A Dwarf planet (as defined by the International Astronomical Union) is "a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite."

This terms was developed in 2006 during the time when a lot of astronomers weren't accepting Pluto as a planet due to it's small size. Some astronomers argued that just because of Pluto's small size, doesn't mean it's not a planet. But later there was a discovery of a new planet, known as Eris, which was found in 2005 by Mike Brown. This new planet was 27% more massive than Pluto. Which meant that we would know have 10 planets. To date, Eris is the largest Dwarf planet in our Solar System that we know of. The discovery of Eris and the small size of Pluto were the two main factors that drove the International Astronomical Union to classify these smaller planets as something else.

As of now, there are a total of 5 Dwarf planets. Ceres, Pluto, Haumea, Makemake, and Eris.

If the International Astronomical Union had not created the term "Dwarf Planets". Then there would be over 20 different planets in our Solar System. As of now, we only have 8 planets.

Some astronomers believe that there are another 40 planets in our Solar System that are Dwarf planets. The majority of them haven't been discovered yet, though. They also believe that there are about 200 Dwarf planets in the Kuiper Belt.