Why are there 8 planets instead of 9?

Answer 1

It was agreed upon by the International Astronomical Union in 2006 that Pluto no longer fits the criteria of what constitutes a planet. The reason for this is because of all the other objects that have been discovered in the Kuiper belt and the asteroid belt that are even bigger than Pluto and those are not classified as planets.

The 8 major known planets in order from the Sun are:

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

If you list ALL 13 known planets in order, including dwarf planets, they are:

1. Mercury
2. Venus
3. Earth
4. Mars
5. Ceres
6. Jupiter
7. Saturn
8. Uranus
9. Neptune
10. Pluto
11. Haumea
12. Makemake
13. Eris

Dwarf planets also fall under the category of minor planets, of which there are thousands in our solar system. As of 2017, the orbits of 734,274 minor planets were archived at the Minor Planet Center, 496,815 of which had received permanent numbers. The largest minor planet that is not considered to be a dwarf planet is Sedna.

Answer 2

The International Astronomical Union redefined the criteria for what constitutes a planet in 2006, leading to Pluto's reclassification as a dwarf planet. This reclassification was due to Pluto's size, location in the Kuiper Belt, and not having cleared its orbit of debris.