No Fixed Point

This post has had a long history (in Internet time), and has been variously known by the titles: An Alternative Model of Classifying Sub-Stellar Celestial Objects; Pluto is NOT a Planet and; There. Are. 7 planets! At no point was I truly satisfied with the results.

The original article was written in early September of 2006, shortly after the IAU had announced their official definition of what a planet was. Rather than including Eris (whose discovery in 2003 had set the whole procedure in motion; it appeared that it was bigger than Pluto, and so was either the tenth planet, or Pluto was not a planet at all), they decided to drop Pluto from the planet category and shuffle it into the newly-created dwarf-planet category (along with Eris, and Ceres, and no doubt more to come), and gave the world this definition of the word planet:

(1) A “planet”¹is 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, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.
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¹ The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
² An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
³ These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
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I had wanted to try my hand at creating a whole new method of ‘classifying sub-stellar celestial objects’, and while the result was a reasonable first almost-draft, it became clear as I worked on it, that there were two many variables, and to little detail regarding extra-solar objects. And so my enthusiasm waned, and the project died.

Then, in late January, I heard the news that Eris was most likely not larger than Pluto (though it is denser—much denser).

My desire to reclassify the planets was spurred again—briefly. At least until I sat down and gave the whole matter some thought. It was at this point that I remembered a comment that Neil deGrasse Tyson made (somewhere—I wish I could remember where) about the whole issue of what a planet was. Briefly, he said that one day, what are now known as planets will (or should) be classified by properties, instead of using a three- to four- thousand year old word imported from ancient Greece. And it makes sense.

Jupiter and Saturn have significantly more in common with each other than with Earth, or Mars. And Pluto has more in common with the comets of the Kuiper Belt (and other objects of a more stayed orbital pattern) than it does with any object lying closer to Sol.

It was this line of thought that eventually lead me to the perfect definition of the word planet: a natural object that, to the unaided eye on the surface of a world, appears to move across the static background of the stars.

This is essentially the definition that was used by the ancient Greeks, for whom the word ‘planetes’, meant wanderer. Certain constraints have been added to account for the development of the telescope, and other detection methods, that helped to redefine the word in the first place, and to prevent the same from occurring again.

So we come back to the first definition of the word, and the planets of Earth are: Sol, Luna, Mercury, Venus, Mars, Jupiter, and Saturn.

But the word is not restricted to Earth. Travel to Pluto, and its planets would be Charon, Nix, Hydra, and Sol (at the least). And Pluto would be a planet of each of its moons.

It becomes both a constrained word, and a limitless one. Anything can be a planet, just not necessarily of the world you are currently standing on.