This highly praised textbook and reference work examines the basic elements of living systems: energy, chemistry, solvents, and habitats in crucial depth. These elements define the opportunities and limitations for life on other worlds. The book argues that life forms we would recognize may be more common in our solar system than many assume. It also considers, however, the possibility of exotic forms of life based on chemistries other than carbon, liquids other than water, and energy other than light. The authors offer an operational definition of life and summarize prevailing thoughts on plausible pathways for its origin on Earth and other worlds. They discuss remarkable adaptations to extreme environments, the nature and detection of biosignatures and possible technosignatures, the future and fate of living systems, and suggestions for the optimization of future exploratory space missions.

The entire text has been thoroughly updated in this fourth edition, with special focus on exoplanets, robotic exploration, synthetic life, detection of life, and life in extreme environments. New sections have been added on international space initiatives, updated plans for human missions to the Moon and Mars, the Fermi Paradox, and current controversies in astrobiology.

The growing impact of artificial intelligence on robotic exploration of the universe and the coevolution of organic and mechanical forms of life in the future are considered. While informed speculation in this emerging field cannot be avoided, the authors have clearly distinguished between known facts and reasonable expectations. They present an objective assessment of the plausibility of life on other worlds that is broad and deep enough for advanced students and experts in astrobiology, while avoiding scientific jargon as much as possible to make this intrinsically interdisciplinary subject understandable to a broad range of readers.

Dirk Schulze-Makuch is a professor at the Technical University Berlin, Germany, and an adjunct professor at Washington State University. He is interested in all aspects of astrobiology, but particularly whether other planets and moons inside and outside our Solar System could serve as a potential habitat for life. He received the Friedrich-Wilhelm Bessel Award from the Humboldt Foundation for extraordinary achievements in theoretical biology  in 2010.  He is best known for his publications on extraterrestrial life, which span more than 200 scientific articles and several books such as Cosmic Biology: How Life Could Evolve on other Worlds (2011),  Megacatastrophes! Nine Strange Ways the World Could End (2012),  The Extraterrestrial Encyclopedia (2016) and The Cosmic Zoo: Complex Life on Many Worlds (2017), as well as earlier editions of Life in the Universe: Expectations and Constraints (2004, 2008, and 2018).

Louis Irwin is a professor emeritus of biological sciences at the University of Texas at El Paso. He has published close to 100 research papers, reviews, encyclopedia entries, and chapters on neuroscience, astrobiology, and evolution. He is particularly interested in evolutionary trajectories under different conditions, including those that lead to intelligent and technologically capable forms of life. He has written two neuroscience memoirs, (Scotophobin, 2006, and Haystack Full of Needles, 2023), and co-authored books on evolution (The Evolutionary Imperative, 2016), and astrobiology (Life in the Universe: Expectations and Constraints, 2004, 2008 and 2018, and Cosmic Biology: How Life Could Evolve on other Worlds, (2011).