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; Jupiter and Saturn are gas giants (giant plants composed mostly of hydrogen and helium) while Uranus and Neptune are the ice giants (containing mainly elements heavier than hydrogen and helium).
Pluto, a dwarf planet, was classified as one of the solar system planets when it was first discovered by Clyde Tombaugh. 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. Other dwarf planets include Ceres, Haumea, Makemake, and Eris.
With an atmosphere, stark surface features, and at least five moons, Pluto is the most complex dwarf planet we know, and one of the most surprising solar system planets. New Horizons flew by our favorite dwarf planet in July 2015 and scientists continue to uncover surprising details about this faraway world.
Learn about our solar system's most ostentatious planet — the ringed giant Saturn and its fascinating family of moons — in our FREE ebook, Saturn's Bounty. Enter your email address to download the ebook, and you'll also receive our weekly e-newsletter with the latest astronomy news.