Why is pluto not known as a planet?
Pluto was reclassified as a dwarf planet.
The word planet means wondering star and was used to describe anything which noticeably moved. Galileo discovered four of the moons of Jupiter. This effectively meant that a planet had to be a body orbiting the Sun.
Ceres was discovered before Pluto. Then Ceres was discovered to be one of many bodies orbiting between Mars and Jupiter. They were named asteroids and Ceres was labelled as an asteroid.
When Pluto was discovered it was given planet status.
As telescopes got better and be had space telescopes, more bodies were discovered. This caused a classification problem.
The International Astronomical Union redefined a planet as having to pass three criteria.
- It must orbit the Sun
- It must be massive enough for gravity to make it spheroidal in shape
- It must have cleared its orbit of other bodies except for moons
A new category of Dwarf Planet was defined to be any body which meets the first two criteria but not the third. This caused Pluto to be demoted to Dwarf Planet. Ceres was promoted to Dwarf Planet.
This decision was controversial. Many, including myself, still regard Pluto as a planet.