Since The Planets was first published in 1993, space exploration has resulted in many new discoveries. There has been the 2004 Mars Exploration Rover Mission, the discovery of additional moons orbiting Jupiter, and more. When Pluto was discovered in 1930, it became the ninth planet in our solar system. Now there are eight planets. Because of its size and unusual orbit, Pluto was classified as a dwarf planet in 2006.