In 2006, International Astronomical Union (IAU) came up with a set of 3 rules which defined a planet. Those 3 rules were-

1) A planet must be spherical in shape.

2) A planet must orbit the sun.

3) A planet must clear it’s orbit.

Pluto followed the 1st and 2nd rule but due to it’s size and therefore gravity, it missed the 3rd rule.

Hence, that year, Pluto was declared a dwarf planet.

Currently, there are 5 dwarf planets in our solar system. They are-

1) Ceres

2) Makemake

3) Haumea

4) Eris

5) Pluto