission intensities they are unobservable with current facilities.