The definition of “planet” has been the subject of intense debate in recent years. Although the word itself dates back millennia, there was no official scientific definition of a “planet” until the early 21st century.
Traditionally, the term was only applied to objects in the Solar System, and any differences there could be dealt with on an individual basis. After 1992, however, astronomers began to discover many additional objects beyond the orbit of Neptune, as well as hundreds of objects orbiting other stars. These discoveries not only increased the number of potential planets, but also expanded their variety and peculiarity. Some are nearly large enough to be stars, while others are smaller than our Moon, and they have challenged long perceived notions of what a planet could be.
The issue of a clear definition for “planet” came to a head in 2005 with the discovery of the trans-Neptunian object Eris, a body larger than the smallest accepted planet, Pluto. In response, the International Astronomical Union (IAU), recognised by astronomers as the international body responsible for resolving issues of astronomical nomenclature, released its decision on the matter. This definition, which applies only to the Solar System, states that a planet is a body that orbits the Sun, is large enough for its own gravity to make it round, and has ” cleared its neighbourhood” of smaller objects.
Pluto does not qualify as a planet under this definition, and the Solar System is thus considered to have eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The new category of dwarf planet was created, currently including Pluto, Eris, and Ceres. The IAU’s decision has not resolved all controversies, however, and some in the astronomical community have rejected it outright. The issue of what constitutes a planet will likely remain contentious at least until 2009, when the IAU holds its next Congress in Rio de Janeiro.
