Fight for Pluto !, A Campaign to Reverse the Unjust Demotion Options

[Ames]

Yet these same astronomers seem quite happy to accept that the terms "moon", "star", and "galaxy" should have no such limit. If there can be dozens of moons in the Solar System why can't that same solar system have dozens of planets?

What is the rationale for restricting the number of planets?

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Stephen

Ok then, for "Star" there is:
Brown Dwarfs
White Dwarfs
Main Sequence
Sub Giants
Giants
Bright Giants
Super Giants
Along with spectral class {O,B,A,F,G,K,M}

And its worse for Galaxies!!

so "Dwarf Planet" is fine.

Nick

[Stephen]

Too many of the elements in the IAU's definition are inherently flawed. Curiously that includes the very one that many people may have the least problem with: ""A planet is a celestial body that (a) is in orbit around the Sun".

Just exactly how do you determine that body goes round the Sun? In many cases the answer is doubtless obvious. However, as with so much else in the universe there are always exceptions. One particularly bizarre example of such an exception is object J002E3, which paid a visit to our vicinity a few years ago. It is believed to be the spent 3rd stage of one of the Apollo missions. Its orbit is such that although it usually goes round the Sun it can perodically be captured by the Earth, orbit it for a while, then escape back into solar orbit again.

A more pertinent example of an exception, though, is Earth's own Moon. Apparently the Sun's gravitation field has a stronger pull on the Moon than the Earth's own field. (For more check out here and here; for a more explicitly worked out version of the maths check out here; and there's an interesting discussion of the problem, together with lots of maths, here; while this page puts a more democratic perspective on the issue: "the moon co-orbits the sun with the earth".)

Since the IAU chose not to define the yardstick(s) on which it determines when a body is in orbit around the Sun (and when it isn't), there seems no reason the balance of gravitation forces could not be used as that yardstick. Yet were that yardstick applied to the Moon it could be used as the basis for arguing that since the Sun has a stronger pull on the Moon than the Earth's own gravitation field, the Moon is therefore (technically) in effect going round the Sun rather than around the Earth and thus (technically) not a moon under the IAU resolution but either a planet or a dwarf planet (depending on the other elements in the definitions).

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Stephen

[Stephen]

Ok then, for "Star" there is:
Brown Dwarfs
White Dwarfs
Main Sequence
Sub Giants
Giants
Bright Giants
Super Giants
Along with spectral class {O,B,A,F,G,K,M}

And its worse for Galaxies!!

so "Dwarf Planet" is fine.

Nick

OK, from the top.

1) "Brown dwarfs"

Brown dwarfs are (at best) seen as failed stars, not stars per se. They are not massive enough for their thermonuclear furnaces to have ignited. (See this Wikipedia page, which dubs them "sub-stellar objects".)

2) "White Dwarfs" "Main Sequence", "Sub Giants", "Giants", "Bright Giants", "Super Giants". Not mention all those spectral classes.

All these are subcategories of stars. That is, subsets of the objects termed "stars".

By contrast, under the IAU's definition a "dwarf planet" is not a kind of "planet". Under the IAU's definition "dwarf planet" and "planet" are different species of astronomical object.

3) Ditto for galaxies. Spiral galaxies and elliptical galaxies, for example, are all classed as galaxies, rather than as objects different to galaxies.

******

IMHO the IAU is going to rue the day it dubbed it's second-best category "dwarf planet". Too many of us lay people are going to mistakenly think that "dwarf planet" is a kind of "planet" rather than a different category of object. Presumably the term was a bone tossed to the Pluto crowd to keep them quiet.

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Stephen

[Jyril]

(PS - Dilo - no, I thought I did get what you meant with your neigborhood comment, but clearly I have it wrong. There is no part of the solar system that can be considered a clean neigbourhood )

The unfortunate wording was selected to make the text clearer to a layman. It's up to the IAU to decide what the orbital clearing means. According to them, all the eight planets have cleared their neighborhoods, whereas Ceres, Pluto, and 2003 UB313 haven't.

Back to polls (Sky & Telescope):

How Many Planets Do You Think There Should Be in Our Solar System?

8: 38%
9: 28%
10: 10%
12: 15%
53: 9%

(Unfortunately they don't show the number of votes.)

[djellison]

According to them, all the eight planets have cleared their neighborhoods,

Complete BULL. I'd have thought SL9 or Tunguska would have reminded us all that clearing is an ongoing process for every 'planet' in the solar system.

Their definition is very very very badly written. Take out the 'cleared' tag, and you make it just about acceptable.

Doug

[David]

The unfortunate wording was selected to make the text clearer to a layman.

I thought they were scientists, scientifically superseding a fluffy floppy cultural-historical definition with a rigorous, technical scientific definition!

[Jyril]

I have no problems with the "orbital dominance" (except for the horrible wording). Why? The IAU decided what is a planet, which is a primarily cultural term. It's more important for non-astronomers. They couldn't fathom the idea of dozens of planets. That's why the number of planets must be limited. Pluto posed a major problem, because it clearly was a member of population of many similar objects. There had to be a way to get it demoted.

Many of you actually think about non-satellite planetary mass objects, which is IMHO a completely different thing, actually. I think it would be wisest to limit the use of planet in non-scientific text and start to use the term "planemo" (or whatever) to refer any round object that does not fuse.

Phil Plait explains this much better:

Which brings me, finally, to my big point. This is all incredibly silly. We’re not arguing science here. We’re arguing semantics. For years people have tried to make a rigid definition of planet, but it simply won’t work. No matter what parameter you include in the list, I can come up with an example that screws the definition up. I’ve shown that already, and I’m just warming up.

The problem here is simple, really: we’re trying to wrap a scientific definition around a culturally-defined word that has no strict definition. Doing this will only lead to trouble. Why? For one thing, it’s divisive and silly. How does a definition help us at all? And how does it make things less confusing than they already are? Charon is a planet? It’s smaller than our own Moon!

[JRehling]

I fully understand Alan is hungry, but, apart historic/cultural reasons, scientifically the Pluto privileges are hardly defendible... Alan's objection about "neighborhood clearing" do not consider that NEO and trojans are less than 1/1000 the size of Earth and Jupiter, respectively!
Pls, do not hate/heat me for this observation inside a pro-Pluto thread...

The whole "neighborhood clearing" issue is bizarre. There are two ways a world can clear its neighborhood: By collision, and by ejecting smaller competitors. It's obvious that ejection, to the extent that it does proceed, take a long time. It's like a radioactive decay of the objects in crossing orbits, but the number doesn't go to zero even after eons. And there are ways that an object can actually attract crossing objects, in the Lagrangian positions, and in 3:2 resonances, etc.

Collision and ejection both take place at a rate determined in part by the mass of the object (collision also pertains to its radius). So this part of the definition is just a muddled attempt to make mass a criterion. Beyond which, it's ridiculous to suggest that Saturn only "became" a planet when it had sufficienly cleared its space. I think if you turn the clock back to the time when Saturn had hundreds of smaller bodies orbiting the Sun in its vicinity, you'd have a hard time looking at that massive globe and saying it's not a planet.

[David]

There are two ways a world can clear its neighborhood: By collision, and by ejecting smaller competitors.

There's also attracting small bodies into its orbit, but perhaps this doesn't count as "clearing". Jupiter and Saturn travel around cloaked in immense clouds of former "competitors".

[odave]

Jupiter: "Dangit, I was just about to become a planet, but then stupid SL9 comes along. Do you know how long it's going to take me to clear this up?"

[dilo]

advice: this post is OT here! sorry for this...

A more pertinent example of an exception, though, is Earth's own Moon. Apparently the Sun's gravitation field has a stronger pull on the Moon than the Earth's own field. (For more check out here and here; for a more explicitly worked out version of the maths check out here; and there's an interesting discussion of the problem, together with lots of maths, here; while this page puts a more democratic perspective on the issue: "the moon co-orbits the sun with the earth".)

Stephen, I must confess that I ignored that ratio between the gravity force from Earth and from Sun is below 1 only for our moon!
Your links are very interesting and, apparently, all figures are right but... while math is right, there is a big flaw in the physics of all of them!
In fact, they forget that Earth-Moon system is not inertial! You cannot ignore that the Earth-Moon baricenter is moving in a circular trajectory, so there are other forces that must be considered in the game, especially the centrifugal force, which is equal to Sun gravity in the baricenter (but, obviously, points in the opposite direction).
The proof of how wrong are conclusions arising from application of inertial rules to such a rotating system is the question posed in the math forum you posted: the asking guy calculated that the equilibrium point between Sun and Earth gravity is about 258000 Km from Earth, so he wonder how the Moon can still orbiting the Earth from a larger distance.
But it is well known that the equilibrium (Lagrange) points are located 1.5 million Km from the Earth, 6 times away and well behind Moon orbit. This result can be ontained only considerning also centrifugal force arising from the rotation around the Sun.
In conclusion, when two body are gravitationally linked (and Earth-Moon are linked without any doubt), you must first consider their movement around their common baricenter and, before to consider Sun attraction, you must subctract the motion of the baricenter around the Sun. All residual forces are very small and can be considered tidal forces or second-order rotational effects...

[David]

Radicalized pro-Plutonians may find this site amusing, or at least consoling.

[mars loon]

Radicalized pro-Plutonians may find this site amusing, or at least consoling.

David,

these are priceless gems

thank you.

in the meantime I have been working to integrate the "Fight for Pluto" campaign into my extensive public outreach efforts. Your contribution will help.

ken

[volcanopele]

Radicalized pro-Plutonians may find this site amusing, or at least consoling.

Sweet! I found a few that would work for my office door. I especially like the "this girl is crying because she was just told that Pluto is no longer a planet. What next? No Santa Clause?" one.

[vexgizmo]

Radicalized pro-Plutonians may find this site amusing, or at least consoling.

I count 4--I repeat: at least 4--of these use Ganymede as a generic icy world to represent Pluto. Now which planetary system ought we to be exploring?