The recent discovery of numerous objects with different properties leads one to question the way in which to define a planet.
The name "planet" has historical origins. Up until 2006, it was arbitrary in its restriction to the nine planets of the solar system. Pluto was among the bodies called "planets" while Eris wasn't among them, even though it is more massive than Pluto.
The International Astronomical Union (IAU) decided in 2006 to change the definition of a planet in order to solve the problems arising with Pluto and Eris, and to use a definition based on physical concepts instead of being purely arbitrary. One now defines a planet as a celestial body that :
A dwarf planet is a celestial body that :
It is important to notice that, according to this definition, a dwarf planet is not a planet ! One can also notice that this definition does not apply to exoplanets, because they do not orbit around the Sun. In other words, according to this definition, an exoplanet (or extrasolar planet) is not a planet despite the fact that an exoplanet is often described as being a planet orbiting a different star than the Sun.
The IAU Working Group on Extrasolar Planets has not yet decided on a definitive definition of the word exoplanet. Its "working definition" (which dates from 2003) is the following : an exoplanet
Unfortunately, the definition of a planet in the Solar System does not specify a limit in mass or size, so this definition cannot be used precisely. One could hope for a more universal definition of the word "planet", that could be applicable to bodies detected around other stars than our Sun. In fact, with current detection methods, it is in general impossible to find out if an exoplanet is spherical or if it has cleared the neighbourhood around its orbit.
The distinction between planets and stars has also been found to be rather vague since the discovery of the brown dwarfs.