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Pluto Falls from the List of Planets

August 24, 2006

Until this week, it was unclear whether we would have more or less than 9 planets within our solar system. A committee of the International Astronomical Union (IAU) had recommended on August 16 to clarify the definition of the word “planet” in a way that increased the number of planets in our solar system to at least 12. The committee suggested this definition: "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."

This definition promoted two trans-Neptunian objects, Charon and 2003 UB313, and one asteroid, Ceres, to the status of planet. A number of other trans-Neptunian objects were expected to be classified as planets as their characteristics became more firmly established. Whether an object is a planet under this definition depends on the precise physics of the body. According to the IAU, a body with a mass greater than 5 × 1020 kg and a diameter greater than 800 km has sufficient gravity to cause itself to be round, although the precise lower limit depends on the elements composing the body.

The General Assembly of the IAU, however, approved today an amended definition for planet; added to the earlier two criteria is a third criterion that bars all known asteroids and trans-Neptunian objects from planet status. This criterion states that the object must have “cleared the neighborhood around its orbit” to be a planet. This criterion eliminates bodies that are members of asteroid belts such as Ceres and the large trans-Neptunian objects in the Kuiper Belt such as Charon, 2003 UB313, and Pluto. So instead of dramatically increasing the number of planets in the Solar System, the approved definition of planet pares the number back to 8 by eliminating Pluto.

With this definition of planet comes a whole set of new terms. The term “classical planet” refers to the 8 planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The objects that satisfy the first-two criteria for a planet, but fail to satisfy the orbit-clearing criterion, are now “dwarf planets.” Everything else is called a “small solar system body.”

While Pluto is now a dwarf planet, it is also the prototype of a new class: the plutonian object. This term covers other large Kuiper Belt objects, such as Charon and 2003 U313. The number of such objects should grow dramatically over the coming years as astronomers continue their systematic search for objects within the Kuiper Belt.

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