What Is a Planet? Planetary Scientists Respond to the International Astronomical Union’s Definition

Artist’s concept of possible planets. Credit: NASA-JPL/Caltech.

Our perception of the solar system was thrown into chaos in 2005. No, it wasn’t the Borg; it was the discovery of the first planet outside the orbit of Pluto. Eris, similar in size to Pluto, raised a disturbing question: How many more planets might there be beyond Pluto’s orbit? This question, and possible ramifications thereof, prompted the astronomers in the International Astronomical Union (IAU) to rush out a set of definitions less than a year later, which resulted in the demotion of Pluto and subsequent discoveries of similar bodies (nine in total) to a category of so-called dwarf-planets. This classification scheme was not without substantial tribulations and controversies.

Sixteen years later, scientists who work on planets have crafted a rigorous response. In an invited paper to the journal Icarus, Philip Metzger and colleagues make the case that the IAU definition is arbitrary and lacks utility. They argue that the definition of a planet should be based on its geologic or atmospheric complexity. A critical consideration then becomes: Is the body large enough to generate a myriad of geologic landforms and/or some form of surface and atmospheric processes?

The definition of a planet becomes one of processes. For example, being in hydrostatic equilibrium (having an essentially round shape) would be insufficient to qualify as a planet because this includes some asteroids. Instead, one argument is that the body must have some form of differentiated mantle and core, which requires substantial internal heating sources. This eliminates asteroids but allows for large satellites to be classified as planets.

The classification of a planet is by nature complex, and no one simple set of rules will be able to satisfy all requirements or needs. However, a conversation has been started to create a framework that allows for utility and complexity. Astronomers and planetary scientists must continue this dialogue to allow for ideas of what a planet is and is not to circulate. READ MORE