Planets and Plutons

I guessed incorrectly about which way the IAU would go in adopting a new definition for “planet”. I thought they would take a more restrictive view that would dump Pluto from the ranks of the planets. Instead they have adopted for a more inclusive definition that is perfectly reasonable from a physical and scientific standpoint (basically: a big round thing that orbits the Sun). In detail:

1) 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.

(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a “dwarf planet.”

(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call “plutons”.

(4) All non-planet objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

I like (1) and (4). I don’t like (2) and (3). Point (2) qualifies the definition based on historical considerations, while point (3) introduces both vague (“highly inclined” and “large eccentricities”) and arbitrary boundaries (200 years). These points seem particularly parochial in a time when most things we think of as planets are actually orbiting stars other than the Sun. While a 200 year orbit neatly separates tiny Pluto from giant Neptune in our system, there are certainly going to be planetary systems where that boundary might fall between two very similar and large planets, like Uranus and Neptune. Where does this leave “Kuiper Belt Objects” with respect to “Plutons”? Are only the large KBOs to be considered Plutons? Why not just call them planets without introducing another taxonomic classification? What is the distinguishing characteristic of a “dwarf planet”? Why isn’t Mercury a dwarf planet?

I guess that the IAU deliberately chose to be vague with the wording, “one may choose to distinguish Ceres … by referring to it as a ‘dwarf planet'”. In that case, I’ll choose not to make that distinction and continue referring to it as an “asteroid”. I’m fine with it being an asteroid that is also a planet. Now it’s an asteroid, a planet, and a dwarf planet, while Pluto is a planet, a pluton and a Kuiper Belt Object.

I cannot see myself using the term “Pluton” in my introductory astronomy classes (or anywhere else for that matter), and I see no pedagogical or scientific benefit in referring to Ceres as a dwarf planet rather than an asteroid.

For more details on the IAU decision, and why Pluto’s largest moon is also now a planet, see the Bad Astronomy blog.

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