“Armchair astronomers and hard SF fans who love a good game of cosmological What If? will find this an entertaining follow-up to physicist Comins's WhatIf the Moon Didn't Exist? Here he presents 10 more intellectual puzzles that explore new worlds, and imagine what life there might be like. Each chapter opens with a vivid glimpse of a hypothetical new world ... This is a lucid, thoroughly accessible presentation of what might have been that is sure to make this volume as popular as its predecessor.” —Publisher's Weekly

“Stretching his scientific imagination, Comins tweaks the astronomical dials that control evolution on Earth, such as the length of the day, the height of the tides, or plate tectonics ... An astronomy professor and author of several successful science books (What If the Earth Had No Moon? 1993), Comins plays out his scenarios in physics-grounded narratives that all posit the same result despite different evolutionary paths: the rise of humans. Depicted in fictional predicaments that preface each chapter, they, warming up Comins' factual explanations, create sf crossover appeal for this entertaining piece of popular science.” —Gilbert Taylor, Booklist