Thanks for inviting me here today to talk about Pluto. When it was discovered in 1930, Pluto was automatically considered to be a planet—the ninth and most distant in our solar system. But, in 2006, astronomers refined their definition'of what a planet is, and Pluto no longer qualified. It was placed in a new category along with a few other small bodies called 'dwarf planets'. The astronomers' new definition specified three qualities of a planet: it must orbit a star, it must be spherical and it must dominate its surroundings. Pluto met the first criterion—it goes around our sun just like the Earth does, but it failed on the other two points. Like many other asteroids in the solar system, Pluto is irregularly shaped; it's simply too small to have enough of a gravitational force to pull it into an even, spherical shape. The third quality that astronomers agreed a planet must exhibit is a command over the space it travels in, drawing in debris from the surrounding area. In Pluto's case, its abnormally long orbit intersects with Neptune's, demonstrating that it does not have the spatial dominance to make the grade. Although a lot of people were disappointed when Pluto lost its planetary status, astronomers now have a lot more information than they did in the 1930s about how planets are formed and their defining characteristics, so, all in all, I think it was the right decision.