In addition to the eight planets in our Solar System, there are a number of Dwarf Planets that do not meet all the criteria required to be a full-blown planet, in the sense that they are not large enough to have cleared their orbits of other similar objects. The official line is that ...

A planet is defined as a celestial body that (a) is in orbit around the Sun, (b) has
sufficient mass that its own gravity pulls it into a nearly round shape, and (c) has cleared
the neighbourhood around its orbit of smaller asteroids.

Currently there are 5 official dwarf planets, Ceres (in the asteroid belt), Pluto, Haumea, Makemake and Eris. In truth there are many similar objects orbiting our Sun well beyond the orbit of Pluto and no doubt these will be considered for addition to the list of dwarf planets in the future. More commonly known as Kuiper Belt objects, they exist in what is believed to be a vast shell of icy and rocky objects that live at the very edge of our solar system.

Background:

The term "Dwarf Planet" came about after disagreements amongst astronomers as to whether Pluto should still be considered a planet. The discovery of other objects, such as Quaoar, Sedna and Eris, led to claims that they should also be known as planets because they were similar in size to Pluto. Finally in 2006, it was decided to tighten up the definition of what a planet is, and this led to a downgrading in the status of Pluto into the dwarf planet category.

That said, these are only names and categories that we choose to use. They don't make the objects any less interesting to observe or to explore.