WRAL WeatherCenter Blog

Definition of a planet, and whether Pluto counts, is still in flux

Posted February 21
Updated February 23

Primary Investigator Alan Stern and others leading the New Horizons Mission, which studied Pluto during a 2015 fly-by, along with researchers from George Mason University in Fairfax, Va., the National Optical Astronomy Observatory in Tucson, Ariz., and the Lowell Observatory in Flagstaff, Ariz., (where Clyde Tombah discovered the once-planet in 1930) aren’t ready to give on planetary status for Pluto. The group will present a paper at the 48th Lunar and Planetary Science Conference in March making the case for a new definition to the word "planet."

The current definition, as adopted by the International Astronomical Union (IAU) in 2005 is based on three criteria:

  1. Is a celestial body in orbit around the Sun
  2. Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
  3. Has cleared the neighborhood around its orbit. (Is massive enough to have sufficient gravity to clear its path around the Sun.)

Stern and the other authors of the paper propose what they describe as a more geophysically focused definition:

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

That is anything round in space that isn’t a star.

While this refreshingly simple definition would give back the planet label to Pluto, this newly broad definition would increase our solar system’s planet count to at least 115. This includes the current eight, Pluto, 18 or 19 moons (including Earth’s), one or two asteroids and 87 mostly unnamed objects beyond Neptune.

This isn’t just Stern's lobbying to get Pluto back into the club. The current definition focuses on size, which these scientists are arguing in favor of a definition which focuses on what really matters in planetary science.

We study planets because of their physical properties, not because of where they are in space or how well they've cleared the path around sun. Planetary scientists study geologic processes both active and historic, storms brewing in the atmospheres, and they search for signs that life might have been survivable there at sometime.

With this in mind a broader definition seems appropriate. I'm not so sure about adding moons though.

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