How to find a habitable planet

Abstract

Instead of diving into the Intergovernmental Panel on Climate Change Reports, astrobiologists interested in the physics behind global warming could start with Jim Kasting’s new book, How to Find a Habitable Planet. Kasting not only describes how to find another habitable planet but also clearly explains the long-term evolution of terrestrial planet atmospheres. This is the long-term context needed to understand short-term global warming. Kasting explains the behavior of terrestrial planet atmospheres on billion-year timescales and shows us how, over the long-term on Earth, greenhouse gases come and go and the increasing luminosity of the Sun eventually humidifies the stratosphere, where H2O is photolyzed and lost to space. Thus, the evolution of the Sun produces a leak in Earth’s hydrological cycle that eventually sinks the boat of life. In blurbs for this book, NASA Ames astrobiologist Chris McKay wrote ‘‘I learned several new things from this book,’’ while MIT’s Sara Seager wrote ‘‘everything you need to know about habitable worlds.’’ As in any dialectical marketing scheme, the truth is in the middle. If you read this book you will learn more than ‘‘several new things’’ but certainly not ‘‘everything you need to know about habitable worlds.’’ Among the new things you might learn are that the Sun’s luminosity increases about 1% every 100 million years; that methane, not ammonia, is the greenhouse gas of choice that most plausibly solves the faint early Sun problem; that tropospheric cold traps keep stratospheres dry. You might learn how to distinguish flashflood outflow channels on Mars from runoff channels due to ground water sapping; that Rayleigh scattering is proportional to l 4 and why this explains how high CO2 abundance acts as an anti-greenhouse gas by increasing Earth’s albedo at short wavelengths; or how the Snowball Earth hypothesis is based on equatorial dropstones, diamictites, and cap carbonates from 710 and 630 million years ago. You might also learn the strengths and weaknesses of the various techniques for finding exoplanets and the important difference between a false positive and a false negative. Kasting writes with the pace and authority of having taught this material many times to undergraduates at Pennsylvania State University. During a Jet Propulsion Laboratory sabbatical he compiled his lecture notes into this book and included the most important insights garnered from NASA Terrestrial Planet Finder meetings in 2004–2005. The extensive set of bibliographic notes are just what the overburdened academic needs to direct an enthusiastic but misinformed classroom of astrobiology students. Kasting has found and organized the essential references that will guide the beginner. For example, for a treatment of the Milankovich cycles, he leads the reader to Imbrie and Imbrie (1979) Ice Ages: Solving the Mystery. So much information is clearly dispensed that it is a pleasure to read. The digressions in the *100 footnotes provide some relief when the text seems too simple. The book includes 8 pages of color figures and an impressive 18 pages of bibliographic notes. In addition to providing an excellent teaching resource, the book has much to offer to the seasoned astrobiologist. In chapter 10, Kasting describes, with authority and familiarity, the history of circumstellar habitable zone research. He traces the increasingly sophisticated research on the circumstellar habitable zone as it moves from Harlow Shapley’s (1953) ‘‘Liquid Water Belt’’ to Hubertus Strughold’s (1953–1955) ‘‘Ecosphere of the Sun’’ to the work of Huang in 1959–1960 to Dole’s (1964) Habitable Planets for Man. Kasting describes how Hart (1978–1979) inappropriately used global glaciations to set the outer boundary of his habitable zone (we now know that we have probably lived through several such Snowball Earth episodes). Kasting reviews and extends the oft-cited habitable zone research of Kasting et al. (1993). He compares the atmospheric evolution of Venus, Earth, and Mars—a comparison that helps provide constraints on the inner and outer boundaries of the habitable zone. He describes how the increasing luminosity of host stars leads to the concept of a continuous habitable zone, and he gives us the pros and cons of habitable zones around M stars. The work of Franck et al. (2000a,b) on the continental-crust-dependent habitable zone is also discussed. In several chapters Kasting describes climate models and their dependence on a zoo of feedback mechanisms: stabilizing negative feedback, runaway positive feedback, feedback at different timescales. Some feedback loops involving the same components act in different directions on different timescales. CO2 seems to be linked to all kinds of complicated feedback. Trying to make a prediction based on a short-term positive feedback superimposed on a long-term negative feedback sounds as confusing and nonlinear as parenting a teenager. Kasting articulately walks us through this minefield and points out some instructive exceptions to this unpredictability. The buildup of CO2 from ongoing