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Formation, Structure and Habitability of Super-Earth and Sub-Neptune

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Formation, Structure and Habitability of Super-Earth and Sub-Neptune Formation, Structure and Habitability of Super-Earth and Sub-Neptune Exoplanets by Leslie Anne Rogers B.Sc., Honours Physics-Mathematics, University of Ottawa (2006) Submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2012 c Massachusetts Institute of Technology 2012. All rights reserved. Author.............................................................. Department of Physics May 22, 2012 Certified by. Sara Seager Professor of Planetary Science Professor of Physics Class of 1941 Professor Thesis Supervisor Accepted by . Krishna Rajagopal Professor of Physics Associate Department Head for Education 2 Formation, Structure and Habitability of Super-Earth and Sub-Neptune Exoplanets by Leslie Anne Rogers Submitted to the Department of Physics on May 22, 2012, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Abstract Insights into a distant exoplanet's interior are possible given a synergy between models and observations. Spectral observations of a star's radial velocity wobble induced by an orbiting planet's gravitational pull measure the planet mass. Photometric transit observations of a planet crossing the disk of its star measure the planet radius. This thesis interprets the measured masses and radii of super-Earth and sub-Neptune exoplanets, employing models to constrain the planets' bulk compositions, formation histories, and habitability. We develop a model for the internal structure of low-mass exoplanets consisting of up to four layers: an iron core, silicate mantle, ice layer, and gas layer. We quantify the span of plausible bulk compositions for low-mass transiting planets CoRoT-7b, GJ 436b, and HAT-P-11b, and describe how Bayesian analysis can be applied to rigorously account for observational, model, and inherent uncertainties. We present a detailed case study of GJ 1214b, the first exemplar of a new class of volatile-rich super-Earth exoplanets. At 6.5 M⊕ and 2.7 R⊕, GJ 1214b must have a gas layer to account for its low mean density. We present three possible scenarios for the origin of the gas layer on GJ 1214b: direct accretion of H/He gas from the protoplanetary nebula, sublimation of ices, and outgassing of volatiles from a rocky interior. We next explore the low-density extreme of the mass-radius relations for volatile- rich super-Earth exoplanets. Using models of planet formation, structure, and survival, we constrain the minimum plausible planet mass for a measured planet radius and equi- librium temperature. We explore both core-nucleated accretion and outgassing as two separate formation pathways for Neptune-size planets with voluminous atmospheres of light gases. Finally, we present a practical method to assess whether a hydrogen-rich sub- Neptune planet with measured mass and radius could potentially harbor a liquid water ocean. Using a one-dimensional radiative-convective model of energy transport through water-saturated hydrogen-rich envelopes, we constrain the combinations of planet properties (mass, radius, equilibrium temperature, intrinsic luminosity) that 3 are conducive to liquid water oceans. The pace of low-mass exoplanet discoveries is poised to accelerate, and this thesis will contribute to constraining the interior properties of newfound planets. Thesis Supervisor: Sara Seager Title: Professor of Planetary Science Professor of Physics Class of 1941 Professor 4 Acknowledgments My doctoral study at MIT has been a formative experience for me as a scientist and as a person. Before arriving in Cambridge MA, I knew MIT would be an amazing place to push my academic development, but I had no idea about the other experiences and challenges that awaited me. Who would have guessed (not me) that I would discover my dream job (pondering the nature of distant planets), find a second home (outside my beloved Nova Scotia), earn a black belt in karate, battle a flying squirrel, start drinking coffee, fall in love, dance on stage again, learn to mediate, and be part of the executive leadership of a 700-person graduate student community. So many people have helped me along the way. The words on the following pages do not live up to the depth of gratitude that I feel. First, I want to thank my PhD advisor, Sara Seager, for her mentorship, guidance, and friendship. She has offered me both intense hands-on collaboration and research freedom when I needed them most. I am especially grateful for how she coached me to develop more confidence in myself. I continue to be inspired by her ambitious vision, dedicated efficiency, keen intuition and strong scientific voice. I would also like to thank the members of my thesis committee, Paul Schechter and Josh Winn, for their sage scientific and career advice. I will always remember (and do my very best to follow) Paul Schechter's maxims (e.g., Don't let better be the enemy of the good, Own your mistakes). Josh Winn's perspective as an exoplanet observer and interested skeptic has been tremendously valuable in anchoring my theoretical pursuits. I am very grateful to have had the honor to collaborate with Peter Bodenheimer, Jack Lissauer, Geoff Marcy, Philipp Podsiadlowski, Saul Rappaport, Max Tegmark, and the Kepler Science Team during my graduate studies. They have each left a positive mark on my scientific development. I also want to send heartfelt acknowledgements to Robin McKenzie and Robert Hawkes, who prepared me for MIT by nurturing my love of physics and by introducing me to scientific research. Thank you to the Seager Exoplanet Theory and Computation Group, the MIT 5 Exoplanets Discussion Group, and the faculty, postdocs, and students at the MIT Kavli Institute for all the camaraderie and invigorating scientific discussions over the years. I also want to thank Paul Hsi, Brian Surette, Arlyn Hertz, Jean Papagianopoulos, Cathy Modica, Crystal Nurazura Hall and the rest of the staff at MKI and the physics department for helping me so much over my time as a graduate student. I am very grateful to George Elbaum for his financial support, which played a pivotal role in making my graduate career such a positive experience. The Whiteman Fellowship during my first year at MIT allowed me the time and flexibility to find my bearings and to start my PhD research on the right foot. I also acknowledge the financial support of the National Science and Engineering Research Council of Canada. My friends deserve special recognition for filling my non-academic life with smiles, laughter, dancing, and fun. I especially want to thank Hannah Chang, Eveline Steine, and Sarah Vigeland for all the meals, conversations, and memories that we've shared. I feel tremendously blessed to have found such kindred spirits. I am also very grateful to my dance teacher Robin Offley, to my karate teacher Johnson Chung, and to the Sidney-Pacific Housemasters Roger and Dottie Mark for supporting me, for pushing me beyond my self-imposed limits, and for helping me to develop my leadership skills. Thank you to everyone in Statmechsocial, Movements in Time, the MIT Karate Club, Sidney-Pacific, the MIT Women in Physics, and the Physics REFS for keeping me happy, productive, and balanced. You have made my time at MIT the best years of my life. Deep heartfelt gratitude goes to my boyfriend, Ben Mares, whose love, patience, kindness and support (of both the IT and emotional varieties) were instrumental in the preparation of this thesis. Finally, I want to thank my family for always being there for me. Their unwavering love and support have carried me through the most challenging patches at MIT. I dedicate this thesis to them. 6 Contents 1 Introduction 15 1.1 Definition of a Planet . 16 1.2 Planet Interior Structure Observations . 17 1.2.1 Cosmological Abundances . 17 1.2.2 Material Properties . 19 1.2.3 Solar System Planet Observations . 20 1.2.4 Exoplanet Observations . 23 1.3 Sub-Neptune and Super-Earth Exoplanets . 27 1.4 Thesis Overview . 30 2 A Framework for Quantifying the Degeneracies of Exoplanet Interior Compositions 33 2.1 Introduction . 34 2.2 Model . 35 2.2.1 Model Overview . 35 2.2.2 Material EOS and Thermal Profiles . 37 2.2.3 Exterior Boundary Condition . 39 2.2.4 Model Parameter Space . 41 2.2.5 Model validation . 45 2.3 Ternary and Quaternary Diagrams . 46 2.4 Results . 47 2.4.1 CoRoT-7b . 48 2.4.2 GJ 581d . 51 7 2.4.3 GJ 436b . 56 2.4.4 HAT-P-11b . 62 2.5 Bayesian Inference Applied to Exoplanet Interior Structure Models . 64 2.6 Discussion . 74 2.6.1 External Constraints on Planetary Composition . 74 2.6.2 Chemical Composition of Interior Layers . 76 2.6.3 Planet Evolution . 77 2.6.4 Beyond Mass and Radius - Further Observational Constraints on Compositions . 78 2.7 Conclusions . 79 3 Three Possible Origins for the Gas Layer on GJ 1214b 83 3.1 Introduction . 84 3.2 Connecting Gas Layer Origins and Planet Interiors . 85 3.3 Structure Model . 89 3.4 Results . 92 3.4.1 Background . 92 3.4.2 Case I: Gas-Ice-Rock Planet with Primordial Gas Envelope . 93 3.4.3 Case II: Ice-Rock planet with Sublimated Vapor Envelope . 94 3.4.4 Case III: Rocky Planet with Outgassed Atmosphere . 97 3.5 Discussion . 102 3.5.1 Atmosphere Observations . 102 3.5.2 Atmospheric Escape . 102 3.5.3 Ongoing Outgassing . 104 3.5.4 Necessity of a Gas Layer . 105 3.5.5 Model Uncertainties . 105 3.6 Summary and Conclusions . 107 4 Formation and Structure of Low-Density Exo-Neptunes 111 4.1 Introduction . 112 4.2 Models of Planets in Equilibrium: Methods . 114 8 4.3 Planet Formation by Core-nucleated Accretion .
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