What Is a Planet?

Scientific American: What Is a Planet? [ ASTROPHYSICS ]
The controversial new official definition of "planet," which banished Pluto, has its flaws but by and large captures essential scientific principles

The interesting point about this article is the philosophical idea that something can be defined by what it does without the necessity of an intrinsic property.

Ancient Classification

In a way, a planet was originally defined by what it did instead of what it was. In the scheme of the ancients, the planets were sources of light that moved relative to other sources of light in the sky.

Here the movement had to be detectable by humans. This meant the timescale for detection was limited to, at most, several years. Saturn, the outermost planet known to the Ancients, has an orbital period of 29.46 years - this means that Saturn moves relative to the "fixed" stars at a rate of about one (1) degree per month.

Although it is now known that all other bodies are in motion relative to each other, this motion was not detectable on the scale of a year with optical technology exiting prior to the 20th Century.

In this ancient scheme, there was the Sun and the Moon along with the other planets: Mercury, Venus, Mars, Jupiter and Saturn. These all moved across the sky as sources of light. The stars were then sources of light that did not move relative to each other although they did move relative to the observer on Earth in a predictable pattern based on the time of day and time of year.

Copernican Classification

The Sun and the Moon were later separated out into their own categories with the general acceptance of the Copernican System when the Earth became a planet. Here the definition focussed on the centre of rotation. The Sun was thought not to orbit around anything. The planets were then bodies that orbited around the Sun, while moons were bodies that orbited around planets.

Modern Classification

With the rapid advances in optical and timing technology in the late 18th and 19th centuries, the number of planets rose to 23. In 1852, these planets were divided into two (2) categories: planets and asteroids. Now the intrinsic property of size is used to classify planets.

So we now have an extrinsic property (orbiting the Sun) and an instrinic property (size) to differentiate between asteriods, suns, moons, and planets.

The discovery of Eris in 2005 meant that under the then current definition, Eris would have to be classified as a planet because its size was comparable to that of Pluto.

Latest Classification

The recent IAU definition of a planet relies on an intrinsic property (roundness) and an extrinsic one (orbital dominance).

The former property is an upgraded version of the size property, while the latter property is meant to convey the idea that the planet controls the orbits of other bodies in its immediate neighbour by flinging them in other orbits, absorbing them through collisions, or by stabilising their orbits.

It is at this point, the author of the article, Steven Soter, wants to remove the intrinsic property altogether and to modify the extrinsic property to be a more measurable one of mass ratio between the largest body and all others in the same orbital zone (see the article for a more precise definition).

His argument is that the largest body would become naturally round when it has absorbed almost all of the matter in its orbital zone, and, as a consequence of that great concentration of matter, is then able to dominate the orbits of all other bodies in its neighbourhood.

Philosphical Implications

This new definition proposed by Steven Sator means that something can be classified by its extrinsic property. This works because there is a theory that explains how planetary formation comes about.

We observe bodies in space near Earth. We can classify them by what they do. The reason for their existence and behaviour derives from the theory of their formation.

The intrinsic properties of a planet then derives from its being a planet. That is, the mass of the planet is determined by how much material it was able to accumulate. (This is awfully vague and ambiguous).

Why Change at All?

The change is needed to create a more precise signifier for astrophysicists to communicate among themselves. This is contradistinction to the cultural norm of keeping the idea of nine (9) planets.

The cultural forces for the retention of nine (9) planets are now impeding scientific understanding of planetary formation and behaviour. It would seem that a lot of people think that the signifier of "planet" is some arbitary and is therefore susceptible to political pressures.

The passion with which some people are defending Pluto as a planet probably reflects upon the uncertain nature of the current political and cultural environment. They the certainity of the past instead of the changing present.