Should Pluto be a Planet or be a Dwarf Planet

[Knight of Clear Skies]

Not completely arbitrary, no. Consider these two statements please:

"The solar system contains eight planets and a number of dwarf planets." (IAU definition.)

"The solar system contains an unknown number of planets. Of these bodies eight are large enough to dominate their orbits." (Rounding definition.)

Both are gross simplifications, but the first sentence is more concise. The problem would be multiplied for an author trying to explain how the solar system formed, it makes a lot of sense to distinguish between planet and dwarf in that context. When considering the bodies themselves the distinction is less useful, there is no physical property that makes one a planet and the other a dwarf. Perhaps dwarf isn't a good choice of word, after all it's possible that there could be one larger than Mercury lurking out there.

A degree of arbitrariness can't be avoided as there will always be edge cases, but a good classification scheme minimises these problems.

[ollypenrice]

Can't argue about science being rewritten, those are factual and replace incorrect theories etc.

You can not improve history...?????

And to write Pluto off is changing History, and I can't agree, as it's based upon an arbitrary decision.

I will now introduce the Trazor planet size identifier (TPSI) for short.

To qualify as a planet, its diameter must be at least 40,000 km, thus we now only have 4 planets, Earth is now merely a lump of rock orbiting the Sun.

History rewritten on the hoof.

The word 'History' contains the word 'story.' The past is the past and history is history. They are quite different things.  What 'really happened' in the past cannot be known perfectly and History, the subject, does not pretend that it can. Nor is searching for the facts all that Historians do. They try to analyse past events, often in the light of new theories (of economics, of psychology, of psychiatry, of climatalogy, of  medicine etc etc). Thus, in telling the story of the past, they bring new insights to bear on past events. It is for this reason (as well as because research uncovers new original sources) that I say that history improves. 

Science also improves as theories are refined. (They are not, all that often, simply found to be 'wrong.' )

Pluto's being called a planet in the first place was an arbitrary decision with a rational basis arising from an error.* Changing the definition was also an arbitrary decision with a rational basis. However, the rationale behind the second arbitrary change was based on a new set of insights. Your rewritten history on the hoof is an interesting example. It is based on an arbitrary measurement and avoiding this was one of the key principles of the reclassification. Astronomers wanted the definition to arise from physical processes or chracteristics, not arbitrary measurements. But suppose (unlikely though it may be) that the Solar System's rocky inner planets turned out to be bodies fomed outside the solar system while the gas giants turned out to have formed from the proto planetary disk, might we not want to create two different terms for what we now know under a single term?

Olly

* The error concerning Pluto is a very specific one. The Lowell Observatory employed Clyde Tombaugh to search for Planet X. Planet X was a hypothetical giant planet thought to be perturbing the orbit of Neptune. Pluto happened to stroll into the region thought likely to contain Planet X and so it was identified as such. But Pluto was not Planet X. Pluto, we now know, is far too small to pull Neptune around. And, so far as we know, Planet X, like Lowell's canals, does not exist.

[DommyDevil18]

I always said when Pluto was a Planet that it should not be a Planet because of how it Orbits and I wondered why its Orbit is highly inclined in a way to make it closer to the Sun then Neptune ever does. Their must be another Planet out their just beyond the Kuiper Belt. The Oort Cloud theory seems plausible I think.

[gkec]

You have to remember that there is no planet X.  The perturbations of Uranus's orbit unaccounted for, after the discovery of Neptune,  were reconciled when Neptune's mass was accurately measured by Voyager.

http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1993AJ....105.2000S

[DommyDevil18]

Thing is though something was theoretically was perturbing Neptune but we do not know. Pluto was meant to be have been the culprit but we know that is highly doubtful because of how small it is. Its been speculated that its the many objects of the Kuiper Belt but tonnes of rock would not perturb a Planet as big as Neptune though. 

Pluto is soon to be visited by the New Horizon probe in the Summer of Next Year so we may get the answer.

[Ags]

As gkec says, there is no perturbation to account for. Once Neptune's orbit was accurately measured, the case for planet X disappeared.

Also, don't forget the trans-Neptunian region has now been surveyed in both hemispheres and we seem to have have found most of the larger, brighter, near-ish objects in that region.

[barkis]

They ought to have left Pluto alone. It had planet status for 70 years since it's discovery, and if for no other reason than respect for Clyde Tombaugh's

tenacious persistence  at a blink comparator, and hundreds of photographic plates tracking it down.

Some reward for his efforts if you ask me. Fame snatched from him, as well as poor Pluto's demise as a Planet.

What harm would it do to have let it be, as a mark of respect for a dedicated man's determination.

That's what I think anyway, although it matters not one jot  to the powers that be.

Nothing sentimental about the IAU. If that's the body who made the decision to change it.

Ron.

[JessicaFaye]

After reading these intriguing, intelligent and interesting post, I feel I should have my two pence worth and contribute... I thought Pluto was a dog...??

[barkis]

After reading these intriguing, intelligent and interesting post, I feel I should have my two pence worth and contribute... I thought Pluto was a dog...??

It is a Dog Jessica, and a very clever one. I used to marvel at what he could do when I watched in the Cartoons at the local flea pit.

Loyal friend of Mickey Mouse as you know, and they made a young lad, (I was young many many moons ago), very happy .

 Both of them.

Ron.

[JessicaFaye]

Well I have to admit Pluto will always be a planet and Dog combined in my mind, as Pluto (the dog) is the visual representation of the planet in my mnemonic technique for remembering the planets of the solar system... so every time I think of the names of planets, there is Pluto (the dog version) sat on the shoulders of the God Jupiter.... I feel It's easier just to remember not to say Pluto at the end, than to systematically restructure my memory palace!

[DommyDevil18]

I must admit Jessica I can call you Jessica cant I that is how I use to remember Pluto by remembering Mickey Mouses Dog but the IAU made the decision must admit I was very shocked as well in 2006 but I reckon that it will help us understand more about Planetary Migration

[jnb]

Would those who advocate Pluto being considered a planet also advocate Ceres still being considered as a planet?

[JessicaFaye]

I must admit Jessica I can call you Jessica cant I that is how I use to remember Pluto by remembering Mickey Mouses Dog but the IAU made the decision must admit I was very shocked as well in 2006 but I reckon that it will help us understand more about Planetary Migration

You may indeed call me Jessica! Yes I fully accept it's not a planet and I don't feel to strongly about it really, it just is imbedded in my memory as a planet!

[DommyDevil18]

Like so many others and I do not have a problem with that But I watched a Documentary with the ever so always brilliant Brian Cox and he quoted a famous Scientist cant remember who and how it went but he said something like "You got to go forward to go back" in other words changing Pluto status at the time was a way forward but with New Horizon on its way to the Plutonian System could reinstate it back alongside Ceres

[Knight of Clear Skies]

They ought to have left Pluto alone. It had planet status for 70 years since it's discovery, and if for no other reason than respect for Clyde Tombaugh's

tenacious persistence  at a blink comparator, and hundreds of photographic plates tracking it down.

Fittingly, an ounce of Clyde Tombaugh's ashes are being carried on New Horizons. Atoms get around, it's likely that each of us at least one which was once a part of William Shakespeare or any other historical personage you care to mention. However, not many escape Earth's gravity well. A little of Clyde will ride past past Pluto and then on into interstellar space - that's some tribute.

I don't agree that reclassifying Pluto diminishes Tombaugh's achievement in any way. His persistence pushed back the boundaries of the solar system - identifying a new piece in the puzzle of its history - and he is respected and remembered for that. However, that does not change that fact that he did not find what he set out to.  "Doctor Slipher, I have found your Planet X." Yet it quickly became apparent that this wasn't the case, even when assuming it was 500 times more massive than it actually was.

I like this quote on the subject of Tombaugh's discovery: "Clyde Tombaugh discovered the Kuiper Belt. That's a helluva lot more interesting than the ninth planet." (Hal Levison)

[davefrance]

OK I accept that Pluto is no longer a planet but in these days of political correctness should we be calling it a "dwarf"? How about "Planet of small stature"? I'll take it up with Brussels.

[DommyDevil18]

Different context it does not necessarily mean small because their are some Dwarf Planets that are about the size of our Moon Its just the definition of Clearing the Neighbourhood that deems it a Planet or Dwarf

[JessicaFaye]

Different context it does not necessarily mean small because their are some Dwarf Planets that are about the size of our Moon Its just the definition of Clearing the Neighbourhood that deems it a Planet or Dwarf

I reckon davefrance gets the context... It looks to me as if he was making a joke! @davefrance would you mind getting with the intellectual stance... I managed to bring it up a notch by bringing Mickey Mouse into this and you just lowered it with a 'dwarf' joke... I mean, really!! ;P

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[davefrance]

I reckon davefrance gets the context... It looks to me as if he was making a joke! @davefrance would you mind getting with the intellectual stance... I managed to bring it up a notch by bringing Mickey Mouse into this and you just lowered it with a 'dwarf' joke... I mean, really!! ;P

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Very sorry, I await my punishment

[gkec]

Like so many others and I do not have a problem with that But I watched a Documentary with the ever so always brilliant Brian Cox and he quoted a famous Scientist cant remember who and how it went but he said something like "You got to go forward to go back" in other words changing Pluto status at the time was a way forward but with New Horizon on its way to the Plutonian System could reinstate it back alongside Ceres

Ceres was classed as a minor planet. Pluto was classed as a planet but that was before it was known that it was effectively a bunch of big rocks orbiting each other.

[JessicaFaye]

Very sorry, I await my punishment

Haha! Food for thought

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[101nut]

Sadly it looks as if the only people really pleased to see the IAU stance on Pluto are the astrologers!

AndyG

[JessicaFaye]

Sadly it looks as if the only people really pleased to see the IAU stance on Pluto are the astrologers!

AndyG

Ooo I think you just swore!! Astrology should be a banned word here... Or just simply banned altogether! (Apologies for the strong opinion)

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[DommyDevil18]

Ceres was classed as a minor planet. Pluto was classed as a planet but that was before it was known that it was effectively a bunch of big rocks orbiting each other.

https://www.youtube.com/watch?v=Z_2gbGXzFbs 

[cantab]

Would those who advocate Pluto being considered a planet also advocate Ceres still being considered as a planet?

Personally, yes.

The current IAU definition has I feel a critical flaw. A body more massive than Mercury, even more massive than the Earth, would be called a "dwarf planet" if it orbited far enough from its star. That is I feel an absurdity.

As long as such a body remains hypothetical, the IAU definition can be got away with. But sooner or later we'll find a real Super-Mercury or Super-Earth dwarf planet orbiting another star, and then I think our definitions will warrant revision.

Simply defining a "planet" as being something orbiting a star, large enough to be gravitationally rounded, and not large enough to fuse deuterium does mean that our solar system has not eight or nine planets, but probably close to a hundred. However, I don't see that as a problem. It's a considerable change in our understanding, but that's happened before; we used to call the Sun and Moon planets and the Earth not one. With the more inclusive defintion of "planet" we would recognise that most planets are pretty small Ceres or Pluto-like objects, and that large terrestrials and gas giants though easy to spot are rare.

Indeed, if we must draw a distinction, then absent any prior prejudices it would not be between planets and dwarf planets, but between gas giants and solid planets. Earth and Mars are a lot more like Ceres than they are like Jupiter. (Realistically though we're not going to make either Earth or Jupiter not a "planet" of course.)