moon? that's no moon »
This month, the International Astronomical Union (IAU) is going to have its 2009 general assembly. One topic that may come up for discussion is Pluto's right to be called a planet. Which is really the topic of how to define a planet. And as we shall see below, although Pluto has been the driver for this redefinition, no matter which way it goes this year, if it is discussed at all, Pluto will never again have the same reverence it once had.
In 2006, the IAU defined a planet to be 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 cleared the neighbourhood around its orbit.
All three of these rules are contentious to some degree or other. Not the least of which Rule Number 1 implies that this definition only applies to our Solar System. Are the increasing number of exosolar "planets" being found not really planets after all?
Rule Number 3, however, has drawn the most ire, as it is that rule which has seen Pluto demoted from planet status to the newly defined (in 2006) dwarf planet status. In fact, it has been asserted that Rule Number 3 was brought in to do exactly that: dump Pluto. Some people weren't happy about it, but then some people don't like change. But then again, there would still have had to be change were Pluto kept as a planet.
If Rule Number 3 was to be retracted, and Pluto was to be re-reclassified in the big league, then there are at least another four bodies in the same boat. Taking the planetary total up to 13 (and requiring another reprint of astronomy books). The planets would then be Mercury, Venus, Earth, Ceres, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake and Eris. Ceres? Haumea? Makemake? Eris? Who? What? Where? When?
02:08:2009 18:36 NZST
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