What is a planet?
According to the International Astronomical Union, a planet is a body that orbits the Sun, is massive enough to be round and has cleared its orbit of other objects. Our solar system has eight planets.
The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are rocky, while the outer four are giants. Jupiter and Saturn are gas giants made mostly of hydrogen and helium, and Uranus and Neptune are ice giants.
In 2006, the IAU adopted this definition, which led to Pluto being reclassified as a dwarf planet. Pluto is round and orbits the Sun, but it shares its orbital region with many other objects in the Kuiper Belt. Other dwarf planets include Ceres, Eris, Haumea and Makemake.
Planets beyond our solar system
Planets orbiting other stars are called exoplanets, and astronomers have confirmed more than 6,000 of them. The word planet comes from the Greek planetes, meaning wanderer, because ancient observers saw planets move across the sky against the background stars.
Did You Know?
A popular memory trick for the order of the planets is My Very Educated Mother Just Served Us Nachos.