It had been discovered many times by astronomers, who did not realize what they found. It was discovered 'for real' in 1930 by Clyde Tombaugh, and was originally considered the ninth planet from the Sun. After 1992, its status as a planet fell into question following the discovery of several objects of similar size, in particular Eris, which is 27% more massive than Pluto. This led the International Astronomical Union to define the term planet formally for the first time. This definition excluded Pluto and reclassified it as a member of the new "dwarf planet" category. The other dwarf planets are Ceres, Eris, Haumea, and Makemake (sic).
The tradition of naming planets after mythological gods was passed continued after Roman names for the five extraterrestrial planets they were aware of.
- Earth is the only planet not named for a mythological god.
- Venus is named after the goddess of love. It is thought
this planet got its name from the fact that it is “pretty”
to look at as the third most bright object in our solar
system in the sky as viewed from Earth (after the Sun and
- Mercury is named after the god of thievery, tradesmen or commerce, and travel. It is thought that the planet probably was named such due to how quickly, relatively speaking, it travels across the sky.
- Pluto, although no longer a "real" planet is named after the god of the underworld. The name was proposed by Venetia Burney, a then eleven-year-old schoolgirl in Oxford, England, who was interested in classical mythology.
- Saturn is named after the Roman god of agriculture. It
followed the Greek designation for Cronus. In modern Greek,
the planet retains its ancient name Cronus—Κρόνος: Kronos.
- Neptune was named after the god of the sea. It got its
name thanks to the fact that it has a blue color.
- Uranus is named after the very early god of the sky (and father to the Titans).
- Mars was named after the Roman god of war. It’s thought that it was labeled such based on the reddish hue of the planet, relating to blood.
- Jupiter is named after the god of thunder and the sky, and king of the gods. It is probable that it was named such as it is the largest non-star in our solar system.