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What is a planet?

The International Astronomical Union (IAU) took on the problem of planet definition and, after two years of consultation, developed a definition of the term “planet” based on physics. This new definition was to be adopted by the IAU, the International Astronomical Is noon, in Prague on August 24, 2006.

solar system
ImageOur solar system (c) NASA / JPL

A planet needs two things

According to this definition, a planet should meet two important criteria: 1. It must circle a star (e.g. our sun) and must not be a moon. 2. It must be large enough to be nearly round due to its own gravity - but should not itself be a star. In short, one could simply define planets as “round worlds orbiting a sun”. But it turned out differently ...

The vote on the planet definition
ImageThe vote on the planet definition (c) IAU / Robert Hurt

The knockout criterion for Pluto

On the last day of the conference, when many of the participants had already left, there was a debate and, surprisingly, another, third criterion was introduced that a planet should meet:

3. A planet must have cleared its own orbit from other objects (i.e. no other large celestial bodies may run on similar orbits).


solar system
ImageOur solar system (c) IAU / Martin Kornmesser

It is this third criterion that Pluto fell victim to in 2006 when voting, because there were other, comparable objects in its area in orbit around the sun. Since then, according to the IAU, there have only been eight planets - the four earth-like planets Mercury, Venus, Earth and Mars as well as the four Jupiter-like gas giants Jupiter, Saturn, Uranus and Neptune.