Sara Seager - Exoplanet Atmospheres: Physical Processes

A new era in planetary science is upon us. Hundreds of extrasolar planets (exoplanets) orbiting normal stars are known. Surprisingly, the exoplanetary systems are very different from our own solar system. Gas giant planets have been detected at a wide range of orbital distances from the parent star, including some much closer to their parent star than Mercury is to our Sun. Many exoplanets have eccentric orbits, some with eccentricities up to 0.9. Planets with no solar system analogs are being discovered including mini Neptunes and super Earths. Now that the existence of exoplanets is firmly established, the adventure of exploring their physical characteristics has begun in earnest. An exoplanets physical properties, such as density, atmospheric composition, and atmospheric temperature, can be measured for a subset of exoplanets.

The first fifteen years of exoplanet discoveries have taught us to expect surprises, because the random nature of planet formation and migration leads to many different planetary system architectures. These include an astonishing range of observed exoplanet masses, semimajor axes, and orbital eccentricities. We similarly anticipate a huge diversity of exoplanet atmospheres; the formation and composition of an atmosphere will depend on where in the disk a planet forms, its evolutionary history (due in part to atmospheric escape), and its present semimajor axis.

To understand the physical characteristics of the potentially wide variety of planetary atmospheres, knowledge of the general physics common to all planetary atmospheresboth exoplanets and solar system planetsis needed. The goal of this book is to present the basic physics that can be used to obtain fundamental atmospheric characteristics and the first-order vertical thermal structure from spatially unresolved spectra and photometry. Emphasis will be on the major physical processes that govern the planetary atmosphere and emergent spectrum. This book is aimed at advanced undergraduate students, graduate students, and researchers entering the field of exoplanet atmosphere studies.

With hundreds of exoplanets now known, and dozens of exoplanet atmosphere observations, there is a need for a book that addresses, in a general way, the basic planetary atmospheric physics for planetary atmospheres in a broad range of environments. Although the physics is the same as for solar system planet atmospheres, traditional planetary atmosphere books are necessarily specific and descriptive in addressing highly detailed observations and phenomenology of solar system planets. While textbooks on stellar atmospheres address many of the issues common to exoplanet atmospheres, they lack topics highly relevant for a nonluminous planet orbiting a star (such as transmission spectra, scattered and polarized radiation, and thermal orbital phase curves). A basic understanding of atmospheric processes is necessary to interpret data and to aid experiment design.

This is a tremendously exciting time for exoplanet atmosphere studies. Dozens of exoplanet atmospheres have been observed, mostly from space. So far, atmospheric observations and interpretation have been limited to a subset of exoplanets: hot Jupiters or hot Neptunes in excruciatingly close orbits to their host stars; and directly imaged young, massive exoplanets in very distant orbits around their host star. This will all change in the not-too-distant future. The anticipated launch of NASAs James Webb Space Telescope in 2014 as well as new ground-based instrumentation will enable a wider range of planets (in terms of masses and orbits) whose atmospheres can be studied. In the more distant future, we hold hope for a space telescope capable of direct imaging, for both discovery and follow-up atmosphere measurements of true Earth analogs.

Good luck on your journey to explore exoplanet atmospheres!

Many people deserve thanks for help with this book. First and foremost a very special thanks to my husband Mike Wevrick, for help with figures, the index, editing, and grammar. His continued patience in many aspects helped tremendously. Second I thank Cindi Fluekiger for her support and encouragement. Many students and colleagues helped with the description of complicated concepts or read drafts of the book for content and accuracy. Thanks to Bjoern Benneke, Renyu Hu, Hannah Jang-Condell, Jim Kasting, Mark Marley, Paul OGorman, Dimitar Sasselov, Feng Tian, Maggie Turnbull, and Neil Zimmerman. A special thanks to Nikku Madhusudhan and Leslie Rogers for their attention to detail. For technical assistance I thank Meghan Kanabay and Eugene Jang for help with figures and Lucy Day Werts Hobor and Steve Peter for their exceptional LATEX expertise. Finally I express my thanks to the Princeton University Press staff Ingrid Gnerlich, Brigitte Peiner, and Dimitri Karetnikov for very professional oversight of this project.

Sara Seager
Cambridge, MA

The search for our place in the cosmos has fascinated human beings for thousands of years. For the first time in human history we have technological capabilities that put us on the verge of answering such questions as, Do other Earths exist?, Are they common?, and Do they harbor life? Critical to inferring whether or not a planet is habitable or inhabited is an understanding of the exoplanetary atmosphere. Indeed, almost all of our information about temperatures and chemical abundances in planets comes from atmospheric photometry or spectroscopy.