Sunday, May 6, 2012

The 6 Planets

In 2006 a group of well meaning but lazy scientists got together to try to prevent more and more celestial objects from being declared planets.  Their goal seems to have been two fold;  First, to exclude everything beyond the original 8 planets from the list.  Second, do so in a way that sounded scientific.

So we should have a definition of what a planet is.  And with this definition we should be able to look at any object in our solar system and be able to conclusively state whether it is a planet by measuring a few criterion.

Give Me A Definition

Here is the definition that they presented:

A 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) shape, and
(c) has cleared the neighborhood around its orbit.
Footnote:   The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
So What's The Problem?

The inclusion of the footnote, for me, is the most telling part.  If we are talking about something else, like the scientific definition of water.  And then included a footnote listing ice, steam, and liquid water, I would wonder what they were talking about.  Should not the definition be enough to find all mater in the form of water and identify it without the cheat sheet?

As a scientist myself, I wonder why we need that footnote.  I hypothesize that there can be only two possible reasons.  One, one or more planet on the list doesn't actually qualify, and they don't want to have defined away any of these 8 as a planet.  Two, there are other objects that would qualify as planets by this definition, and we are trying to keep the planet club exclusive.

I don't have data, on all of the objects in the solar system, so testing my second hypothesis would be difficult.  But I can test the first one.  I have data on the 9 planets to examine.

According to this definition, (ignoring the footnote), Venus, Earth, Mars, Jupiter, Saturn, and Uranus qualify as planets.  That is assuming you don't worry about comets and Asteroids and other small objects as things not cleared from the "neighborhood".

Pluto and Neptune have a mutual problem.  They cross each other's path.  So if we are really being honest, Neptune can't be a planet.  Especially if Pluto is disqualified.

And now lets look at one more.  Mercury.  Mercury has an entirely different problem.  Its neighborhood and orbital path is clear.  And it is in orbit of the sun.  But there is one other qualifier that if fails on.  Mercury is not round.

This definition really is troublesome.  It tries to use a form of a scientific definition.  And to appear straightforward.  However, one can't look at an object in the sky and decide if it is a planet.  We have to scan the entire solar system to see if there is another object out there that might disqualify it.  Just plain silly.

How About This Idea?

Just like defining other things, the process of creating a definition is to list what things have in common.  Like defining a tree would start with taking a couple known trees and listing attributes that they all share.  (being on the same footnote listing tree names should not be one of these.)

The next step is to take other things and compare them and see how our definition holds up and if it needs changes.  Whatever changes to the definition we make, must still work to define our known trees as trees.  (And our known planets as planets.)

Looking at Earth, Mars and Venus, what do they have in common.  (Round, Orbit the Sun, have an Atmosphere.)  And with these we can make a definition like this.


A planet is a celestial body that:
 (a) is in orbit around its Sun or closest star(s),
 (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 an atmosphere.

Footnote:  This should be all you need to tell if it is a planet, not an pre-defined list that makes the definition irrelevant.
Why This Is A Better Definition.

Aside from my snarky footnote, this definition defines planets in a way that is not exclusive to our solar system.  The blatant planetary bigotry that there are no planets outside of ours is truly astounding.  We really need a definition that helps us recognize planets around other stars.

We don't have to look everywhere for this cleared orbital neighborhood to figure out if it qualifies.  (We don't look at the plant or rocks around a tree to see if it is a tree.)  This definition holds just by looking at the celestial body itself.

What Would Happen If We Used This Definition?

Pluto would be back as a planet.  So would 50 to 150 other known objects that are round, and have atmospheres out there orbiting the sun.

Mercury would become a moon orbiting the sun.  Or something else.  But with no atmosphere and not being round it would lose it's status as a planet.