Atmospheres & Climate

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

By Keith Cowing
Status Report
May 30, 2024
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Exoplanet Aeronomy: A Case Study of WASP-69b’s Variable Thermosphere
The main plot shows the estimated WASP-69 spectrum (black line), which was established from Swift observations and scaling HD 85512’s spectrum (grey line) to solar X-ray/UV flux ratios. Each bandpass-of-interest is shown with background shading. The inset shows the TIMED/SEE data from Solar Cycle 24 from the Fx and Feuv,2 bands along with the best-fit power-law. — astro-ph.EP

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.

W. Garrett Levine, Shreyas Vissapragada, Adina D. Feinstein, George W. King, Aleck Hernandez, Lia Corrales, Michael Greklek-McKeon, Heather A. Knutson

Comments: 25 pages, 15 figures, 6 tables, accepted to The Astronomical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2405.19177 [astro-ph.EP] (or arXiv:2405.19177v1 [astro-ph.EP] for this version)
Submission history
From: Garrett Levine
[v1] Wed, 29 May 2024 15:18:37 UTC (7,573 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻