The Atmospheres and Orbital Dynamics of Hot Jupiters

Pryal, Matthew, Astronomy - Graduate School of Arts and Sciences, University of Virginia
Arras, Phil, Astronomy, University of Virginia

The detection of the first exoplanets in the early 1990s brought along many questions about their characteristics, including on their atmospheric structure and the dynamics of their planetary systems. One of the most puzzling types of exoplanets were the so-called `hot Jupiters.' Hot Jupiters are Jupiter sized objects, orbiting very close to their host star, that absorb a large amount of radiation, making their atmospheres very hot. Their short orbital periods and hot, extended atmospheres are some of the many characteristics that make these objects unlike any planet in our Solar System. In this thesis, I present models of the electrical conductivity of the upper atmosphere of hot Jupiters as well as the migration of hypothetical moons orbiting hot Jupiters due to tidal friction effects.

In Chapter 1, I give an introduction and provide context to the research outlined in the thesis. Specifically, I give an overview of the detection of exoplanets and discuss HD 189733b, the hot Jupiter at the focus of my atmosphere model. I also introduce the interaction of a stellar wind with a planet's magnetosphere which is central towards setting the current that can run through the atmosphere, which is then affected by the atmosphere's electrical conductivity. Finally, I overview the dynamics of star-planet-moons systems and tidal friction as it relates to the hypothetical moons of hot Jupiters.

In Chapter 2, I detail the star-planet interaction between the charged particles of the stellar wind with a planetary magnetic field that set the current in the atmosphere. Importantly, the electrical conductivity of the atmosphere governs the rate at which the charged particles can flow and release energy into the atmosphere through collisions. I also review the formulas for electrical conductivity of an atmosphere and discuss the collision rates between charged particles and other species of the atmosphere which is then implemented into the HD 189733b atmosphere model in Chapter 3.

In Chapter 3, I present a hydrostatic model of the dayside upper atmosphere of hot Jupiter HD 189733b. With this model I compute its electrical conductivity and conductance, which determines the amount of energy that can be transferred to the atmosphere through Joule heating. I find that the conductance of hot Jupiters is much greater than the conductance of the Earth, and therefore Joule heating does not significantly impact the thermal structure of the atmosphere for Jupiter sized magnetic fields (B ~ 10 G).

Finally, in Chapter 4, I examine the orbital migration of hypothetical moons of hot Jupiters. I find that, even in the case of synchronous rotation and circular orbits of the moon, gravitational perturbations from the star create a forced eccentricity in the moon that makes tidal friction never cease. This tidal friction then causes the inward migration of the moon towards the planet where it can be tidally disrupted. In the case of hot Jupiter systems, this inward migration is fast enough to explain the lack of large moons detected orbiting hot Jupiters. Additionally, I place an upper limit on the mass of moons that are able to survive this orbital migration effect within 5 Gyr, to be many orders of magnitude smaller than the mass of the Earth's moon.

PHD (Doctor of Philosophy)
astronomy, astrophysics, exoplanets
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