The previous classification system was based on historical knowledge and size. The eight planets in our solar system today, plus Pluto, were known to scientists and were believed to be large—at least, all larger than Earth’s moon. Other objects that were known to orbit the Sun—but were smaller than about two thousand miles across—were called asteroids (or, more generally, minor planets). So until August 24, 2006, the International Astronomical Union officially recognized Pluto as the ninth planet. It does not anymore.