ABSTRACT
Medieval European scholars spent little time thinking about whether and where extraterrestrial life may exist. e Aristotelian cosmology adopted by most medieval scholars held that the Sun, Moon, planets, and stars all comprised perfect spheres orbiting the Earth in concentric circles. Because the celestial bodies were thought to be perfect spheres
CONTENTS 1.1 Extraterrestrials from the Scientic Revolution to the Space Age 1 1.2 Drake Equation and the Anthropic Principle 4 1.3 Essential Features of Life 5
1.3.1 Reproduction and Heredity 5 1.3.2 Evolution 6 1.3.3 Cellularity 7 1.3.4 Metabolism 7
1.4 Origin(s) of Life 8 1.4.1 Prebiotic Chemistry 8 1.4.2 Ancient Life 9 1.4.3 Unanswered Questions about the Origins of Life 11
1.5 Looking for Life beyond Earth 11 1.5.1 Environmental Limits of Life on Earth 11 1.5.2 Extraterrestrial Habitable Environments 12 1.5.3 Habitable Planets around Other Stars 13 1.5.4 Detecting Extraterrestrial Life 14
1.6 Future of Astrobiology 16 Acknowledgments 16 Glossary 16 Review Questions 17 Additional Reading 17
without geologically complex terrains, their habitation was rarely considered. In the midsixteenth century, Nicolaus Copernicus’s mathematical model of the Earth and the ve other known planets orbiting the Sun led to an enormous shi in the broader understanding of the Earth’s place in the cosmos.
