IAU Resolution: Definition of a Planet in the Solar System
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.
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.
IAU Resolution: Pluto
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.
So what does this mean for Pluto?
Well, had Resolution 5B passed, we would have had the overall classification of a "planet" that would have had two sub-classifications: "classical planet" and "dwarf planet." By those classifications, we could have used the sentence "Pluto is a planet" with complete scientific accuracy.
But the IAU rejected Resolution 5B. What this means for our solar system is that there are now two separate categories: "planet" and "dwarf planet." Pluto is now a dwarf planet; and it would be technically incorrect to refer to it as a "planet" without the preceding adjective of "dwarf."
I have to say that I am disappointed with this news. I grew up with a nine-planet solar system, and as I was growing up I marveled at the Voyager spacecraft missions that brought us new knowledge and those spectacular pictures of the planets. I was disappointed that we didn't know what Pluto looked like, and I went on record for many years as hoping for a mission to Pluto. I was delighted when the New Horizons spacecraft launched back in January, as we would finally get to see pictures of the planet Pluto.
But by relegating Pluto to "dwarf planet" status, the IAU has changed the emotional impact that such a mission can have.
I believe the IAU's vote could conceivably stifle the imagination of those of us who still wonder at the glories of our solar system, and who reach for the stars. And I hope that in 2009, when the IAU meets again in Rio de Janeiro, Brazil, they will reconsider the vote that they have taken today in Prague.