Why was Pluto reclassified as a dwarf planet?
1) The definition of a planet was clarified: a planet must have a radius more than 1500 km. Pluto is not large enough and is a dwarf planet.
2) The definition of a planet was clarified: a planet must have a radius more than 1500 km. Pluto satisfied this condition when it was discovered, but has become smaller over time.
3) The definition of a planet was clarified: a planet must be able to clear away objects orbiting nearby. Pluto is not large enough for this.
4) The definition of a planet was clarified: a planet must have a spherical shape. Pluto is not large enough to assume a spherical shape.