On August 24, 2006, the International Astronomical Union officially defined the term "planet.” A planet orbits a star and is large enough to form itself into a nearly round shape (due to gravity). A planet must also “clear the neighborhood” of space debris in its orbital zone. Our solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Astronomers have also identified nearly a thousand planets outside our solar system, called extrasolar planets.

Celestial bodies that have not “cleared the neighborhood” are called “dwarf planets.” Dwarf planets in our solar system include Pluto, Ceres, Eris, Haumea, and Makemake. Hundreds of more distant objects may still be classified as dwarf planets in the future.

debris
Noun

remains of something broken or destroyed; waste, or garbage.

gravity
Noun

physical force by which objects attract, or pull toward, each other.

orbit
Noun

path of one object around a more massive object.

Noun

large, spherical celestial body that regularly rotates around a star.

solar system
Noun

the sun and the planets, asteroids, comets, and other bodies that orbit around it.

star
Noun

large ball of gas and plasma that radiates energy through nuclear fusion, such as the sun.