Author ORCID Identifier

https://orcid.org/0000-0003-1337-723X

Date of Award

2024

Document Type

Thesis (Ph.D.)

Department or Program

Physics and Astronomy

First Advisor

Elisabeth Newton

Second Advisor

Hans Mueller

Abstract

The astronomy community has spectacularly achieved the first step in characterizing exoplanets: discovery. Over three decades of exoplanet detection have populated the Milky Way's exoplanet mosaic with thousands of planets, and more are on the way. Key trends within this mosaic reveal atmospheric escape as an underlying evolutionary process that has significantly shaped the population. While we are aware that atmospheric escape plays some role in exoplanet evolution, we do not know the extent of its influence, how it compares to other processes, and its specific physical mechanisms. Detailed studies of individual systems at known, young ages are direct tests of the initial state of exoplanet evolution. Within this thesis, I investigated a sample of young planets theorized to be experiencing catastrophic levels of atmospheric escape using the far-ultraviolet capabilities of HST/STIS. These planets --- K2-25 b, AU Mic b, and AU Mic c --- show that there is a rich field of questions to be explored regarding the impact of stellar environment on the presence and behavior of atmospheric escape. In particular, I showed that the photoionization of a young planet's outflow is important for, but does not solely govern, its observability, and there is the potential for extreme variability in an individual planet's mass loss over relatively short timescales. Motivated by my results, I have attempted the first steps towards thoroughly characterizing these young systems with a 3D hydrodynamic model of atmospheric escape that pays attention to the details of stellar-planetary wind interactions.

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