Dwarf+Planets

Ceres, Pluto, Eris.... and others?
For many people the defining characteristic of a planet is that it is spherical (ball shaped) and orbits the Sun. However, as we have become better able to look in detail at the smaller objects in our solar system we have discovered more spherical objects, and we expect to find many many more. By some estimates there may be as many as 200 ball shaped objects in the Kuiper belt (pronounced like "Viper"), and as many as 2000 further out. What to do with all these "planets" - leaning the order of eight planets is quite enough, no one wants to learn thousands!

What is a dwarf planet?

The International Astronomical Union (IAU) came up with the definition of a dwarf planet in 2006. A dwarf planet is something that is:
 * 1) big enough that its own gravity is enough to make it roughly spherical.
 * 2) small enough that it lacks the gravitation to sweep up or scatter objects near its orbit.

Where are the dwarf planets?

Others (maybe) - Haumea, Makemake, Quaoar, Orcus, Ixion, Sedna...
 * Ceres** - was defined as an asteroid until 2006. It is the largest object in the asteroid belt (between Mars and Jupiter), and we now know that it is large enough to be roughly spherical.
 * Pluto** - was defined as a planet until 2006 (although many people disputed its classification as a planet in 1930, when it was discovered). As we have been able to understand more about the distant solar system, it has become clear that Pluto is just one of many objects (mostly) outside the orbit of Neptune, called the Kuiper belt (or Edgeworth-Kuiper belt). Pluto has three moons called Charon, Hydra and Nix, with a four, as yet unnamed moon discovered in 2011. Charon is about half the size of Pluto, and orbits close by, so they are sometimes thought of as a double planet.
 * Eris** - Orbits nearly twice as far out from the Sun then Pluto, and is larger than Pluto. Has one known moon - Dysnomia.

It is worth remembering that these dwarf planets are small compared to the size of the major planets, with the largest being Eris which is 1/6 the size of the Earth and half the size of Mercury (the smallest of the major planets).