Rhett Zollinger Thesis Defense 07/08/14

Thesis Defense

Rhett Zollinger

Tuesday, July 8, 2014
10:00am (110 INSCC)

Title: Exomoon Habitability & Tidal Evolution In Low-Mass Star Systems

Abstract:

Current technology and theoretical methods are allowing for the detection of sub-Earth sized extrasolar planets. In addition, the detection of massive moons orbiting extrasolar planets ("exomoons") has become feasible and searches are currently underway. Several extrasolar planets have now been discovered in the habitable zone (HZ) of their parent star. This naturally leads to questions about the habitability of moons around planets in the HZ.

Red dwarf stars present interesting targets for habitable planet detection. Compared to the Sun, red dwarfs are smaller, fainter, lower mass, and much more numerous. Due to their low luminosities, the HZ is much closer to the star than for Sun-like stars. For a planet-moon binary in the HZ, the close proximity of the star presents dynamical restrictions on the stability of the moon, forcing it to orbit close to the planet to remain gravitationally bound. Under these conditions the effects of tidal heating, distortion torques, and stellar perturbations become important considerations to the habitability of an exomoon.

Utilizing an evolution model that considers both dynamical and tidal interactions, I performed a computational investigation into long-term evolution of exomoon systems. My study focused on satellite systems in the HZ of red dwarf stars and the dependence of exomoon habitability on the mass of the central star. Results show that dwarf stars with masses less than about 0.2 M_Sun cannot host habitable exomoons within the stellar HZ due to extreme tidal heating in the moon. Perturbations from a central star may continue to have deleterious effects up to ≈ 0.5 M_Sun , depending on the host planet's mass and its location in the HZ. These results suggest that a host planet could be located outside the stellar HZ to where higher tidal heating rates could act to promote habitability for an otherwise uninhabitable moon. In addition to heating concerns, torques due to tidal and spin distortion can lead to the relatively rapid inward spiraling of a moon. The effects of torque and stability constraints also make it unlikely that long-term resonances between two massive moons will develop in the HZs around red dwarf stars.

My study showed that moons in the circumstellar HZ are not necessarily habitable by definition. In addition, the HZ for an exomoon may extend beyond the HZ for an exoplanet. Therefore, an extended model is required when considering exomoon habitability in comparison to exoplanet habitability.

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