Permanent Human Settlement of the Earth, Space and Ocean Frontiers

Friday, August 25, 2006




Planetary Matters

Ralph Buttigieg
Sydney NSW
Australia

The eggheads of the IAU have come up with a definition of a planet that's completely scrambled. Lets have a look at it in more detail. The resolution said:

The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A planet1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects3 orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

So “dwarf planets” are not planets. But if thats the case why call them “planets”? It makes no sense. Astronomers call some small stars dwarfs but a red dwarf is as much a star as a red giant.

The problem with Pluto seems to be it has company. It has not “cleared the neighbourhood around its orbit.” This makes me wonder about the personal issues these people have, but lets leave that aside for the moment. How clear do orbits have to be? Some asteroids come close to Earth after all and Pluto's orbit crosses Neptune's.

Then there is Sedna. A few years ago astronomers found a little world way past the Kuiper Belt called Sedna. Its about 80-1100 miles in diameter and appears to be round. It might be part of the the inner Oort Cloud but we don't know that. Nothing else has been found that far yet. So by the IAU's current definition Sedna should be a planet.

There a likely to be more round objects past Pluto and Xena. What happens if the Oort Cloud is confirmed and they find objects as big and round as Mercury or Mars? They can hardly call them dwarf planets.

The whole mess arises because astronomers refused to accept that the definition of a planet is as much cultural and historic as scientific, yet they were themselves limited by those same cultural requirements. In my view the original proposal made sense, it was to accept all round objects orbiting the Sun as planets but that would have given us potentially dozens of planets. They rejected that proposal but why? We don't worry about the number of stars or islands so why should planets be any different? They felt uncomfortable because everyone is used to having a relatively small number of planets so they rebelled against the original proposal.

A simple solution would be to call the nine traditional planets some thing like the Classical Planets or Original Nine and accept the rest as new planets no matter their size or the company they keep.