It’s official…

Xena**UPDATED**: A previous version of this post contained inaccurate information (thank you, BBC and NPR). What follows is straight from the horse’s mouth - the IAU.)

We have 12 planets now.

As I wrote a couple of months ago, the International Astronomical Union met this summer to finally define what a “planet” is. At issue is the quandary that many astronomers find themselves in today. Objects bigger than Pluto have been found, but to date, they’ve been assigned identifiers that classify them with asteroids, comets, and the like.

This created a problem for astronomers: If there are objects bigger than Pluto that are not considered planets, then why is Pluto still considered one?

The IAU has proposed a resolution that will be refined and voted on during the meeting of 24 August. Here are a few relevant snips from that resolution:

The IAU therefore resolves that planets and other Solar System bodies be defined in the following way:

  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”.

What this means is that we now have the following 12 planets:

  1. Classical planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune
  2. Plutons - Pluto, Charon, and 2003 UB313 (proposed name: Xena)
  3. Dwarf planets - Ceres

Our new solar system, courtesy the IAU. Click to embiggenate.

I used to be split on this one. I never felt that it was appropriate to classify an object incorrectly just because it was sentimental to do so. Otherwise, what is the purpose of classifying objects at all?

But I am pleasantly surprised that, instead of wasting time arguing over whether or not certain objects should be ‘promoted’ or ‘demoted,’ the IAU addressed the root of the problem by coming up with a proper definition for a planet, and creating new sub-classes that define its members. It’s an opportunity to be more precise, and specific.

And that’s cool.

Image credit & copyright: “Artist’s Representation of Xena,” BBC News. Original caption: “At 3,000km (1,860 miles), the object is significantly bigger than Pluto.”

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