There are eight planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The four inner solar system planets (Mercury, Venus, Earth, and Mars) fall under the category of terrestrial planets, while the outer four (Jupiter, Saturn, Uranus, and Neptune) are the gas and ice giants.
Pluto, a dwarf planet, was once classified among the aforementioned eight as a solar system planet; however, it is now considered to be one of the largest known members of the Kuiper Belt—a collection of icy bodies on the outer fringes of the solar system. Pluto was demoted from its planetary status in 2006 when a body of scientists decided a formalized definition for the term “planet.” According to the International Astronomical Union's definition, a planet is “a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.” Because Pluto is part of the Kuiper Belt, and therefore has not met the third criterion, it is no longer considered a planet. Instead, it is classified as a dwarf planet. With an atmosphere and at least five moons, Pluto is the most complex dwarf planet we know.