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The Great Planet Debate

Nov 12, 2007

So it's been over a year since the International Astronomical Union demoted Pluto to what they term a "dwarf planet". I must say that this decision was a long time coming. Astronomers knew for decades that Pluto wasn't by any means massive enough to have a significant affect on other bodies within its vast and highly inclined orbit. Essentially, all theories and estimates concluded that it was only a matter of time before other bodies were found within the same area of space.

And starting in the 1990s, these bodies began being found. One after the other, with great anticipation, the lists of astronomical discoveries were lengthened. And every eager face with an eye pressed to a telescope hoped to be the one to find the Grail: a solar system body larger than Pluto.

Then, on October 21, 2003, it was found by a team headed by the unfortunately blandly named Mike Brown (in contrast, how could you forget names like Clyde Tombaugh or Percival Lowell!). This body was Eris, larger than Pluto and orbiting the Sun at an average distance 70% further than Pluto. It was Eris' discovery which finally brought to the attention of the mainstream, a question which had been plaguing astronomers for years: What is a planet?

Where We Stand

On the surface, it seemed obvious to most people. The Planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto; what's the big deal? Well, that list was The Planets, but the big question was rather: what set of criteria makes body A a planet and body B a non-planet? Also, should any bodies currently referred to as "planets" and "non-planets" be reclassified because of this change?

In the end, the IAU came up with three guidelines and a new set of naming conventions.

IAU said:

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

(1) A "planet"[1] 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) shape[2], (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects[3] except satellites orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

Footnotes:

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.

The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognised as the prototype of a new category of trans-Neptunian objects.

While it divided the objects in the solar system into neat categories, it also divided the astronomical community against itself. There were, and still are, significant camps of people who disagree with the new definition. And these people raise a few very legitimate concerns.

First of all, the definition cripples itself by limiting its scope to bodies orbiting the Sun. The objects we are now finding orbiting other stars are being called "extra-solar planets"; so is this wrong or right? The current definition can't say.

Second, the eight objects in the solar system which fulfil all the criteria in item (1) of the IAU proposal, are called "planets" while those fulfilling all the criteria in item (2) are called "dwarf planets". Confusingly, this means that "dwarf planets" can't be called "planets" because there is a category which already has this single name officially. At the same time, "dwarf stars" are still "stars" and "dwarf galaxies" are still "galaxies". However, "dwarf planets" are not "planets".

Finally, there is an instance of an object which technically fulfills criteria (a) and (b) of both items (1) and (2) yet is a satellite. This object is Luna, Earth's moon. A study of Luna's path around the Sun reveals that it is never concave as one might conceive in a mental image. Rather, it follows a wobbly orbit around the Sun influenced by Earth's gravity. To prevent the Moon from becoming its own dwarf planet, the extra criteria (d) was tacked on to the second definition.

An Alternate Proposal

Obviously, the definition reached by the IAU is not perfect, but it was the product of much wrangling between groups determined to implement their own view of the cosmos on the public. That the conflicting items above were introduced and retained, is only to be expected when trying to find a compromise between two groups determined to have their own way.

Well, the definition has stood for over a year now and the world hasn't fallen apart. Personally, I think it's time the definition was refined, and this time a more universal approach should be taken.

First off, it should be noted that I am a universalist when it comes to measurements. If we ever meet life out there, their systems of measurement are going to be based on their own culture and because we base our own systems on the eccentric properties of Earth and Earth's orbit, any such systems will clash in a hopeless mess. We need to devise systems of measurement which do not change, which hold true from one end of the universe to the other. Our measurements of distance are in a particularily incompatible and sorry state of affairs in this regard.

The meter? Based on the circumference of the Earth, so it's out.

Well, how about the light-year? That depends on the definition of year which depends on Earth's orbit.

Okay, then the parsec has to be it! Wrong. The parsec is how far away from the solar system you need to be for the orbit of Earth to appear 1 arcsecond in diameter, so it is also based on Earth's orbit.

To be fair, things get a little better with SI volumes and masses, which are based on water, a substance which should be ubiquitous wherever life is found.

In any case, I believe the definition of planet should be made such that it holds true no matter which cultural viewpoint is used. Thus we need to prepare a definition based solely upon observations, eschewing any cultural bias. Whereby I offer the following, new definition of "planet" and also "moon".

The New Definition

(1) A planet is a celestial body that: (a) is in orbit around a star, (b) is not itself a star, and (c) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape. This set is further subdivided into major and minor planets.

(2) A major planet is a celestial body that: (a) fulfills all the criteria of item (1) and (b) has cleared the neighbourhood around its orbit.

(3) A minor planet is a celestial body that: (a) fulfills all the criteria of item (1) and (b) has not cleared the neighbourhood around its orbit.

(4) An asteroid/comet is a celestial body that: (a) is in orbit around a star, (b) is not itself a star and (c) has insufficient mass for its self-gravity to achieve hydrostatic equilibrium.

Furthermore:
(1) A moon is a celestial body that: (a) orbits a planet (the primary), (b) forms a barycenter with the primary which is contained within the volume of the primary, (c) absent any third-party gravitational influence would continue to orbit the primary, and (d) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.

(2) A binary (star|planet|asteroid/comet) is a pair of celestial bodies that: (a) are of the same basic type (star|planet|asteroid/comet), (b) which orbit each other forming a barycenter not contained within the volume of either body, and (c) absent any third-party gravitational influence would continue to orbit each other. The exact term depends on the largest of the system's components. For example, Pluto and Charon shall be known as a binary planet, or as a minor binary planet since at least one of the system components fulfils the minor planet criteria (in this case, both components do).

(3) All other celestial bodies which do not fulfil the criteria of item(2), but fulfil criteria (a) and (b) but not (c) of item (1) shall be collectively known as satellites. "Moon" is defined as a type of satellite.

Consequences

As a result of this new definition, Ceres, Pluto/Charon, Eris, Sedna, and a host of other solar system bodies become "planets", but the "major planets" remain the traditional eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. In effect, the term planet becomes synonymous with hydrostatic equilibrium, something which is easily observed and is a property of mass and gravity which holds true throughout the universe.

A second consequence is that the number of "moons" orbiting the planets will change. Earth will keep its one moon, but Mars will lose its count of two since both Phobos and Deimos haven't enough mass to acheive hydrostatic equilibrium. Instead, Mars will have two satellites, but no moons. In effect, the term moon also becomes synonymous with hydrostatic equilibrium as "planet" does.

Next, the terms "major" and "minor" describe whether or not the body co-orbits with many other bodies, or has cleared a swath due to a considerable gravitational influence.

I feel the use of these terms will assist people in visualizing what astronomers are talking about while they begin finding many more objects within other star systems. For example: "a major planet with one moon and two satellites", would be immediately interpreted to mean: a round body large enough to have cleared its orbit, with three satellites, one of which is large enough to be round.

However, the best property of this new definition is that it is based on laws of gravitation and physics which (as far as we know) hold true throughout this universe. So if we ever do meet another civilisation out there, our definitions of celestial bodies should be quite easily translatable since they have not been defined within any cultural context.

It's just too bad that we'll also have to deal with 1 meter being approximately equal to 8.28672065901862533 oogolbleks.

The Great Planet Debate PlanetWerks 2 - Now Available!

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