Why are Pluto and charon smaller than the other planets?

Answer 1

Pluto, and it's moon Charon, are bodies known as Dwarf Planets. These objects are small bodies of mostly ice and rock that are believed to be leftovers, so to speak, from the original formation of the planets out of a proto-planetary disk that orbited our sun. The aforementioned bodies are in a category of objects called Trans-Neptunian objects, obviously meaning objects that are found outside of the orbit of Neptune, the farthest of the Major planets. They are further classified as Kuiper Belt Objects -KBOs-. The Kuiper Belt is a ring of icy bodies also circling far outside the furthest orbit of Neptune.

When it was first discovered, Pluto's size was miscalculated to be similar to that of Mars, thus it was easy to mistakenly classify it as a true planet. It's size was later re-calculated, but it's status remained. When a series of objects in this Kuiper belt were recently discovered and found to be similar in size to, and larger in some cases, Pluto itself, a debate arose as to whether or not Pluto was indeed a true planet. At a meeting of the International Astronomical Union in mid-2006, a new definiton of the word Planet was put forth, and as a consequence, Pluto was demoted to Minor Planet status.

Though their status has been demoted, there is still great interest in studying and imaging Pluto and Charon up close, as it is believed that these and other Kuiper Belt Objects are pristine examples of material present at the earliest stages of the formation of the planets, a virtual scrapbook of our early solar system, if you will.