When I was an astronomy-obsessed kid, I learned that most of the stars in our galaxy and beyond are very similar to our Sun. No less an authority than Carl Sagan wrote that “the Sun is an ordinary, even a mediocre star.” If that insight diminished the importance of our place in the universe, it also made it seem likely that there must be many other living worlds around us. If the Sun is a typical star, couldn’t the Earth be a typical planet?
Except that Sagan was wrong, or at least misleading. More than 90 percent of the stars in the Milky Way are cooler and dimmer than the Sun. A full three-fourths are red dwarfs, the smallest of the small. Furthermore, dwarf stars seem to be especially likely to have rocky planets. Together, those statistics indicate that there are a trillion planets around red dwarfs in our galaxy, including at least 100 billion potentially habitable Earth-size worlds. The big unknown is whether those planets are actually habitable — that is, if the genuinely typical Earth-size worlds out there are really anything at all like our own.
