IAU Definition of Planet - The History of The Definition

The History of The Definition

Because a new planet is discovered infrequently, the IAU did not have any machinery for their definition and naming. After the discovery of Sedna, it set up a 19-member committee in 2005, with the British astronomer Iwan Williams in the chair, to consider the definition of a planet. It proposed three definitions that could be adopted:

Cultural

a planet is a planet if enough people say it is;

Structural

a planet is large enough to form a sphere;

Dynamical

the object is large enough to cause all other objects to eventually leave its orbit.

Another committee, chaired by a historian of astronomy, Owen Gingerich, a historian and astronomer emeritus at Harvard University who led the committee which generated the original definition, and consisting of five planetary scientists and the science writer Dava Sobel, was set up to make a firm proposal.