"Whoa! Pluto's dead!"
-- Mike Brown, California Institute of Technology

Whoa whoa whoah! What's with all this? Planets have been rearranged, you say? There are only eight of them?

Yes, Pluto has been stripped of its "planet" title, and is now filed under the not-quite-so-majestic "dwarf planet". Following a lengthy series of resolutions held in Prague - so lengthy, in fact, that only a sixth of the 2700 astronomers attending were there for its final day - "planet" was finally given its very own definition, ending a debate that's lasted for... well, a few years. Also downgrading Pluto in the process.

What exactly is a "dwarf planet", though? A planet... but smaller? Does it have a beard and eyeglasses and work in the mines to sell gems? It's the first one, but I'll get to all that later. Firstly, some history for you.

We've had the term "planet" for thousands of years. If you want to know where the original "planet" term came from, it was coined by the Greeks. To the Ancient Greeks, a Planet - well, a πλανήτης, meaning "wanderer" - was one of Mercury, Venus, Mars, Jupiter and Saturn, as well as the Sun and our Moon. And, without telescopes, these are the only planets in the sky that you can see.

After we discovered the gigantic ball of iron that we're standing on orbited the Sun, the Sun and our Moon became not-a-planet anymore and the Earth became a planet. Also, curiously enough, four of Jupiter's moons - the Galilean moons - were thought of as planets until the modern definition of moon was created; before that, the only true Moon was Luna.

In 1781, a comet was spotted around Taurus. But it was a strange comet at that - with a circular orbit, it wasn't a comet at all, it was the planet Uranus. With the six-planets system having stood for over a hundred years, no-one would think that another could be found - it would destroy the then-fundamental beliefs.

Neptune was discovered in 1846 while looking for a reason behind the irregularities in Uranus's orbit. This same search eventually yielded Pluto in 1930, and harkening to the idea of more planets further and further away from the sun, Pluto was given the title of a Planet. And, until recently, these nine were the only planets there were - no more, no less.

They haven't found the supposed large planet causing irregular orbits in Uranus and Neptune yet. I wonder if they will.

The largest asteroids in the belt were also known about: 1 Ceres, 2 Pallas, 4 Vesta and 10 Hygiea make up just over half of the entire belt, all of which were discovered by the 1850s. But these were not classified as planets - their position in the belt confined them squarely to just asteroids.

But the kicker came in 2003, when a Mike Brown discovered the "tenth planet" 2003 UB313, although you probably know it as Xena 136199 Eris. As it was larger than Pluto, and near it, too - just on the edge of the Kuiper Belt - its status was uncertain. Was it a new planet? Was it not? Maybe the whole definition has been built up incorrectly.

But they looked, and they searched, but they couldn't find a proper definition for what is a planet and what is not. And so the International Astronomical Union decided to clarify this by creating such a definition.

Easier said than done? Of course. Anything that applied to this new Xena Eris also applied to Pluto. It was unusually small compared to the other planets, and with its elliptical orbit, some astronomers questioned whether it should be classified as a planet at all. So, what to do? Shatter people's beliefs that there are only nine planets, leaving us with eight, or anything up to twelve - or enforce a definition that isn't a definition at all?

Reasons against dwarf planets

"I think they voted primarily on scientific grounds and were not sensitive to the historical and cultural role that Pluto has played"
-- Owen Gingerich, Harvard University

Reasons against the new definition are mainly social. Not only does the classic "textbooks will have to be rewritten" apply here, but models, pictures, diagrams - a lot of things. And some people won't like that. The American Museum of natural History received a furious letter from an 11-year-old because they didn't show his favourite planet on their model. And that was before this whole controversy even began.

There are also a few technical complaints. A dwarf planet should have a "hydrostatic equilibrium" - that is, it's round. But how round is round enough for the definition? There are long asteroids, thin asteroids, too. Look at poor old 2003 EL61: It is round, but still vaguely egg-shaped. This same can apply to the definition of "clearing the neighbourhood around a body's orbit" - not specifically enshrined in the new definition, to allow for common sense judgement. Couldn't the same judgement be used to classify Pluto as a proper planet again?

And the new planet mnemonics. MVEMJSUNP. My Very Excellent Mother Just Sent Us Nine... nine what? planets? plutons? And no, MVEMCJSUNPCX isn't any better. I mean, there's an X in it.

Reasons for dwarf planets

"This tiny thing in this oddball orbit - a planet? Give me a break!"
-- Richard Conn Henry, American Museum of Natural History

What about the other side's arguments?

The principal argument is that this redefinition is correcting an error that should never have been made in the first place. If we had known more about Pluto, the argument goes, we would have realised that it shouldn't be classified as a planet, rather than having to correct it later on. (And we are learning more about it - see the New Horizons spacecraft).

Pluto is fundamentally different from the other planets. Its orbit is out of whack, being tilted, and occasionally becoming closer to the Sun than Neptune. It is also much smaller, as though it failed to accrete into a larger planet somehow. The other planets have all managed to "clear the neighbourhood" by accumulating small pieces of rock around it - save the asteroid belt, which was forced apart under Jupiter's influence - and Pluto, which is the matter at hand.

These are the same people that say that changing a scientific definition to suit the needs of the public is folly.

The 16th August draft

The first proposed draft looked like this:

A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

With this, Ceres, Charon and Xena Eris would all be considered planets, along with a potential "candidate list" of a dozen others. Its advantage was that a planet's roundness can be easily measured, without the need to go up there and check. Most other drafts involved limits tailored for our solar system, such as mass, or orbit, or maybe inclination.

There would be no minor planets, just "small solar systems bodies" and "plutons", which means "a planet with a very inclined orbit". The current (well, old) system of planets would be renamed the "classical planets".

That said, some argue that this definition would leave us with too many planets. Mike Brown, the same Mike Brown who discovered Sedna and Xena Eris, said that fifty-three celestial bodies are known to fit the definition, and we could probably find about two hundred if we tried hard enough. And that's a lot of planets.

The 18th August and 22nd August draft

So they tried again, and came up with this proposition:

  1. A planet is a celestial body that (a) is by far the largest object in its local population, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) does not produce energy by any nuclear fusion mechanism.
  2. According to point (1) the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane are the only planets of our Solar System. All the other objects in orbit around the Sun are smaller than Mercury. We recognize that there are objects that fulfill the criteria (b) and (c) but not criterion (a). Those objects are defined as "dwarf" planets. Ceres as well as Pluto and several other large Trans-Neptunian objects belong to this category. In contrast to the planets, these objects typically have highly inclined orbits and/or large eccentricities.
  3. All the other natural objects orbiting the Sun that do not fulfill any of the previous criteria shall be referred to collectively as "Small Solar System Bodies".

The name for a "Pluto-like planet" was a "pluton", which was in the original draft but was dropped out of it four days later. Not only is it also the geological term for a form of magmatic intrusion, but Pluton is Pluto's name in French and Spanish, making astronomical translations... interesting, to say the least.

But how the term was coined was of little value compared to the other debate going on - specifically, about the planetary definition. The two viewpoints collided, and there was arguing over whether Pluto should keep its planetary title, or demoted into a "dwarf planet". The argument for Pluto's demotion gained the upper hand, and that carried over into the next (and final!) draft two days later.

Resolution 5A

Said draft two days later was the one that was accepted.

Here lies the official new definition of planet, as confirmed by the IAU; these three categories should apply to everything in the Solar System.

  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) shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
  3. All other objects3 except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".
  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 Three Dwarves

"We know of 44 dwarf planets so far," "We will find hundreds. It's a very huge category."
-- Mike Brown

Due to their being the planets that sparked the whole redefinition controversy, there are three bodies that have been classified right away:

There are also a few more that might be dwarf planets, but the IAC is being undecided about due to lack of information on them. Here they are, with where they are and their relative diameter to Earth:

  • 90482 Orcus, in the Kuiper Belt, relative diameter 0.14
  • 90377 Sedna, in the Scattered disc, relative diameter 0.11
  • 50000 Quaoar, in the Kuiper Belt, relative diameter 0.12
  • 20000 Varuna, in the Kuiper Belt, relative diameter 0.07
  • 28978 Ixion, in the Kuiper Belt, relative diameter 0.06

There are also some without official names yet:

  • 2005 FY9, in the Kuiper Belt, relative diameter 0.14
  • 2003 EL61, in the Kuiper Belt, relative diameter 0.12
  • 2002 TC302, in the Scattered disc, relative diameter 0.09
  • 2002 UX25, in the Kuiper Belt, relative diameter 0.07
  • 2002 TX300, in the Kuiper Belt, relative diameter 0.07
  • 2002 AW197, in the Kuiper Belt, relative diameter 0.05

I don't want to make this sound like some sort of death list, I really don't. There'll be updates if any of these are decided upon.

Interestingly enough, Pluto's largest moon Charon is also on that maybe-a-dwarf-planet list. Under the current new definition, natural satellites are referred to as such, rather than a planet; however, the IAU have not yet said whether they consider Charon as a satellite or not, due to Pluto's small size and the fact that they orbit round each other. Interesting to see how this one turns out.

Farvel, Pluto the Planet...

"It's going to be hard to find a new planet," "You'd have to find something the size of Mars. Finding a new planet will really mean something."
-- Mike Brown

Well, there we have it. By the imperious order of the IAU, Pluto is now a dwarf planet, not a planet.

I hope that the other planets do not gang up on it. Mars could send its fastest armies 28 AU to taunt it. Jupiter could relegate it down and down into its own little "tiny weeny dwarfy planet" status. Neptune would be still be happy, though - a definition change isn't enough to keep it from visiting for 20 years.

Naah, I'm sure they wouldn't do that, and life will carry on as normal with our new dwarf planets. Besides, the IAU probably has regulations against planetary wars. Or draft resolutions of them.

Sources from the IAU: From elsewhere:

Update, September 2006: We now have Eris and Dysnomia, instead of Xena and Gabrielle. What ho. I have strikedthrough accordingly.

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