There has been a lot of discussion in recent years, and in particular the past week, about the definition of “planet”. This is the result of, among other things, the discovery of more objects similar to Pluto in the Solar system. Astronomers have been debating furiously about whether to call these new objects planets, like we did Pluto.

The International Astronomical Union has proposed the following “official” definition for “planet”:

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.

I do have to say it would be nice to have a definition that everyone uses… but I don't like this definition so much. First of all, I didn't think Pluto should be a planet. It doesn't resemble any of the other current planets in any way. This definition not only keeps Pluto a planet, but makes its largest moon, Charon, a planet as well. It also adds Ceres (an asteroid) and 2003 UB313 (a Kuiper Belt object slightly larger than Pluto).

Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, 2003 UB313

This part isn't so annoying, really. We could change the textbooks to have these 12 planets and be happy. But we are going to discover more objects that fit this definition. As we do so, we'll have to change the textbooks again and again. Ugh.

Personally, I thought we should drop Pluto from the list of planets and add Earth's moon (which I'd prefer to call Luna). Pluto probably wouldn't have been called a planet except we first thought it was the size of Earth, in 1930. Then I'd make a definition for “planet” something like: “Any object larger than or equal in mass to Luna which orbits a star and is not a star.”

I dunno… people were so worried about Pluto being dropped from textbooks, but now we just have to add 3 more to textbooks, and keep adding over time.

We should recognize our mistakes and correct them instead of making more mistakes.

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