There was worldwide outcry in 2006 when the International Astronomical Union redefined the term planet and in doing so shifted the small icy world known as Pluto from that prestigious group. From then on, Pluto has been classified as a dwarf planet, along with other objects in its solar neighbourhood.
Why was Pluto reclassified? It was, simply put, too small.
Pluto is part of an area in the Solar System known as the Kuiper Belt, a region beyond the orbit of Neptune, but before the beginnings of the Oort Cloud (to be explained later), that is crowded with billions of icy bodies. These bodies range from small to, well, Pluto-sized.
One of the most notable of these bodies is Eris.
Eris was discovered in 2005 and nicknamed Xena. At first it appeared to be bigger than Pluto, which was the catalyst for the debate about the classification of a planet. On closer examination it was found that whilst Eris wasn’t bigger than Pluto, they were almost identical to each other in terms of size. That was the final nail in the coffin of Pluto’s planethood. Eris was classified as a dwarf planet, which clearly meant that Pluto must be classified the same, thus reducing the number of planets in the Solar System from nine down to eight.
But let’s get back to Pluto. Its core is rocky, but its surface is covered with icy formations; these include plain ice water, and also some more obscure frozen substances such as methane, nitrogen, and carbon monoxide.
To give a representation of its (and Eris’) size Pluto’s diameter is 2230km, which is slightly less than the distance between Adelaide and Perth. It’s also one-third of our Moon’s volume and one-sixth of its mass.
But size isn’t everything and when the New Horizons craft flew by Pluto in 2015, it returned some incredible images and data.
Pluto had long been thought of as a wasteland, far too distant from the Sun for anything of true interest to be occurring there. But New Horizons has abolished that notion. It discovered that Pluto was geologically active. Despite its distance from the Sun, the internal structures of the dwarf planet held enough heat to induce constant changes on the surface, most notably the Sputnik Planum, a nitrogen ice-plain that showed signs of recent change.
Further to this, Pluto breathes. Due to the dwarf planets highly elliptical orbit, for a small time during its 248-year orbit around the Sun, Pluto is close enough that its atmosphere heats up and expands, and then, as it flies away back to its far distant orbit, the atmosphere cools, and contracts, until it comes around again.
Pluto is a fascinating object and excitingly is just one of many such objects in the incredibly large and populated Kuiper Belt.
References:
http://pluto.jhuapl.edu/Pluto/The-Pluto-System.php
https://solarsystem.nasa.gov/missions/new-horizons/in-depth/
Physics and Fiction 
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