A key question that is often posed is, ‘Are we alone in the universe?’ If the answer to this question were simply ‘Yes’ or ‘No’, this book would probably not have been written. But what looks like a simple question is actually a complex and multifaceted set of issues which can hopefully be elucidated by discussing the many aspects involved.

First, by ‘the universe’ we can only mean the small corner of the galaxy which we inhabit, say within about a 100-light-year or, at maximum, 1,000-light-year radius. This is a tiny fraction of our Milky Way galaxy (at least 100,000 light years in diameter) and we cannot even see half of the galaxy which is beyond the central bulge of the disk on the other side. The Milky Way is an infinitesimally small part of the entire universe.