“If we consider just those worlds similar in size to the Earth—let’s say, in a range from half its diameter to about four times its diameter—it is obvious that there must be anywhere from a few billion to a few tens of billions of these planets in the Milky Way. In fact, if we consider only those orbiting their stars at the right distances to allow for moderate surface temperatures and liquid water, some studies suggest a galaxy-wide population of more than 20 billion and even as many as 40 billion.”