Definition

What Is a Planet?

Most of us grew up with the conventional definition of a planet as a body that orbits a star, shines by reflecting the star's light and is larger than an asteroid. Although the definition may not have been very precise, it clearly categorized the bodies we knew at the time. In the 1990s, however, a remarkable series of discoveries made it untenable. Beyond the orbit of Neptune, astronomers found hundreds of icy worlds, some quite large, occupying a doughnut-shaped region called the Kuiper belt. Around scores of other stars, they found other planets, many of whose orbits look nothing like those in our solar system. They discovered brown dwarfs, which blur the distinction between planet and star. And they found planetlike objects drifting alone in the darkness of interstellar space.

These findings ignited a debate about what a planet really is and led to the decision last August by the International Astronomical Union (IAU), astronomers' main professional society, to define a planet as an object that orbits a star, is large enough to have settled into a round shape and, crucially, "has cleared the neighborhood around its orbit." Controversially, the new definition removes Pluto from the list of planets. Some astronomers said they would refuse to use it and organized a protest petition.

source: http://www.scientificamerican.com/article.cfm?id=what-is-a-planet

 definition provided: defines a planet

Glossary:

asteroid: An asteroid is a rocky body, carbonaceous or metal smaller than a planet larger thana meteoroid, that orbits the Sun in an orbit inside that of Neptune.

Kuiper belt: Region of the solar system with a few thousand rocky objects that are locatedbetween 38 and 200 AU (astronomical units). It is believed that also includes kites,many of which comets are visible at this time. You might think that Pluto may be thelargest Kuiper Belt object.

brown dwarfs: are also called failed stars. They lack enough energy to be true stars but are also too massive and hot to be planets.

blur: A smear, smudge or blot; Something that appears hazy or indistinct.

interstellar space: the space between stars

planetlike: Resembling a planet or some aspect of one