The Sun’s corona extends millions of miles beyond the Sun’s surface. The plasma of the solar wind, however, extends billions of miles farther—well beyond the orbit of Pluto. Beyond there, the plasma density continues to drop. There is a limit, called the heliopause, where the influence of the solar wind dwindles to just about nothing. The region inside the heliopause—which is thought to be some eight to fourteen billion miles (thirteen to twenty-two billion kilometers) from the Sun—is called the heliosphere.