A First Reconnaissance of the Atmospheres of Terrestrial Exoplanets Using Ground-Based Optical Transits and Space-Based UV Spectra The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Diamond-Lowe, Hannah Zoe. 2020. A First Reconnaissance of the Atmospheres of Terrestrial Exoplanets Using Ground-Based Optical Transits and Space-Based UV Spectra. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences. Citable link https://nrs.harvard.edu/URN-3:HUL.INSTREPOS:37365825 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA A first reconnaissance of the atmospheres of terrestrial exoplanets using ground-based optical transits and space-based UV spectra A DISSERTATION PRESENTED BY HANNAH ZOE DIAMOND-LOWE TO THE DEPARTMENT OF ASTRONOMY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN THE SUBJECT OF ASTRONOMY HARVARD UNIVERSITY CAMBRIDGE,MASSACHUSETTS MAY 2020 c 2020 HANNAH ZOE DIAMOND-LOWE.ALL RIGHTS RESERVED. ii Dissertation Advisor: David Charbonneau Hannah Zoe Diamond-Lowe A first reconnaissance of the atmospheres of terrestrial exoplanets using ground-based optical transits and space-based UV spectra ABSTRACT Decades of ground-based, space-based, and in some cases in situ measurements of the Solar System terrestrial planets Mercury, Venus, Earth, and Mars have provided in- depth insight into their atmospheres, yet we know almost nothing about the atmospheres of terrestrial planets orbiting other stars. I present an observational reconnaissance of the atmospheres of terrestrial exoplanets orbiting nearby, low-mass stars, opening the door for an atmospheric branch of comparative terrestrial planetology. I studied three worlds, LHS 3844b, GJ 1132b, and LHS 1140b, which have equilibrium temperatures of 805 K, 580 K, and 235 K, respectively. All three planets transit stars with masses less than 20% the mass of the Sun, and lying within 15 parsecs. I employed the technique of ground-based transmission spectroscopy using the Low Dispersion Survey Spectrograph (LDSS3C) on the Magellan Clay Telescope at the Las Campanas Observatory in Chile. To observe transits of LHS 1140b I used the the Inamori-Magellan Areal Camera & Spectro- graph (IMACS) on the Magellan Baade Telescope concurrently with LDSS3C. I searched for chromatic differences in the amount of light attenuated as the planets transited across their host stars. I disfavored cloudless, hydrogen- and helium-dominated atmospheres on LHS 3844b to 5.5s confidence, and on GJ 1132b to 3.7s confidence. I disfavored cloudless, water steam atmospheres to 3.5s confidence on both LHS 3844b and GJ 1132b. The cool equilibrium temperature and high surface gravity of LHS 1140b render any atmosphere around this world below my detection limits. iii Planetary atmospheres are sculpted and in some cases removed by the high-energy radiation from their host stars. Low-mass stars spend an extended amount of time in the highly-active pre-main sequence phase compared to Sun-like stars. It is not known if ter- restrial planets orbiting low-mass stars can maintain atmospheres at all, let alone provide hospitable conditions for abiogenesis. To unite my constraints on terrestrial exoplanet atmospheres with the influence of their low-mass stellar hosts I present a first look at the ultra-violet spectrum of LHS 3844 taken with the Cosmic Origins Spectrograph on the Hubble Space Telescope. I detected prominent emission lines in the ultra-violet spectrum of LHS 3844, and used them to estimate the Lyman-a and extreme ultra-violet luminos- ity. These data will inform models of atmospheric photochemistry on LHS 3844b and constrain rates of atmospheric escape from this world. My reconnaissance of the atmospheres of several terrestrial exoplanets excluded low mean molecular weight atmospheric cases around these worlds, and pushed current in- strumentation to its limits. The pursuit of thin, secondary atmospheres similar to those on Venus and Earth must await future facilities, notably the James Webb Space Telescope and the ground-based giant segmented-mirror telescopes. Detections of terrestrial exoplanet atmospheres, in conjunction with studies of the high-energy stellar flux they encounter, will allow us to place the terrestrial worlds of the Solar System in the context of terrestrial planets as a whole. iv ACKNOWLEDGEMENTS This has been a long trek on a bumpy trail, and I could not have reached the summit with- out the support, advice, love, and tough love from my family, friends, and community. First and foremost, to my advisor Dave, who read every word of my papers, proposals, conference abstracts, and this thesis, thank you for your mentorship. Your patience and insight have made me the scientist I am today, and that is something I can be proud of. A big thank you to my committee members, Dimitar, Mercedes, and Robin, whose varied expertise has strengthened this work, and to David Sing for serving as my external reader. Thank you to Jacob and Dorian, my college advisors. Thank you to the extended MEarth team who I have gotten to know over the years, Jonathan, Jen, Raphie, Jayne, Laura K., Caroline, Laura M., Joey, Suri, Ryan, Amber, Nick, Juli, Kristo, Adrianna, and Emily, I’m glad we will never again have to meet at House of Chang, but I will miss the group conversations. To the broader Center for Astrophysics community, who keep the science, the servers, and the building running, thank you for making the CfA a warm and functioning place to work, even as a pandemic keeps us home. Christine, a very special thank you to you for being my administrative rock, for making sure I got reimbursed at lightning speed, and for sharing where to get the best Mai Tais. I have to thank some incredible friends who have been around through this process. To the past and present residents of 12 Lawrence, my grad school home, Oliver, Ali, Jas- mine, Cassandra, Katie, Allison, Molly, and Christina, thank you for every late night con- versation, every walk to the store, and of course all the backyard barbecues and holiday parties. I learned so much from all of you, and I’m so glad our paths crossed in a creaky old Cambridgeport house. Laura, thank you first for being a friend and confidant, and second for being a top- shelf astronomer. You amaze me always and I’m going to miss not working across the hall, or a floor down from you. Adrien, my oldest friend, thank you for visiting me and keeping our friendship going, now 20 years strong and counting. Erik, Sam, Jay, and Mike, from France to Norway to Chile and both coasts, we’ve been on some great (mis)adventures. Let’s do it again sometime. Zoe, my best friend, most things we talk about I probably can’t write in a Harvard thesis, so I’ll just say, I can’t wait until the next time we can sit out in the sunshine with some white wine and talk until the sun goes down. And then go to Van Kleef. v Dan, it turns out I leaned on you more than I ever thought I would. Thank you for the day-to-day support while I finished this thesis, and for all the music, laughter, and Miller High Life a girl could want. I’m looking forward to our next adventure. Amber, I could double the length of this thesis with how many thank yous I have for you. Imagine? I’d calculate how many hours we’ve spent together over the last five years but I’d need you to do it with me on the whiteboard and we’re not allowed in our office right now. And let’s face it, it would take us like all day to work out. For all I learned about astronomy, I learned just as much about New Mexico, pecans, the whole Medina family, and your work-out regimen. Te quiero mucho chica. And finally the biggest thank you to my family. Mom and Dad, you showed me the world from day one, and you remain my fiercest supporters to this day. I am so lucky to have such cool parents. To the Diamonds — Grandma Randa, Aunt Celia, Uncle Jon, Aunt Val, Dave, Caryn, Phillip, Eve, and Noah, it is always such a treat to visit with you, whether it’s in California, Hawai’i, or Ohio. To the Chicago Diamonds, Linda, Mark, Sam, and Michael, thank you for giving me a home away from home during my college years. To the Lowes — Uncle Brian, Aunt Mary, Nick, Alexa, Liz, Tim, Miles, and Una, it’s always a good time when we get to hang out and see some art. To my extended Highland Park family, Wendy, Colin, Olivia, and Eliot, thank you for being a part of my life since the Plainfield days. To Jill and Stacy, traveling with you in Ireland is still one of my favorite vacations, and I hope we can make it happen again. And of course to Auntie Wen, Uncle Ron, and Mischa, thank you for making every Hanukkah my favorite holiday celebration. I could not ask for a more talented, more intelligent, or kinder extended family, and I love you all so much. vi Contents Copyright ii Abstract iii Acknowledgementsv List of figures xi List of tables xii 1 Introduction1 1.1 What is a terrestrial exoplanet?..........................1 1.2 Small planets: terrestrial planet vs. enveloped terrestrial planet.......3 1.2.1 What causes the small planet radius valley?..............6 1.2.2 Secondary atmospheres on terrestrial planets..............9 1.3 M dwarfs as terrestrial exoplanet hosts.....................