[dreadgeek]

When I read this story (and then watched Rachel Maddow actually go over some *really* cool science related to it) I laughed. People have always guessed I was an Aquarius (whatever that means) although I was born under the sign of Pisces (whatever that means).

As far Pluto is concerned, it's not a planet. It doesn't fit the definition of a planet according to the consensus of astronomers.

The IAU...resolves that planets and other bodies, except satellites, in our Solar System 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.

Pluto is nearly round but it is clearly not dominant in its gravitational neighborhood. The actual dominant gravitational force in that neighborhood (the Sun notwithstanding) is Neptune. Pluto and Charon look more like the two innermost Kuiper Belt objects. (The Kuiper belt is a region beyond Neptune comprised of small bodies most of them ice of some form (water, methane or ammonia specifically) left over from the formation of the solar system) Pluto is what is known as a dwarf planet.

While it might *look* entirely arbitrary it isn't quite so arbitrary when looked at closely.

Pluto has less mass than the Moon. Is the Moon a planet? No, it's a satellite of Earth. Why? Because Earth dominantes the gravitational neighborhood. Pluto doesn't dominate its gravitational neighborhood. While Pluto orbits the Sun (like every other object in the Solar system) it is *also* orbiting Neptune. In fact, it orbits Neptune three times for every two orbits it makes around the Sun. So Pluto looks more like a satellite of Neptune with a very weird periodicity. Calling Pluto a planet makes as much sense as calling the Moon a planet. And if your question is "why was Pluto initially considered a planet" the reason is that it was large enough to be seen and there was, until recently, no scientific definition of a planet. A definition became necessary after we started discovering extrasolar planets.

The astrological implications I'll leave to others to discuss.

Cheers
Aj