Compare Planet Mercury, and Dwarf Planet Pluto: CLICK HERE!

Our new understanding of Pluto considerably raises the importance of
NASA's "New Horizons" Mission.

Rather than simply being "yet another mission" to "yet another planet,"
New Horizons has now become the first mission ever to a Dwarf Planet!

Dwarfs are important! Our own Sun is a dwarf star!

Victory!

IAU Resolutions: Pluto

To be voted on momentarily

RESOLUTION 5A APPROVED!

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

(1) A planet1 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) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects3 orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

RESOLUTION 5B DEFEATED!

Insert the word "classical" before the word "planet" in Resolution 5A, Section (1), and footnote 1. Thus reading:

(1) A classical planet1 is a celestial body . . .

and 1The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

RESOLUTION 6A APPROVED

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.

RESOLUTION 6B DEFEATED!

The following sentence is added to Resolution 6A:

This category is to be called "plutonian objects."

Listen to the Pluto story on National Public Radio
Think about it this way:

Suppose we originated on a planet (one that is essentially identical to Earth) of Alpha Centauri, ... and we've finally mounted an expedition to Sol! Think of the excitement as we approach that nearest star to our own! Will it have planets? How many? As we get closer, we spot Jupiter, Saturn, Uranus, and Neptune. Hard to miss these gas giants! Well, that's four! But of course we are even more interested to find out if there are additional planets, similar in size to our own planet (that orbits Alpha Centauri)!

We are overjoyed to find Mercury, Venus, Earth, and Mars. And we also note the presence of a "failed planet" between Mars and Jupiter, the Asteroid belt.

Of couse there a many hundreds of comets as well.

And in the outer part of the system there are large numbers of Kuiper belt objects of various sizes.

Finally, the expedition dimwit says, "let's pick one among all those objects that are in the outer part of Sol's system, and pretend it is a planet, too!"

Richard Conn Henry, professor in the Department of Physics and Astronomy:

"I think the notion that Pluto is a planet is absurd. When it was initially discovered, it was thought to be vastly more massive than it turned out to be. Its orbit is radically different from that of all the other planets. Down with Pluto, is what I say!"

But now that rationality has prevailed, I say "Hurrah for Pluto, most fascinating of all the Dwarf Planets!"

The purple line is Pluto's orbit!

All illustrations should be credited to:

updated 2006 August 24     by our Webmaster