This book investigates the scientific probability of extraterrestrial life and the conditions required for biological development on other worlds. Ben Bova, a veteran science writer and former editor of Analog Science Fiction and Fact, utilizes current astronomical data and biological theory to argue that the universe is likely populated by various forms of life. He presents a framework that balances the vast scale of the cosmos against the specific chemical and environmental constraints necessary for life as we understand it.

Experts and readers recognize this work as an accessible entry point into the field of exobiology for a general audience. The prose is noted for its clarity and ability to distill complex astrophysical concepts into understandable arguments.