Definition of planet

At the turn of the 21st century, the definition of "planet" became the subject of intense debate. Although the word itself dates back millennia, it had never had a formal definition until, beginning in 2005, a confluence of circumstances forced the astronomical community to take action.

From the end of the 19th century, "planet" had, without being defined, settled into a comfortable working term. It only applied to objects in the Solar System; a number small enough that any differences 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 were nearly large enough to be stars, while others were smaller than Earth's moon. These discoveries 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 then-accepted planet, Pluto. In its 2006 response, the International Astronomical Union (IAU), recognised by astronomers as the world body responsible for resolving issues of 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.