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Exomoon Habitability and Tidal Evolution in Low-Mass Star Systems

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EXOMOON HABITABILITY AND TIDAL EVOLUTION IN LOW-MASS STAR SYSTEMS by Rhett R. Zollinger A dissertation submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics Department of Physics and Astronomy The University of Utah December 2014 Copyright © Rhett R. Zollinger 2014 All Rights Reserved The University of Utah Graduate School STATEMENT OF DISSERTATION APPROVAL The dissertation of Rhett R. Zollinger has been approved by the following supervisory committee members: Benjamin C. Bromley Chair 07/08/2014 Date Approved John C. Armstrong Member 07/08/2014 Date Approved Bonnie K. Baxter Member 07/08/2014 Date Approved Jordan M. Gerton Member 07/08/2014 Date Approved Anil C. Seth Member 07/08/2014 Date Approved and by Carleton DeTar Chair/Dean of the Department/College/School o f _____________Physics and Astronomy and by David B. Kieda, Dean of The Graduate School. 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 < 0.2 M© cannot host habitable exomoons within the stellar HZ due to extreme tidal heating in the moon. 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. Perturbations from a central star may continue to have deleterious effects in the HZ up to « 0.5 MSun, depending on the host planet’s mass and its location in the HZ. In cases with lower intensity tidal heating, stellar perturbations may have a positive influence on exomoon habitability by promoting long-term heating rates above a minimum for habitable terrestrial environments. 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. iv To Stefanie, Missy, Bella, and Conner---- if I’ve learned one thing from this experience it’s that your support is worth more than any success. CONTENTS A B S T R A C T ................................................................................................................................... iii LIST OF FIG U R ES .................................................................................................................. ix LIST OF T A B L E S.......................................................................................................................xiii A C K N O W L E D G M E N T S ...................................................................................................... xv C H A PT ER S 1......IN T R O D U C T IO N ............................................................................................................. 1 1.1 Exoplanet Detection M ethods...................................................................................... 1 1.1.1 Historical Perspective.......................................................................................... 2 1.1.2 Radial Velocity...................................................................................................... 3 1.1.3 Transiting Planets................................................................................................. 5 1.1.4 Direct Im aging...................................................................................................... 6 1.1.5 Additional Detection M ethods.......................................................................... 7 1.1.5.1 Gravitational Microlensing....................................................................... 7 1.1.5.2 A strom etry.................................................................................................... 8 1.1.5.3 Pulsar Timing............................................................................................... 8 1.2 Extrasolar Systems Overview ...................................................................................... 8 1.2.1 An Explosion of Discoveries.............................................................................. 9 1.2.2 Apparent Detection Bias...................................................................................... 10 1.2.3 Exoplanet Composition ........................................................................................ 12 1.2.4 Additional Orbital Properties ............................................................................ 14 1.2.5 Some Interesting Examples ................................................................................. 15 1.3 Habitability ....................................................................................................................... 16 1.3.1 The Habitable Zone ............................................................................................. 16 1.3.2 Additional Considerations for the Habitable Zone ...................................... 18 1.3.3 Detected Planets in the Habitable Zone ......................................................... 18 1.4 From Exoplanets to Exomoons ................................................................................... 19 1.5 Dissertation Outline ...................................................................................................... 22 2. EX TRA SO LA R M O O N S ............................................................................................... 24 2.1 Motivation ......................................................................................................................... 24 2.2 Formation ......................................................................................................................... 24 2.3 Predicted Properties of Exomoons ............................................................................ 25 2.3.1 Stability .................................................................................................................. 25 2.3.2 Evolution.................................................................................................................. 27 2.3.3 Composition ........................................................................................................... 28 2.4 Exomoon Detection Methods ...................................................................................... 28 2.4.1 Dynamical Effects on Transiting Exoplanets (TTV and TDV) .............. 29 2.4.2 Eclipse Features Induced by Exom oons......................................................... 30 2.4.3 Direct Detection of Exomoons .......................................................................... 30 2.4.4 Detection Outlook................................................................................................. 31 2.5 Exomoon Habitability.................................................................................................... 31 2.5.1 Mass Requirements............................................................................................... 31 2.5.2 Global Energy Flux............................................................................................... 32 3. IN T E R E ST IN LOW -M ASS S T A R S ....................................................................... 34 3.1 Red Dwarfs ....................................................................................................................... 34 3.1.1 Detected Planets.................................................................................................... 35 3.2 Habitable Zones Around Red Dwarf S ta rs .............................................................. 36 3.2.1 Concerns for H abitability................................................................................... 39 3.3 So Why Bother with Exom oons................................................................................
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