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When is a Cow a Cow?

The very question "When is a Cow a Cow?" is preposterous, yet this is exactly the situation astronomers are in today if you simply change the word cow to planet. We don't demand that an animal must live on a dairy farm before we call it a cow. However we not only require that a world be in its stellar system but that it must circle its sun before we deem to call the world a planet. Should Bossie take a notion to circle Ferdinand rather than the barn, or should Bossie decide to kick up her heels and skedaddle, we would unhesitatingly still call Bossie a cow. Not so a planet. A world that circles another planet is merely a moon, and a world that takes off for parts unknown has no name at all!

A few decades ago, the whole point was moot. We could unhesitatingly specify a planet, it was one of nine specific worlds that circled our Sun. Worlds that circled some of these planets were given the lesser rank of moons. Everything else we called asteroids or comets depending on their appearance. We had a neat and tidy set of labels for everything. The only problem was that our picture was naive with labels that told us nothing significant about the worlds we saw.

As we learned more about the Solar System we ran into a number of embarrassments. The lesser category of moons included huge worlds. Ganymede, the largest of these moons has a diameter greater than two of the planets. Pluto once thought to be a gas giant rivaling Neptune or Uranus proved to only a sixth the size of our Moon. Worse yet, Pluto far from being the "outpost of the Solar System" turned out to be one of a huge swarm of similar objects that circle beyond Neptune [Kuiper Belt Objects or KBOs]. Several KBOs rival Pluto in size (Charon, Ixion and Varuna), and we may yet find other KBOs larger than Pluto out there.

We know that nearby star systems have circling worlds. We have chosen to call them planets. Our new infrared telescopes can peer through gas clouds and dust, revealing reveal free roaming worlds in the Orion Nebula. Why do we hesitate to call them planets when we certainly would if they circled stars?

The definition of a planet should depend on the world itself. Extraneous criteria like requiring planets circle a star, or that they be formed as the star coalesces, or that they be made of specific materials seem a weak basis for a definition. Some attribute of a planet should be the deciding factor and it should apply across the board. The attribute should allow us to distinguish the planets from the smaller worlds (planetesimals) as well as from larger bodies (stars). The transition category between planets and stars called brown dwarfs can be placed in either group or define a new group as we learn more about them.

The most self sufficient criteria to distinguish planets is mass. Worlds with a mass about 4100 times that of Earth are so large they kindle heavy hydrogen nuclear reactions so they become brown dwarfs. [Worlds with a mass of about 25500 times that of Earth are big enough to form normal stars.] The lower boundary on mass is somewhat subjective. One proposal is that any world massive enough to shape itself into a ball by gravity alone should be called a planet. This mass is about 0.001 times the Earth or about 6 billion billion tons. Huge as this sounds, it is really quite small for a world. A great many small worlds would suddenly be granted planet status in the Solar system, including lots of moons, several asteroids and several KBOs. However any other mass choice is arbitrary. Currently this "mass" is artificially set at 15 billion billion tons. Why? Well that number is just small enough to retain Pluto as a planet while excluding the asteroids and the other known KBOs. As I said at the outset, we need a definition of a planet and we could use its soon.

Leslie Coleman
Author:
Leslie Coleman
Entry Date:
Aug 1, 2002
Published Under:
Leslie Coleman's Columns
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