The solar system today is recognized as having eight planets. In order from the Sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. These eight worlds vary enormously — from small rocky planets close to the Sun to vast gas and ice giants in the cold outer regions. For most of the twentieth century, however, students were taught that there were nine planets, with Pluto as the ninth and most distant.

Pluto was discovered in 1930 and was considered the ninth planet for over seventy years. The change came in 2006, when the International Astronomical Union, the body responsible for naming and classifying objects in space, agreed on a formal definition of what a planet is. Pluto did not meet every part of that definition, and it was reclassified as a "dwarf planet." This decision was widely discussed and remains famous, but it was based on how Pluto compares to the other eight worlds.

Under the official definition, an object must do three things to be called a planet. It must orbit the Sun, it must be large enough for its own gravity to pull it into a roughly round shape, and it must have "cleared its neighborhood" — meaning it is gravitationally dominant and has swept up or pushed away other objects in its orbital path. Pluto meets the first two conditions but not the third, because it shares its region of space with many other icy bodies.

Although there are only eight planets, the solar system is far from empty. It also contains several dwarf planets, including Pluto, Eris, and Ceres. Beyond the planets lie countless asteroids, comets, and icy bodies, along with the many moons that orbit the planets themselves. The eight planets are the largest and most dominant members of the Sun’s family, but they are only part of a much richer and more crowded cosmic neighborhood.

This article was written using information from Wikipedia.