Definition of planet

The definition of the term planet has changed several times since the word was coined by the ancient Greeks. Greek astronomers employed the term ἀστέρες πλανῆται (asteres planetai), 'wandering stars', for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different celestial bodies, from the Sun and the Moon to satellites and asteroids.

In modern astronomy, there are two primary conceptions of a planet. A planet can be an astronomical object that dynamically dominates its region (that is, whether it controls the fate of other smaller bodies in its vicinity) or it is defined to be in hydrostatic equilibrium (it has become gravitationally rounded and compacted). These may be characterized as the dynamical dominance definition and the geophysical definition.

The issue of a clear definition for planet came to a head in January 2005 with the discovery of the trans-Neptunian object Eris, a body more massive than the smallest then-accepted planet, Pluto. In its August 2006 response, the International Astronomical Union (IAU), which is recognised by astronomers as the international governing body responsible for resolving issues of nomenclature, released its decision on the matter for the IAU definition of planet during a meeting in Prague. This definition, which applies only to the Solar System (though exoplanets had been addressed in 2003), states that a planet is a body that orbits the Sun, is massive enough for its own gravity to make it round, and has "cleared its neighbourhood" of smaller objects approaching its orbit. Pluto fulfills the first two of these criteria, but not the third and therefore does not qualify as a planet under this formalized definition.