The nearby red dwarf star TRAPPIST-1 has a system of seven Earth-size planets, all jammed into a space much smaller than the gap between the Sun and Mercury. (Credit: ESO/O. Furtak)

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