Exoplanet Aeronomy: A Case Study of WASP-69b's Variable Thermosphere

TL;DR

This study investigates how stellar XUV variability influences the thermosphere of the exoplanet WASP-69b, demonstrating that changes in stellar activity correlate with observable variations in planetary atmospheric signals.

Contribution

It provides the first multi-epoch analysis linking stellar XUV flux changes to variations in exoplanetary metastable helium absorption, advancing understanding of star-planet atmospheric interactions.

Findings

WASP-69's X-ray flux decreased by over 50% from 2016 to 2023.

Metastable HeI absorption from WASP-69b was lower in 2023 compared to 2017-2019.

Atmospheric modeling suggests stellar magnetic activity influences planetary thermosphere signals.

Abstract

Aeronomy, the study of Earth's upper atmosphere and its interaction with the local space environment, has long traced changes in the thermospheres of Earth and other solar system planets to solar variability in the X-ray and extreme ultraviolet (collectively, "XUV") bands. Extending comparative aeronomy to the short-period extrasolar planets may illuminate whether stellar XUV irradiation powers atmospheric outflows that change planetary radii on astronomical timescales. In recent years, near-infrared transit spectroscopy of metastable HeI has been a prolific tracer of high-altitude planetary gas. We present a case study of exoplanet aeronomy using metastable HeI transit observations from Palomar/WIRC and follow-up high-energy data from the Neil Gehrels Swift Observatory that were taken within one month of the WASP-69 system, a K-type main sequence star with a well-studied hot Jupiter companion. Supplemented by archival data, we find that WASP-69's X-ray flux in 2023 was less than 50% of what was recorded in 2016 and that the metastable HeI absorption from WASP-69b was lower in 2023 versus past epochs from 2017-2019. Via atmospheric modeling, we show that this time-variable metastable HeI signal is in the expected direction given the observed change in stellar XUV, possibly stemming from WASP-69's magnetic activity cycle. Our results underscore the ability of multi-epoch, multi-wavelength observations to paint a cohesive picture of the interaction between an exoplanet's atmosphere and its host star.