Dwarf planets are solar bodies that are too small to be considered actual planets, but are too large to be considered something else. The best known dwarf planet is Pluto, which was originally considered to be a planet, but later, when its size was compared to other dwarf planets, was deemed a dwarf planets, leaving 8 actual planets in orbit around our sun. Pluto is about half the size of Mercury and is mostly rock and ice, and is believed to be covered by a layer of frozen nitrogen and have a thin atmosphere of methane. Being so far from the sun, Pluto has a temperature of -235°C (-391°F). It also has a moon, which is a little more than half the size of Pluto.
There are, of course, other dwarf planets, even in our solar system, like Ceres. There are even more outside of our solar system that we haven't discovered yet. To be a dwarf planet instead of an actual planet, a dwarf planet must be too small to clear other objects in its path. Pluto falls under this classification. Eris, another body recently discovered, was found and declared a dwarf plant. Since Eris is bigger than Pluto, scientists decided on what a dwarf planet and planet technically were, and where the line was drawn between the two. Thanks to this, we now know what planets are dwarf planets and which ones are actually considered to be planets.