The smallest planet in the solar system, according to the International Astronomical Union's classification, is Mercury, with a radius of approximately 1,516 miles, which is 38 percent the size of Earth. The next smallest is Mars, with a radius of approximately 2,460 miles, or 53 percent the size of Earth.
Pluto, with a mean radius of almost 736 miles, was classified as a planet prior to 2006, when the IAU established a new set of conditions for an object to be considered a planet. This redefinition led Pluto to be reclassified as a dwarf planet. As of 2014, the IAU recognized five dwarf planets in the solar system, including Eris, which is slightly larger than Pluto.