There has not been a definition of a planet for thousands of years. The ancient Greeks talked about the planets as being the following: Saturn, Jupiter, Mars, Venus, Mercury, the moon and the sun were the 7 planets that stood for a long time. Since then we have dumped the moon and the sun as planets and added earth and others.
There was never a firm meaning on what a planet was, that is until 2006. In August of that year the International Astronomical Union declared the following, "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."
On the surface this sounds very simple but if you take a closer look you'll soon discover that the definition opens the door to a number of objects being called planets when they were originally something totally different. There are now dwarf planets and what are called the classical planets. This definition, according to some astronomers, opens the door for potentially hundreds of planets being discovered in our solar system once further looks at the The Kuiper belt are had.
The Kuiper belt is a band of celestial objects which is similar to the asteroid belt. There are a couple of dwarf planets already in the Keiper belt. The first is the most well known because we all learned it was the 9th planet; Pluto. The other dwarf planets in this region are; Eris, Makemake, and of course Haumea.
Also now making the list of planets albeit a dwarf planet is Ceres, found in 1801 it was originally considered a planet but its designation was changed. It was soon decided that Ceres was nothing more than an asteroid until after 2006 and the International Astronomical Unions' definition of planet.
There are several other asteroids which, like Ceres, might later become planets after further study. Pallas, Juno and Vesta are current asteroids that have an orbit but at this point they don't seem round enough to be included within the dwarf planet category. But as I said, further study could well change that.
Even though this definition was passed 2 years ago by the International Astronomical Union (IAU) there are still those that see the definition as messy and not definitive enough. Some of these scientists don't like Ceres being called a dwarf planet. The same goes for the dwarf planets in the Kuiper belt.
To confuse the matter even further the IAU has determined that there are a class of dwarf planets that can be referred to as Plutoids. IAU definition of a plutoid: "Plutoids are celestial bodies in orbit around the Sun at a semi-major axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighborhood around their orbit. Satellites of plutoids are not
So plutoids would only exist out in the Kuiper belt beyond the non-plutodian Neptune. It becomes a question of time before more of the Kuiper belt gets explored by our ever powerful observatories and their telescopes or perhaps the Hubble telescope, being situated above the atmosphere of the earth, would have an easier time seeing into the Kuiper belt. Regardless, the next few years could prove to be the most prolific for finding planets in human history.