Audio Guide：

The International Astronomical Union on August 24, 2006, announced a new classification schemata for the solar system. A new distinction of a ‘dwarf planet’ was added, and the term ‘planet’ was given a clear definition. A ‘planet’ was defined to consist of three characteristics: it is in orbit around the sun, has sufficient mass for its self-gravity to allow it to form a spherical shape, and has cleared the neighborhood of its orbit. This new definition meant that Pluto, once a planet, would be reclassified as a dwarf planet. And other objects, such as Ceres and Eris (located in the Kuiper belt), joined the ranks of dwarf planets. Many other such dwarf planets on the way are sure to be added in the future.