How to Build a Habitable Planet: The Story of Earth from the Big Bang to Humankind

by Charles H Langmuir and Wally Broecker, Princeton University Press, Princeton and Oxford, 718 Pages, 2012, Price $43.95 ISBN: 9780691140063 (Hardcover), ISBN: 9781400841974 (eBook)

Carl Sagan once said, “These are some of the things hydrogen atoms do given 15 billion years of cosmic evolution.” This book describes those things and the path that the hydrogen atoms have taken from before they were hydrogen shortly after the Big Bang to human civilization. It is an ultimate historical and scientific perspective of our planet and the universe.

Although the narrative is full of rich scientific description, the authors assume no prior knowledge from the reader. Concepts from a variety of sciences (astronomy, physics, geology, chemistry, biology, etc) are introduced as they are needed. They gradually build on one another to provide a thorough and complete story of how our universe, planet, and civilization have come to be bit by bit. Overall it is very accessible and could even be used as an introductory text for a college astrobiology course.

Each chapter starts with a motivating illustration and brief narrative to set the scene. This is followed by a formal introduction to the chapter, a thorough discussion, a summary, and a list of supplemental readings. Many illustrative black and white figures are provided in each chapter, along with 16 color pages with select color figures as an inset.

The stage is set with a discussion of the power and limitations of scientific reductionism, the idea of a system, and the need to take different approaches to finding answers over a wide range of spatial and temporal scales. A scientific theory rating system (0-10) is introduced and referred back to many times throughout the text. For example, the formation of the moon by a giant impact gets a 5-6 on the scale whereas plate tectonics is a perfect 10.

The story starts with the beginning of the universe describing the transition from particles to atoms and looking carefully at how stellar processes and nuclear stability affect chemical abundances.  Key concepts of density and volatility are introduced and related to the structure and functionality of inorganic and organic molecules.  On a molecular level the composition of Earth is intertwined with the overall structure of the planet as a whole.

The formation of the solar system and Earth in particular is discussed in detail. The history of the planet from its formation onward is intimately connected to the history of life.  In order to start the timeline, it is necessary to establish the age of Earth, and we are presented with conflicting views from a Biblical perspective, a naive scientific perspective, and a geological perspective. Ultimately the conclusion of a 4.5 billion year old Earth uses evidence from many sources (Earth’s geological record, meteorites, radiometric dating, etc). Puzzling together a timeline of the co-history of life and planet with what has survived since ancient times is a work in progress, but the authors layout the pieces and fit them together.

While discussing volatiles in Earth’s atmosphere we begin to see important modifications life has made to the planet to keep it habitable over time. An important part of keeping volatiles mixing is the global jigsaw puzzle of plate tectonics. The evidence for plate tectonics is presented in detail including the paleomagnetic record, earthquakes, and volcanic activity. The theory with its strong evidence only took about 5 years for a practically complete acceptance in the scientific community. From the understanding of plate tectonics, mantle flow can be studied. Mantle flow drives chemical fluxes between the surface and interior of the planet. This determines the chemical composition of the atmosphere and oceans and long term climate stability and is therefore important for both physical and biological properties of the planet.

As the narrative progresses the connection between our physical planet and life is made stronger. For several chapters the story turns the focus toward life and its origin and evolution. This is never addressed as a stand-alone question, but rather in the context of the planetary history because the evolving planet and evolution of life are completely co-dependent. Through biological processes, life reduces carbon and adds O2 to the atmosphere. This creates a surface environment in a high oxidation state and reservoirs of reduced carbon that get locked up in the subsurface by geological processes. The separation of these two reservoirs creates a planetary fuel cell that provides energy for modern life.

The evolution of life depends on many external factors. Mass extinction events are important because, while they reduce biodiversity on short time scales, they also open up opportunities for other life to survive. This increases biodiversity in the long run. Life also depends on the climate (which it also affects). To this end the authors take a close look at climate cycles on various timescales.

Over time the evolution of life has led to an increase in energy utilization. Low on the scale of energy utilization is the first life, anaerobic single-celled organisms. With the production of O2 aerobic life with higher energy utilization became possible.  At the top of this ladder is our human civilization that has found ways to exploit the reduced carbon reservoirs, quickly draining the planetary fuel cell and other resources. The authors argue that humans are on the way to using billions of years of banked resources in just a few centuries.

The penultimate chapter focuses on the consequences that this great and rapid energy expenditure have on the planet:  climate change, ocean acidification, and loss of biodiversity. Humans are bringing about rapid change in a system that has been slowly changing for billions of years. The authors put our current state into the context of the entire history of the planet and project forward to a dismal future outlook. The book suggests possible solutions such as population control, alternative energy, and carbon sequestration and storage, and discusses pros and cons of each.

The final chapter proposes that the habitability of Earth is not unique since the history of the planet has been governed by physical laws. Examples of failed habitability (Mars and Venus) are given and extra-solar planet detection techniques are explained in hopes of understanding the number of Earth-like planets in our galaxy. This is a rapidly growing field. By the authors' own admission their words are outdated immediately upon publishing; for example the number of habitable planets currently known is already an order of magnitude up from the ~200 in the book. Finally, there is a look at the terms in the Drake equation that allow a probabilistic estimation of the number of intelligent civilizations we might be able to contact. A key term is the lifetime of the civilization. How long do intelligent civilizations last? With our recent history we are left to wonder about our own.

Hillary Stephens
Associate Professor of Physics and Astronomy
Pierce College Fort Steilacoom
email: hstephens@pierce.ctc.edu

These contributions have not been peer-refereed. They represent solely the view(s) of the author(s) and not necessarily the view of APS.