Exoplanets

Preface

xvii Preface This is a unique time in human history — for the first time, we are on the technological brink of being able to answer questions that have been around for thousands of years: Are there other planets like Earth? Are they common? Do any have signs of life? The field of exoplanets is rapidly moving toward answering these questions with the discovery of hundreds of exoplanets now pushing toward lower and lower masses; the Kepler Space Telescope with its yield of small planets; plans to use the James Webb Space Telescope (launch date 2014) to study atmospheres of a subset of super Earths; and ongoing development for technology to directly image true Earth analogs. Theoretical studies in dynamics, planet formation, and physical characteristics provide the needed framework for prediction and interpretation. People working outside of exoplanets often ask if the field of exoplanets is like a dot.com bubble that will burst, deflating excitement and progress. In my opinion, exciting discoveries and theoretical advances will continue indefinitely in the years ahead, albeit at a slower pace than in the first decade. The reason is that observations uncover new kinds and new populations of exoplanets — and these observations rely on technological development that usually takes over a decade to mature. For example, in the early 2000s almost all exoplanets had been discovered by the radial velocity technique. At that time, many groups around the world were working on wide-field transit surveys. But it was not until recently, a decade into the twenty-first century, that the transit technique is responsible for almost one-quarter of known exoplanets. The planet discovery techniques astrometry (as yet to find a planet) and direct imaging have not yet matured; when they do, they will uncover planets within a new parameter space of planet mass and orbital characteristics. In addition, people are working hard to improve the precision for existing planet discovery techniques to detect lower-mass planets and those further from their star. All in all, technology enables slow but sure progress, and this fuels ongoing discovery. Theory, like observations, also takes time to unfold and mature. We can anticipate an “ultimate” planet formation model similar to the “millenimum simulation” for galaxy formation and evolution. In time, incorporating detailed physics as well as being able to reproduce the generic outcome of planet populations (mass, radius, and orbital characteristics, including period) will enable a deeper understanding of planet formation and migration. Similarly, the ideal exoplanet atmosphere code of the future could be a three-dimensional Monte Carlo code that includes radiative transfer with inhomogeneous cloud coverage and surface features, a code that also solves for the temperature structure and combines with a hydrodynamical simulation to calculate the three-dimensional temperature and wind structure. Classical orbital mechanics, already reinvigorated by interesting exoplanet systems (e.g., planets in resonant orbits, hot Jupiter exoplanets that orbit in the direction opposite to the stellar rotation), also has a role to play in explaining fundamental mechanisms of how planetary system configurations came to be. Orbital dynamics modeling is driving the search for moons and other unseen planet companions by their perturbations on transiting planet signatures. Exoplanets is a unique science because it involves so many disciplines within and beyond planetary science and astrophysics. The other disciplines include geophysics, high-pressure mineral physics, quantum mechanics, chemistry, and even microbiology. While exoplanet observations clearly belong under the branch of astronomy, for many years the whole discipline of exoplanets lacked a true home. Physics departments have said “Exoplanets: It’s interesting, but is it physics?” xviii Planetary and Earth science departments used to collecting real data in their hands from Earth and in situ measurements from solar system planets were, in the early 2000s, reluctant to believe there would ever be enough high-quality data to take the research field of exoplanets seriously. With hundreds of known exoplanets and many fascinating observational and theoretical discoveries, the whole world is embracing exoplanets, a field that can now find its home both in and spanning across planetary science, astronomy and astrophysics, and astrobiology. We use the term “exoplanets” and not “extrasolar planets” in this book. Exoplanets is the most direct derivative from the Greek language, from which the word “planet” originates. (Note that the word “extrasolar” is derived from Latin.) It is most fitting that the field be named as traditionally as possible, and there is none more traditional than Greek. The goal of this volume is to cover the range of topics in...