25 August 2006
The International Astronomical Union (IAU) voted Thursday on a definition of the word planet. The proposed definition we reported on last week was rejected and the IAU defined a planet as a celestial body that
is in orbit around the Sun,
has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
has cleared the neighborhood around its orbit.
According to the IAU, this leaves our solar system with eight planets, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. By this definition, Pluto is not a planet because it has not cleared its neighborhood.
The IAU also rejected use of the proposed term pluton, for the class of objects similar to Pluto. That term is also commonly used by geologists for an igneous mass that forms when molten rock cools underground and it was thought that there could be confusion between the geological and astronomical senses–although that doesn’t seem very likely. Context would rule out any chance of confusion in most cases. After all, there isn’t any confusion of the geological and anatomical uses of vein.
More problematic for pluton is that in French and Italian this is the name for the former-planet Pluto. This could cause much confusion between the class and the specific body in those languages.
Instead of pluton, the IAU decided on another linguistically problematic term, dwarf planet, which is defined as a celestial body that:
is in orbit around the Sun,
has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
has not cleared the neighborhood around its orbit, and
is not a satellite.
Benjamin Zimmer over at Language Log has a good discussion as to why this is a questionable form in English. In English compound nouns, the more general term is usually the second noun. Catfish are fish, not felines and mountain lions are cats, not masses of rock. Although there are exceptions, like sea lion. Although a dwarf star, arguably the most similar term to dwarf planet, is most definitely a star.
But perhaps the most cogent commentary on the subject is by Ruben Bolling who penned this cartoon. The third example is the best.
