there'd be eight little planets sitting on the wall
I am so very sorry; despite being a dynamicist with a natural affinity for Resolutions 5A and 6A, demoting Pluto was still bloody foolish.
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".
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.
So, what is going on?
First of all, this is semantics, a planet is a label and is whatever we can darned well like it to be.
A word means exactly what we want it to mean, no more, no less.
Secondly, this is payback by mostly east coast astonomers who are still annoyed that Tombaugh discovered it. Some vendettas go down the generations.
But mostly it is a deluded chase for consistency because some people, of a type not uncommon in science, just can't stand it if things don't fit together ever so neatly.
The problem is, as with all committee resolutions, that it just makes thing messier.
And I just don't mean the millions of elementary school children in tears, or the Wrath of Colbert, or the very large number of "but The Book says Pluto IS a planet, but your notes say it isn't, which will it be on the exam" e-mail from Astro 001 students...
The problem is that the Resolutions are not consistent or complete.
We've already covered the notion that hydrostatic equilibrium depends on composition, and in principle thermal history!
Also, the "nearly" round begs future controversy. Quantify it people! No object is perfectly round, or even a perfectl figure of hydrostatic equilibrium. How much fractional divergence from the ideal shape is acceptable before it is no longer a planet?
But, the dynamical dominance clause is the worst of all.
What were you people thinking?
First of all, Trojans, - Jupiter is dynamically dominant, but has not "cleared" the neighbourhood around its orbit!
Secondly, "clearance" is time dependent! This definition would have an object not be a planet initially (debris lingering for many dynamical time scales); then become a planet - but the inner planets become planets before the outer planets! And then it may stop being a planet, since, for example, mass loss late on the main sequence can induce dynamical instabilities and repopulate inner orbits cleared earlier.
Oh, and this means Neptune is NOT a planet. Hello! Pluto - it crosses Neptune's orbit. Clearance is incomplete...
Also, "clear orbit" does not mean "the planet did it". Mercury orbit is pretty clean, but I suspect Mercury was the least important factor in making it so.
Ultimately, then, Jupiter is probably the only real planet, since N-body systems are all unstable on large enough time scales (ok, we're talking over 1000 trillion years, but you gotta be consistent); on that time scale all planets will eject or collide, and since Jupiter is by far the largest, the rest can't get rid of it. Ergo, Jupiter is the only planet.
Pah.
A foolish consistency is the hobgoblin of little minds
If we want consistent, go with my plan:
a planet is 1) 1000 km or greater mean equatorial radius (Pluto in)
or 2) 2000 km or greater mean equatorial radius (Pluto out).
For sub-stellar bodies orbiting stars. That is it. Consistent, just as arbitary, and it works.
One excellent ploy though.
The resolution only applies to objects around the Sun, not stars in general.
The opening shots in the Real War for planetary semantics are fired, and we wait for the Battle of Rio in 2009
Ooh, Rio, maybe I will go...
Got to fight them on the beaches.
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Pluto, like Neptune, was predicted from irregularities in the orbits of existing planets. Ceres was not. So if we have to have a mathematically regular definition, surely it has to be that a Planet's orbit can be inferred from irregularities in the orbit(s) of adjacent bodies already thusly determined to be planets? (by inferential proof, where case 1 is Earth, one cou;d work put from there by the maths of Earth's actual measured orbit). This i admit is also a moiving target - perhaps Ceres will become a Planet when the day arrives that our instrumentation becomes sensitive enough?
As I understand it, the irregularities that led to the discovery of Pluto were (a) too large to be caused by a body of Pluto's mass and (b) artefacts of some sort of calculation error, so it was all a bit of a fluke really (which is not to lessen Clyde Tombaugh's achievement in detecting it).
Ceres' perturbations on Mars and Earth can already be detected. In fact, its mass has been determined that way. Ceres turns out to be rather icy, like a smaller version of a Galilean satellite. Some other asteroids' perturbations can be detected, too.
I think there might be ways to define the "cleared the neighborhood around its orbit" part, if arbitrarily. For a candidate planet X:
1. Bound a shell around the Sun, with inner radius of 0.8 * b, and outer radius 1.25 * a, where a and b are X's semi-minor and semi-major axes. We'll call it X's neighborhood. Fudge those factors as needed.
2. Sum up the mass of all the non-satellite bodies that are instantaneously in X's neighborhood.
3. If that sum is less than one third of X's mass, it has cleared the neighborhood of its orbit. Again fudge the 1/3 as needed.
Using that, I'm pretty sure the 8 current planets would fit. Jupiter is a lot more massive than all the Trojans, so it qualifies. Neptune is a lot more massive than all of its Trojans and the Kuiper Belt, so it qualifies. I think Ceres would also fail. But since Neptune orbits within Pluto's neighborhood, Pluto fails. Even if the Moon were to be ejected from Earth, Earth would still qualify (though the Moon might not). Long term chaotic effects would eventually mess things up.
On a somewhat different note, do dwarf planets follow the planet naming convention or the asteroid naming convention or something else? That was the whole reason for this debate in the first place - to name 2003 UB313. Can dwarf planets have the same name as asteroids, like "Persephone"? Will 1 Ceres lose the number, and will we have to renumber all the asteroids? Will Pluto get a number?
If we must have an actual definition of planet, I personally would prefer something simpler, like If it's roundish and has lots of rocky stuff in it somewhere, it's a planet. Planethood for Enceladus!
Hi,
You might well be interested in this new wrinkle then:
http://asymptotia.com/2006/08/25/a-glimmer-of-hope-for-pluto/
-cvj
Pluto was not predicted -- "Planet X" was predicted by Lowell, and never found. Pluto explains nothing about Neptune's orbit; it's insignificant.
Tombaugh's reputation has been enhanced by this decision. He didn't discover another planet -- he discovered a whole other zone of the solar system, a whole other type of object.
The resolution is entirely consistent; a sensible, objective set of criteria has been put forward that do not rely on arbitrary is-it-bigger-than-a-breadbox criteria. Crucially, the value Lamda measures whether a planet has cleared the neighborhood or not. There is a gap of five orders of magnitude in Lamda between the planets and Pluto. It's just not close.
http://arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf
No-one can objectively look at these tables and figures and say that this data does not unambiguously support the recategorisation of Pluto. If we define the word "planet" at all, then Pluto isn't one. When you look at the cold facts there just isn't any doubt at all.
It seems that all we've had from Gingerich and the other inclusionists is invective about the voting process (which they had no problem with when they thought their silly scheme was going to get through), conspiracy theories (trying to slander Clyde Tombaugh? What?), and appeals to unscientific populism and emotion.
For cooly rational, empirical science of the sort that grown-up professional scientists do, you have to look at the eight-planet proposal. Which won, because thankfully professional science won out over shrill tribalism and turf warring. The vote was as overwhelming as the data and empirical evidence is.
Mercury: small, round, ecentric orbit, no atmosphere, no moons.
Pluto: small, round, excentric orbit, atmosphere, three moons.
Let's demote Mercury.
Earth: 10,000 objects cross it's path, flat (just look outside), you have to look down to see it, it isn't a planet.
Sun: Wanders the skies like the other planets. It's a planet.
Uranus: visible to the naked eye. Wanders like other planets. It's a planet.
Neptune: not visible to the naked eye. Not a planet.
New mnemonic: Vigilant Mom Just Served Us Nine Pizzas.
Hey, it's better than:
My Very Energetic Mother Just Served Us Nothing.
To look at the change in the planet designations a slightly different way, my two young boys (aged 6 and 8) have taken this in their stride and understand quite well that "Dwarf Planets" are not quite up there with the planets but are close enough. The debate in their eyes now is "how many dwarf planets might there be?". If nothing else, they have learned a lot about the planets and such from having this debate going on in the news recently. (Visits to the Astronomy Picture of the Day site have added to their interest.)
http://antwrp.gsfc.nasa.gov/apod/astropix.html
Browse the archive a bit if you need a new background pattern on your PC.
(signed) marc