11/04/2025
By Lukas Hanson

The Kennedy College of Science, Department of Physics & Applied Physics, invites you to attend a doctoral dissertation defense by Lukas Hanson on “Probing the Cosmic Shoreline - The Role of Extreme Ultraviolet Radiation on Exoplanet Atmosphere Retention.”

Candidate Name: Lukas Hanson
Degree: Doctoral
Defense Day: Friday, November 14, 2025
Time: 9:30-11 a.m.
Location: Wannalancit Business Center Conference Room 305, 600 Suffolk Street, Lowell

Committee Members:

  • Advisor: Ofer Cohen, Department of Physics & Applied Physics, University of Massachusetts Lowell
  • Silas Laycock, Department of Physics & Applied Physics, University of Massachusetts Lowell
  • Viktor Podolskiy, Department of Physics & Applied Physics, University of Massachusetts Lowell
  • Aaron Ridley, Department of Climate and Space Sciences and Engineering, University of Michigan

Abstract:
The identification of Earth-like planets outside our solar system is a leading research goal in astronomy, but determining if candidate planets have atmospheres, and more importantly if they can retain an atmosphere, is still out of reach. The Cosmic Shoreline is a concept used to estimate the likelihood of an exoplanet’s ability to retain its atmosphere, using historic EUV flux compared to the escape velocity of a planet to determine if that planet can retain an atmosphere. An alternative method to study these exoplanets is through the use of atmospheric models which can simulate the response of an atmosphere to the space weather conditions it is exposed to. There are a number of models being developed and maintained for this purpose, with one such model being the Global Ionosphere and Thermosphere Model (GITM). GITM was originally developed to study the upper atmosphere or Earth, with additional branches of it developed for Mars, Venus, and Titan. In this thesis, a means of generalizing GITM for use in treating Earth as an exoplanet is described. There are two phases to this research, the first of which is simply scaling the EUV intensity that is provided to the model in order to test the atmospheric response to enhanced EUV radiation levels, looking for signs of hydrodynamic escape. This is relevant because many rocky exoplanets are observed orbiting M and K stars at orbital distances much closer than Earth is to the Sun, resulting in EUV radiation levels that can exceed an order of magnitude greater than what Earth receives. The second half of this research focuses on a case study of 6 potentially habitable exoplanets which orbit host stars that have been observed in the MUSCLES Treasury Survey and therefor have available EUV spectra. These EUV spectra are used as input for GITM in order to study how the selected exoplanets’ atmospheres may behave if they are Earth-like in their composition. Additionally, stellar wind parameters in the model are modified to match the estimated parameters of each star. Results for the scaled EUV intensity are presented, notably showing an inflation of the ionosphere and a transition towards hydrodynamic escape for EUV radiation equivalent to 10 times what Earth normally experiences. In the case study of 6 potentially habitable exoplanets, results for each planet’s upper atmospheric composition are shown, as well as a time evolution of the 3 planets which exhibit clear atmospheric escape: Trappist-1e, L 98-59 c, and Proxima Centauri b. In particular, Trappist-1e and L 98-59 c are interesting as they are regarded as promising candidates for habitability, but the inclusion of stellar wind parameters and the impact those parameters have on the joule heating in the atmosphere suggest that the magnetic activity of their host stars present a significant barrier to atmosphere retention.