THE HABITABILITY AND STABILITY OF EARTH-LIKE PLANETS IN BINARY STAR SYSTEMS by Nicholas W. Troup A thesis submitted to the Faculty of the University of Delaware in partial fulfillment of the requirements for the degree of Honors Bachelor of Science in Physics with Distinction May 2012 c 2012 Nicholas W. Troup All Rights Reserved THE HABITABILITY AND STABILITY OF EARTH-LIKE PLANETS IN BINARY STAR SYSTEMS by Nicholas W. Troup Approved: John E. Gizis, Ph.D. Professor in charge of thesis on behalf of the Advisory Committee Approved: James MacDonald, Ph.D. Committee member from the Department of Physics and Astronomy Approved: James Glancey, Ph.D. Committee member from the Board of Senior Thesis Readers Approved: Michael Arnold, Ph.D. Director, University Honors Program ACKNOWLEDGMENTS First and foremost, I would like to thank the members of my thesis committee for their guidance and support: Professor James Glancey foe proving a forum for peer feedback and provided a valuable lay-person's view on my work; Professor James MacDonald for providing me with valuable information on computational methods that proved crucial in the completion of my thesis; and many thanks to Professor John Gizis for always finding the time to meet with me, keeping me on track, providing advice, pointing me towards numerous valuable resources, and for being an overall marvelous mentor. I would also like to thank the University of Delaware for the free use of their UNIX computing clusters and the software package MATLAB. In addition, I must acknowledge the University of Delaware Undergraduate Research Program for their financial support, and for the people there who allow the Senior Thesis program to run smoothly. Many thanks to my fianc´eeChrissy for always being my cheerleader and moral support, and for reminding me that what I was doing was interesting and worthwhile. Finally, I would like to thank my parents. Without their unconditional love and sup- port, I would not be here today writing this thesis. iii This thesis is dedicated to the glory of God, the creator of the Heavens and the source of my passion. The heavens declare the glory of God; the skies proclaim the work of his hands. Day after day they pour forth speech; night after night they reveal knowledge. - Psalm 19:1-2 iv TABLE OF CONTENTS LIST OF TABLES :::::::::::::::::::::::::::::::: viii LIST OF FIGURES ::::::::::::::::::::::::::::::: ix ABSTRACT ::::::::::::::::::::::::::::::::::: xi Chapter 1 INTRODUCTION AND BACKGROUND :::::::::::::: 1 1.1 Criteria For Long-Term Habitability :::::::::::::::::: 1 1.1.1 The Habitable Zone :::::::::::::::::::::::: 1 1.1.2 Dynamical Stability :::::::::::::::::::::::: 2 1.2 Orbital Configurations :::::::::::::::::::::::::: 2 1.3 Lagrange Points :::::::::::::::::::::::::::::: 3 1.4 Overview and Goals ::::::::::::::::::::::::::: 5 1.4.1 Habitable Zone Geometry and Stability ::::::::::::: 6 1.4.2 Determining Critical Separations :::::::::::::::: 6 1.4.3 Investigation of Lagrange and Transfer Orbits ::::::::: 6 1.5 Restrictions and Scope :::::::::::::::::::::::::: 7 1.5.1 Stellar Type :::::::::::::::::::::::::::: 7 1.5.2 Orbital Inclination :::::::::::::::::::::::: 8 1.5.3 Orbital Eccentricity :::::::::::::::::::::::: 8 2 GENERAL SIMULATION METHODS :::::::::::::::: 9 2.1 Simulator Overview :::::::::::::::::::::::::::: 9 2.1.1 Simulation Set-Up :::::::::::::::::::::::: 9 2.1.2 Simulation Time Steps and Wrap-Up :::::::::::::: 11 v 2.1.3 Premature Ends to Simulations ::::::::::::::::: 11 2.2 Dynamical Simulation :::::::::::::::::::::::::: 12 2.2.1 KDK Method ::::::::::::::::::::::::::: 12 2.2.2 Lagrange Point Calculation ::::::::::::::::::: 13 2.3 Thermal Simulation :::::::::::::::::::::::::::: 13 2.3.1 Planetary Surface Temperature Calculation :::::::::: 14 2.3.2 Habitable Zone Determination :::::::::::::::::: 15 2.4 Verification :::::::::::::::::::::::::::::::: 15 2.4.1 Dynamics ::::::::::::::::::::::::::::: 16 2.4.2 Habitable Zone and Surface Temperature Verification ::::: 17 3 SIMULATIONS AND HYPOTHESIS ::::::::::::::::: 19 3.1 Critical Binary Separation for Planet Habitability in S-Type Systems 19 3.1.1 System Parameter Initialization ::::::::::::::::: 20 3.1.2 Simulation Time Step :::::::::::::::::::::: 20 3.2 Lagrange Point Habitability ::::::::::::::::::::::: 21 3.2.1 L4 Point Temperature Simulations ::::::::::::::: 21 3.2.2 Expected Behavior :::::::::::::::::::::::: 22 4 RESULTS AND DISCUSSION :::::::::::::::::::::: 25 4.1 Habitable Zone Geometry :::::::::::::::::::::::: 25 4.2 Transfer Orbits :::::::::::::::::::::::::::::: 27 4.3 S-Type Binary Results :::::::::::::::::::::::::: 27 4.3.1 Sensitivity of Stability and Planetary Eccentricity Perturbation 28 4.3.2 Critical Binary Separations ::::::::::::::::::: 28 4.3.3 Climate Effects of Abnormal Day-Night Cycles in S-type Binaries :::::::::::::::::::::::::::::: 31 4.4 Lagrange Point Habitability ::::::::::::::::::::::: 33 4.4.1 Comparison to Expected Behavior :::::::::::::::: 33 vi 4.4.2 Discussion ::::::::::::::::::::::::::::: 34 5 CONCLUSIONS AND FUTURE WORK ::::::::::::::: 37 5.1 Summary of Results ::::::::::::::::::::::::::: 37 5.2 Future Work :::::::::::::::::::::::::::::::: 38 5.2.1 Longer-Term and More Efficient Simulations :::::::::: 38 5.2.2 Testing the Odds of Planet Formation ::::::::::::: 39 5.2.3 Exploring Planetary Parameters ::::::::::::::::: 39 5.2.4 Orbital Eccentricities ::::::::::::::::::::::: 40 5.2.5 P-type Habitability :::::::::::::::::::::::: 41 5.2.6 L4/L5 Orbit Stability ::::::::::::::::::::::: 42 5.3 Conclusion ::::::::::::::::::::::::::::::::: 42 REFERENCES :::::::::::::::::::::::::::::::::: 43 vii LIST OF TABLES 2.1 Orbital parameters of the Earth, acquired from NASA's Planetary Fact Sheets[12], used to verify the correct simulation of orbital dynamics. :::::::::::::::::::::::::::::::: 16 3.1 Sample of time steps used to initialize simulations for the equal mass case. ::::::::::::::::::::::::::::::::::: 21 4.1 Comparison of the fitting coefficient C from Equation 4.4, and the function H(M; η) from Equation 3.11 for various mass ratios, η (right), and combined stellar masses, M = M1 + M2 (left). ::::: 34 viii LIST OF FIGURES 1.1 The two common orbital configurations for binary systems with planets. The heavier lines indicate planetary orbital paths, and lighter lines, the orbital path of the stars. :::::::::::::: 4 1.2 The 5 Lagrange Points of the Earth-Sun System with the James Webb Space Telescope orbiting at the L2 point (Not to Scale). Image Credit: NASA :::::::::::::::::::::::::::::: 5 2.1 Simulation Overview :::::::::::::::::::::::::: 10 2.2 Habitable Zone of the Sun, indicated by the green ring, with the position of Venus, Earth, and Mars plotted as small circles. Red indicates the area where the planet would be too warm, and blue, as well as white, indicates regions which are too cold. The Sun would be located at the bullseye of this target shape. All distances are in Astronomical Units (AU). ::::::::::::::::::::::: 17 2.3 Distance between the Earth and the Sun over the evolution of the test system for various time steps dt. ::::::::::::::::: 18 3.1 Overview of Lagrange Point simulation set-up :::::::::::: 23 4.1 Two main classes of Habitable Zone geometries(a and b), with two special cases of each (c and d). :::::::::::::::::::: 26 4.2 Surface Temperature of an Earth-like planet in an S-type orbit near Critical Binary Seperation with M1 = 1M , η = 1, and R = 3:43AU. 29 4.3 Minimum Habitable S-type Binary Separation plotted with the distance the planet needs to be from the host star to have T=288K with η = 1. ::::::::::::::::::::::::::::::: 30 4.4 Minimum Habitable S-type Binary Separation plotted for varying mass ratios. The line represents the equation of best fit. :::::: 31 ix 4.5 Overview of how a planet in an S-type binary orbit would experience day and night. ::::::::::::::::::::::::::::: 33 4.6 Surface Temperature of an Earth-like planet at the L4 point for various equal-mass binaries. The upper and lower horizontal lines represent the boiling point and freezing point of water, our criteria for planet habitability. ::::::::::::::::::::::::::: 35 4.7 Surface Temperature of an Earth-like planet at the L4 point for M1 = 1 and a variety of mass ratios. The upper and lower horizontal lines represent the boiling point and freezing point of water, our criteria for planet habitability. ::::::::::::::::::::: 36 5.1 Examples of eccentricity of orbits, with e = 0 representing a perfectly circular orbit, and larger e representing increasingly elliptical orbits. Image Credit: HyperPhysics, Georgia State University. ::::::: 41 x ABSTRACT In 2011, NASA's Kepler spacecraft observed the first planet to be detected in a known binary star system, Kepler 16-b. Its discovery has sparked new discussion on the potential of binary systems supporting habitable Earth-like planets. In this study, by using relatively simple computation models, we are able to shed new light and place new limitations on this discussion. Habitable Zone geometry in binary systems is discussed as a part of this study, in which we are able to classify binary habitable zones into two major classes of merged and unmerged, with important special cases of each class presented. We were able to learn a good deal about the behavior of planets in S-type binary orbits, and the possibility of life on those planets. The effect of the orbit of the planet on the climate as well as how the planet experiences day and night are discussed, and we also present a lower limit for the binary separation at which a binary system would fail to have