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Blog Introduction

From the earliest times it was noticed that some stars appear fixed against the background of the heavens and that others appear to be moving against it. The ones that moved were the five "naked eye" planets of Mercury, Venus, Mars, Jupiter, and Saturn. The sun and moon were likewise referred to as planets. Later under the impact of the Copernican revolution when the sun was considered the center about which the planets revolve, it and the moon were no longer referred to as planets and the earth became one.

Bode's law or as it is most correctly called the Titius-Bode rule as it is not really a law and was first pointed out by Johann Titius and later popularized by Johann Elert Bode, pointed out that all the planets known at the time had a mean distance from the sun that was roughly deducible using the formula X x 3 + 4. The X can be a 0, a 1 or any number twice as large as the one used for X in a previous such equation. After multiplying X by 3 and adding to it 4, the resultant amount can be divided by 10 or as Titius had done by 100 representing 1/100 the distance between Saturn and the Sun. The number 4 can also simply be the exact distance between the sun and the first planet from it, Mercury. The 10 can alternatively represent the distance from the earth to the sun, an AU or Astronomical Unit but whatever the units are made to represent the point of Bode's law is that the spaces between the planets are determined roughly by a mathematical progression.

His law held up well and seemed further confirmed with the discovery by William Hershel of Uranus, the first new planet to be discovered since antiquity and even led to the search for the only missing planet in the progression in the solar system, a planet between Mars and Jupiter. Titius felt that the space of the missing planet would be filled with undiscovered satellites of Mars. Bode felt that an actual planet revolving around the sun should inhabit the space instead.

The discovery of the dwarf planet Ceres appeared to confirm the law though it's absoluteness was marred by the subsequent discovery that Ceres was one of many objects subsequently called asteroids in a belt of material between Mars and Jupiter. Ceres and the asteroid belt of which it is a part lies where Bode's law says a planet should be and so whatever significance Bode's law may have for planetary spacing applies to the asteroid belt as well. The planets could still be the objects orbiting the sun at a distance dictated by Bode's law but eventually if an object lay in a belt of material it was excluded from planethood. A holdover, however of the days when asteroids were called planets is the reference to them as minor planets in contrast to major planets which were consequently thought of as more lone figures.

While Bode's rule survived the discovery of asteroids it was dealt a mortal blow as a full literal description with the discovery of Neptune. The planet Uranus the farthest known planet at the time was not orbiting according to prediction. The astronomers Leverrier and Adams used Newton's theory of universal gravitation to predict the existence of a new planet farther from the sun, Neptune. Neptune though is not where Bode's law says it should be. If Bode's rule is really significant why should a body as major as Neptune violate it but if it isn't telling us something at all why did it hold up so well from Mercury to Uranus? In any event there was then no physical definition of a planet, only a way to decide if there were more planets beyond Neptune.

Later Uranus was again thought to have some disturbance, or perturbation, from its orbit and Neptune was thought to as well. The search was on for another planet whose gravitational force would explain the perturbations. Pluto was subsequently discovered by astronomer Clyde Tombaugh and thought to be the missing planet. With recognized planets being massive enough to be rounded by their own gravity, the fact that Pluto to many did not seem to be causing a significant disturbance on any planets after all did not detract from its planethood.

Asteroids can be as big or bigger than a small moon, as Stephen P. Maran says in his book Astronomy for Dummies a nuclear explosion would not be enough to destroy one as it would be a collection of still threatening rocks. Still it would be harder to find an asteroid or one of its ice cousins massive enough to be rounded by its own gravity. With a picture of a planet in those days as opposed to earlier periods being only such a rounded body, only such a body beyond Neptune's orbit, would be called a planet and threaten Pluto's planet status for others unwilling to countenance more planets of less mass than the more traditional ones. Pluto though was an ice world just beyond the gas giant planets and its defying of easy categorization in a neighborhood of gas giant planets would have enabled it to remain for want of any easy description, a planet even if no other planets would have been accepted.

The first gravity rounded TransNeptunian objects since Pluto were finally spotted culminating only several years after Tombaugh's death after almost a lifetime as the discoverer of a planet, in the International Astronomical Union's 2006 General Assembly resolution on the definition of a planet stating http://www.iau.org/public_press/news/release/iau0603

"Contemporary observations are changing our understanding of planetary systems and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation "planets". The word "planet" originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

RESOLUTION 5A

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".

IAU Resolution: Pluto

RESOLUTION 6A

The IAU further resolves:

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

[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."

Seeing that the word dwarf planet has been used by some for outside the solar system I came up with a definition for such ones in Merriam Webster's Open Dictionary http://www3.merriam-webster.com/opendictionary

"Exodwarf planet (noun) : An object orbiting a star other than the sun (b) with a mass insufficient for core nuclear fusion but enough to mostly round it (c) that has not cleared its own orbit in the lifetime of its star to make it more massive than the total mass of the objects crossing its orbit and also not be subject to more than perturbations from its usual course."

If an object really has to fully clear its neighborhood to be called a planet, we would lose planets, but it is true that Ceres and Pluto do lie in belts and if Ceres was excluded for that reason it is consistent on that ground to eliminate Pluto if one defines the Kuiper belt as starting beyond the orbit of Neptune rather than Pluto as others define it. But Ceres and the asteroids were collectively called planets until later when asteroids were called instead minor planets, while Pluto was a singularity. Even in the face of new discoveries it alone was officially a planet beyond Neptune. Its hold as a planet over many therefore is stronger than Ceres'. A vote however is not enough to eliminate an object from planethood that was officially considered a planet and accepted as such by the general public. The vote was only partially successful as some though by no means all astronomers and laypeople no longer call Pluto a planet.

At the 2006 IAU conference the first proposal for a planet definition recommended as an informal term "classical planet" to refer to the eight planets before Pluto. Though the term was rejected by the conference, like much of the results of it, confusion reigned about what was said, and this has led to use of the term classical planets to indeed refer to the eight ones before Pluto.

There are arguments in Pluto's favor. Pluto fits where Bode's rule says it should if Neptune were not where it is. Further the fact that Neptune's gravity is powerful enough to reach into the Kuiper belt where Pluto and other icy bodies reside and exert control over Pluto's orbit, shows that Neptune's gravitational relationship to the Kuiper belt may provide the solution to why it does not conform to Bode's law.

Bode's rule may not dictate precisely where you will find bodies in the solar system but it involves too many planets and predicts too well a distribution of mass to just be a coincidence. Bodian like structure has even been seen with moons.

Pluto is also unique in another way with its satellite system. Moons were at first referred to as planets. Surprising discoveries concerning planets outside the solar system have revived this to an extent as aside from the moons of our solar system's planets, what one may refer to as a moon rounded by its own gravity, another refers to as a terrestrial planet. Such ambiguity was found beforehand in our own solar system's backyard with the discovery that Pluto and its moon Charon, the only moon of Pluto known at the time actually orbit each other causing some to call Pluto and Charon a double planet.