8,9 or 12 Planets?

[Johnny Pixels]

The International Astronomical Union have been debating recently over the definition of a planet. Under their proposed new rules, there will be 12 planets in the Solar System, the 8 "classical planets", and then Pluto becomes head of a new class of planets called "Plutons". This new class includes Pluto's current moon Charon, 2003 UB313, nicknamed Xena, and the largest asteroid in the Main Belt between Mars and Jupiter, called Ceres.

The rules for planets are proposed to be:

1) 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.
(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a “dwarf planet.”
(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call “plutons”.
(4) All non-planet objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.


More:

http://news.bbc.co.uk/1/hi/sci/tech/4795755.stm

http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html

http://www.badastronomy.com/bablog/2006/08/15/congratulations-its-a-planet/

Obviously I don't have a lot of say in all this, but I don't know why things can't stay as they were. Is there any major problem in not having a strict definition of planet?

[TobiasTheCommie]

if i had to make a definition i think it would be.
1) must be a product of an accretions disk in relation to a protostar -> star transformation.
2) must have a core.(not necesarily active)
3) must be in the same plane as the other planets in the system.
4) must not be a satelite of a planet.

We would loose pluto, but i'm fine with that.

[Starthinker]

Also more here: http://forums.randi.org/showthread.php?t=61704

[sphenisc]

I must admit I didn't like their use of the term 'classical planets', that's best kept for the 'planets' known from antiquity. I'd prefer 'major planets'.

I'd also like some kind of margin of error attached to the idea of hydrostatic equilibrium say, a maximum discrepany of x% of any point from the spheroid with the same principle axes.

It's not clear in category 3 whether all the properties are required for a pluton, i.e. high eccentricity and inclination and long orbital period.

It's not clear from the quote why Charon is a planet, though I think it's because the Pluto-Charon system has a barycentre external to the primary. It's a double planet.

[ponderingturtle]

Obviously I don't have a lot of say in all this, but I don't know why things can't stay as they were. Is there any major problem in not having a strict definition of planet?

Yes. If it does not have a precise meaning it is not useful as a scientific term, and there is no definition of a planet that you can get that will keep pluto as a planet but not add in lots more.

So regardless of definition there is no way to say that there are 9 planets in the solar system, you can make a case for 8 you can make a case for more than 9 but 9 is impossible.

[drkitten]

It's not clear from the quote why Charon is a planet, though I think it's because the Pluto-Charon system has a barycentre external to the primary. It's a double planet.

Close. Technically, both Pluto and Charon orbit the sun, not each other. (See my posting earlier about "retrograde" motion.) Therefore both Pluto and Charon are planets.

[GreedyAlgorithm]

Bad Astronomy (http://www.badastronomy.com/bablog/2006/08/15/congratulations-its-a-planet/) has a good post on this announcement. Edit: Oops, that's one of the original links.

No, there shouldn't be a problem with not having a strict definition of a planet. Scientists don't find that "planet" is a useful scientific term (since there's no strict definition!), so they use other terms. "Planet" is just another everyday word now, and we know how well those are defined, but the public perceives it as scientific, so we have to define it for them.

[sphenisc]

Close. Technically, both Pluto and Charon orbit the sun, not each other. (See my posting earlier about "retrograde" motion.) Therefore both Pluto and Charon are planets.

From here
http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_resolution.html

Footnote 2 indicates that the barycentre criterion is the relevant one. My astrophyscis isn't up to determining whether this is equivalent to the no-retrograde motion criterion.

Footnote 2 also doesn't address systems with tertiary bodies or beyond..

[drkitten]

From here
http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_resolution.html

Footnote 2 indicates that the barycentre criterion is the relevant one. My astrophyscis isn't up to determining whether this is equivalent to the no-retrograde motion criterion.

I stand corrected.

Footnote 2 also doesn't address systems with tertiary bodies or beyond..

I have a hard time envisioning such a system that could not be resolved by looking at the bodies in pairwise fashion.

[Johnny Pixels]

How about having two classes of planet, without defining the word any more than it is already, but have say cored planets and non-cored planets? That way sufficiently large asteroids that effectively are planets from a size, shape, orbit point of view etc could be called planets, but they would be differentiated from cored planets for a development and structural behaviour point of view?

If you're just looking at a star system as a orbital system then calling all the objects planets won't make a world of difference, but if you're looking at how the system grew, then defining the difference between planets which grew, and maybe planets that are broken off chunks of others, might be useful.

Or I could be wrong. I'm only really just starting to get into astronomy :)

[pgwenthold]

I love the classification of "pluton"

I want to be a pluton!

[Soapy Sam]

I was under the impression Shakespeare settled this question some time back:-

Fool. "The reason why the seven stars are no more than seven is a pretty reason."

LEAR. "Because they are not eight?"

Fool. "Yes indeed. Thou wouldst make a good fool."

[Outhere]

They seem to have paraphrased Humpty Dumpty in "Alice in Wonderland". A planet is whatever they say it is, which is OK with me.

I'm just hoping this will put the astrologers out of business. But no doubt they are up to the challenge.

[Marquis de Carabas]

So regardless of definition there is no way to say that there are 9 planets in the solar system, you can make a case for 8 you can make a case for more than 9 but 9 is impossible.
For now...

[cackles maniacally]

[WagnerianDisciple]

For now...

[cackles maniacally]

This brings more attention to astronomy I believe - cackling, shrugging, how about simply saying that technology allows our understanding that there are more than the original nine?

"The asteroid Ceres, which is round, would be recast as a dwarf planet in the new scheme.
Pluto would remain a planet and its moon Charon would be reclassified as a planet. Both would be called "plutons," however, to distinguish them from the eight "classical" planets.
A far-out Pluto-sized object known as 2003 UB313 would also be called a pluton.

That would make Caltech researcher Mike Brown, who found 2003 UB313, formally the discoverer of the 12th planet. But he thinks it's a lousy idea.


"It's flattering to be considered discoverer of the 12th planet," Brown said in a telephone interview. He applauded the committee's efforts but said the overall proposal is "a complete mess." By his count, the definition means there are already 53 known planets in our solar system with countless more to be discovered.


Brown and other another expert said the proposal, to be put forth Wednesday at the IAU General Assembly meeting in Prague, is not logical. For example, Brown said, it does not make sense to consider Ceres and Charon planets and not call our Moon (which is bigger than both) a planet."

Fifty three planets? Hold everything - change the science texts and throw them out. It will be interesting to see the result of this resolution.

Check this: http://www.space.com/scienceastronomy/060816_planet_resolution.html

Plutons - Trans-Neptunian Objects. Cackling is cool Marquis.

Stay tuned.

WD

[Wolverine]

For those interested: Mike Brown, leader of the team which discovered 2003 UB313, has posted a page expressing his thoughts (http://www.gps.caltech.edu/~mbrown/whatsaplanet/) on the IAU's proposal.

More reading at The Planetary Society: Proposed Definition of "Planet" Shakes Up the Solar System (http://www.planetary.org/explore/topics/pluto/new_solarsystem.html) (More (http://www.planetary.org/news/2006/0816_The_IAU_Redefines_Planet__Pluto_is_a.html))

In the meantime, I'm still mulling over the situation. It'll sure be interesting to see how the vote goes.

[Dark Jaguar]

Those of you who read 8-bit Theater should appreciate the author's opinion on this:

It seems the intellectual elite are at it again. Just lookit 'em. Sitting there. All smug. Eatin' fancy food. Probably wearin' fancy suits. And trying to add planets to our Solar System (http://www.sci-tech-today.com/story.xhtml?story_id=032001T499J4). It's an afront to science, is what it is.

I for one will not stand for this aggression against our moral and solar traditions.

I mean, you can't just invent -- or worse, change -- a definition based on new evidence. That's unscientific! You have to bury your head in the sand and divorce yourself from reality. That's what gets things done. How? I don't know. I don't ask questions, because these things work themselves out. Remember: head in sand, divorce from reality. Asking questions is just going to rock the boat.

Are you a boat rocker?

Like some kinda Commie?

You live in America, pal. Those of you who don't live in America? You don't count. And 'round here it's Mom, Apple Pie, and Nine Planets. Not Adam and Steve.

[sphenisc]

I have a hard time envisioning such a system that could not be resolved by looking at the bodies in pairwise fashion.


2 For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto-Charon a double planet.


The situation I was considering was an apparently twin planet system in orbit round a larger planet, let's call them A, B and C in order of mass, all satisfy the 'sphericity' criterion.
A is a large planet. The (A-(B,C)) barycentre lies beneath the surface of A. The B,C barycentre lies between B and C, closer to B.

By pairwise comparison.
B is a satellite of A as the subsystem's (and therefore B's) barycentre lies within A. Therefore B is not a planet.
Within the B, C subsystem, C is a planet, since its barycentre relative to its immediate primary (B) is outwith B.

This mean that a planet and a satellite are gravitionally bound, with the planet as the smaller of the two. This is counterintuitive and unhelpful in attempting to bring 'clarity' to the definition of planet.

Alternatively, if you look at the A-C system then C is a satellite, which means that depending on the system under consideration C can be a planet or a satellite. Again not helpful in producing a clear definition.

I'm not trying to say this is a necessary interpretation of the definition, merely that it is one, not unreasonable, interpretation which leads to confusion.

I think the ambiguity comes in the use of the the word 'also' in
"A secondary object satisfying these conditions is also designated a planet..".
This assumes the first criterion is met, and doesn't address the situation where it isn't, rather than including it as part of the definition.

The definition would, I think, be improved by adding the phrase:

"A secondary object satisfying these conditions is designated a planet if the primary is a planet and the system barycentre resides outside the primary.