Thirty-three light-years away, in the constellation Leo the lion, astronomers say they have found a world considerably smaller than Earth, orbiting a dim red-dwarf star. That’s something to think about. While scientists have confirmed the existence of more than 700 so-called exoplanets since 1995, most of them have been giant — many considerably larger than Jupiter. This new world, say the researchers who found it, may be only 5,200 miles across, about two thirds as large as Earth.