MTU (112) said: “Because an orbit doesn’t define a planet as a planet is – or should be called – a planet wherever its found. (Aside from not being a moon which I do include.)”
This is contradictory.You count the thing at the focus of the object’s orbit as relevant (i.e. does the object orbit a body that sustains nuclear fusion or not?), yet you dismiss what else shares the orbital region.

Well there’s a pretty huge and clear difference between directly orbiting another planet and not doing so. That’s just axiomatic really.

Also, no I don’t require a planet to orbit a star at all let alone our Sun like the IAU do so I’m not sure what you’re meaning there and you seem to (again) be misunderstanding me.

This, too, is an invalid analogy. Any manufactured item can be categorised according to sctrictly-known parameters. Solar system objects are naturally-formed, not manufactured.

So what? The analogy was illustrative of the general principle. Which is that something reemains thatsomething whether it is loctaed inone spot or another. If you prefer we could use ducks or birds instead for exactly the same point. A duck is a duck whether it is flying amidst a flock in the air or swimming alone in the creek or has been injuredand is sitting ina cage at the vets!

Same way Pluto is – or should be – a planet whether it orbits where it doers with its flock of fellow ice dwarfs or was somehow found orbiting alone around another star or was not orbiting a star at all.

And they [solar system bodies] naturally form divisions. The four inner planets; the four gas / ice giants; a whole bunch of asteroids in the main belt; and a whole bunch of icy bodies in the EKB. Plus some other small stuff in eccentric orbits (mostly comets) or at stable points on the orbits of larger bodies (Trojans).

Or as I like to describe things we have three main types of planet in our solar system – the rocky, the gassy and the icy!

Sure, there are divisions. Most of the inner planets form their own belt close into our star, theditto the gas giants have their zone and luckily don’t stray fromitas eccentric orbiter typeexoplanets do and the icedwrafs have their own realm beyond that. I don’t see otr think thatmake sice dwraf planets not planets. Planet is a very broad class just as star and galaxy are.

We define it pragmatically, in the same way we draw an arbitrary line between objects that are round and those that are not round.

Point is defining “orbital region” raises superfluous unnecessary questions and issues and thereby violates Occam’s razor. Why have it as a condition if we don’t have to and it unduly complicates things?

But it [Pluto] does share orbital space with other EKBs that it does not dominate, and Pluto is by far the junior partner in its relationship with Neptune. So, to a certain extent, Pluto’s orbital behaviour is dictated by Neptune’s gravity.

And toacertrainextent our Sun’s trajectory around the galaxy is dictated by Jupiter which doesn’t stop our Sun being a star!

You think that sharing orbital space matters, I don’t.

I am sure that the junior partner of the exoplanet pair exerts more influence over its sibling than Pluto exerts over Neptune.

Perhaps but I don’t think that is a key factor. They do have the same basic orbital relationship and are analogous.

In the same way, it is easy to list similarities between Pluto and the eight planets, but the same argument works both ways. The similarities between Pluto and comets don’t make it a comet. In exactly the same way, the similarities between Pluto and the eight planets don’t make it a planet.

Similarities are suggestive NOT definitive. I think we’re agreed on that, yes?

What makes Pluto a planet isn’t its similarities although they are indicators of what it may be, what defines planet, well that’s the question! You know my preferred answer to that and why I disagree with the IAUs faulty one.

@115. Nigel Depledge :

the superficial resemblances to some of the planets (such as moons and weather that you often cite as planetary traits).

You saying us having weather, oceans and an atmosphere is just superficial? Geez I’d hate to be without ‘em!

Serious answer, well, see above. Suggestive indicators versus defining traits.

@116. Nigel Depledge :

Arguments from authority do not wash. Their arguments may be no stronger than the ones you propose.

Or, worse yet, the ones *you* use!

Argument from authority eh? Funny, that’s how I see the IAU – unsuccessfully because fallaciously – justifying its rubbish definition!

What the quote goes to show is that other professional expert astronomers like me think the IAU got it wrong and that smaller planets are planets nonetheless. I am not alone in my thinking here.

Hey, if a majority of IAU members disagreed with the definition, then I’m sure it would have changed in the last 6 years. The fact that it has not is strongly indicative that the majority of IAU members accept it.

Or that the IAU is simply ignoring the majority who disagree and that the IAU leadership is unrepresentative of the majority opinion.

If the Royal Zoological Institite decreed that a duck was actually classed as a type of fish (y’know coz there’s a lot of ducks and fish and they all live in or on water!) would you just accept that even if it was only one loopy president of the RZI and a small cadre of his friends forcing that classification on the rest of us?

This [the rushed undemocratic nature of the anti-Pluto IAU defining meeting-ed.] is a separate issue that has nothing to do with whether or not the definition is valuable.

Yes so far as it goes.

However, it does suggest that the process of coming up with the IAU definition was flawed and a flawed process is likely to lead to a flawed result as in this case it clearly did.

Seems another planetary scientist supports Pluto’s planetary status too with this comment :

“If you’re important enough to have aquired five satellites you’re a planet.”
- Page 11, New Scientist , 21st July 2012.

By Kevin Baines, a planetary scientist at JPL recorded in the latest issue of New Scientist magazine on a page long article on the discovery of Pluto’s fifth moon.

Maybe the best definition of planet if we have to have one is to say that an object is a planet if it isn’t directly orbiting another planet (ie not a moon) and is between the mass of Vesta and the minimum mass for deuterium fusion of about eleven Jupiter’s allowing for metallicity?

Sound good to y’all?

Because an orbit doesn’t define a planet as a planet is – or should be called – a planet wherever its found. (Aside from not being a moon which I do include.)

This is contradictory.

You count the thing at the focus of the object’s orbit as relevant (i.e. does the object orbit a body that sustains nuclear fusion or not?), yet you dismiss what else shares the orbital region.

If you were to be purely logical, then you must either include or exclude consideration of the object’s orbit. It is not logical to cherry-pick what aspects of an object’s orbit matter enough to be included in a definition. Either the object’s orbital context matters (including what thing the object orbits) or it does not.

This allows for a planet being a planet wherever it is whether orbiting close to its star, far from its star or not around a star at all – just as an aircraft is still an aircraft whether it is in a hanger, on the runway or in flight and that’s true whether we’re talking about a small cessna or a jumbo jet.

This, too, is an invalid analogy. Any manufactured item can be categorised according to sctrictly-known parameters. Solar system objects are naturally-formed, not manufactured. And they naturally form divisions. The four inner planets; the four gas / ice giants; a whole bunch of asteroids in the main belt; and a whole bunch of icy bodies in the EKB. Plus some other small stuff in eccentric orbits (mostly comets) or at stable points on the orbits of larger bodies (Trojans).

It’s the main reason that, even before Eris et al. were discovered, some people were questioning Pluto’s status as a planet – because (as far as anyone could tell) Pluto was more like other KBOs than it was like other planets, and unlike Mercury to Neptune, Pluto did not dominate the behaviour of other objects in the same orbital region.

So how big is an orbital region? How do we define that? Why should it matter?

We define it pragmatically, in the same way we draw an arbitrary line between objects that are round and those that are not round.

Alternatively, we could define that region by drawing two circles, with radii equal to the aphelion and perihelion distances of the object. At the end of the day, it doesn’t really matter very much, because any sensible definition of the object’s orbital region will give us the same result in terms of the definition of a planet. This is because the solar system has a natural and obvious demarcation between the eight objects that dominate their orbital regions and all the other objects that do not.

Ask not “why should it matter?”. Ask “why should we dismiss it from consideration?”.

Pluto does control or influence a lot of space – it has five moons one of them relatively large

But it does share orbital space with other EKBs that it does not dominate, and Pluto is by far the junior partner in its relationship with Neptune. So, to a certain extent, Pluto’s orbital behaviour is dictated by Neptune’s gravity.

as well as being in an orbital relationship with Neptune analogous to that of a pair of massive gas giants around the orange dwarf star HD 45364. Should the junior of those exoplanets then be discounted from planetary status despite having thrice Neptune’s mass?

I am sure that the junior partner of the exoplanet pair exerts more influence over its sibling than Pluto exerts over Neptune. Neptune has something of the order of 10,000 times Pluto’s mass. Neptune quite obviously dominates Pluto, and Pluto will have almost no influence at all over Neptune’s movement.

Or better yet maybe we should ask astronomers who specialise in the area of planets like Jim Bell, professor of Astronomy and Planetary Science, University of Arizona who I quoted in comment #42 here saying Vesta is the smallest terrestrial planet!

Or how about Alan Stern an expert on Pluto who stated the IAU definition was “idiotic” – his words not mine?

Arguments from authority do not wash. Their arguments may be no stronger than the ones you propose.

Or the many other astronomers who strongly disagree

Hey, if a majority of IAU members disagreed with the definition, then I’m sure it would have changed in the last 6 years. The fact that it has not is strongly indicative that the majority of IAU members accept it.

with what one dubious and rushed and not very democratic or representative meeting decreed?

This is a separate issue that has nothing to do with whether or not the definition is valuable.

(Hmm .. Earth has water and ice, so do comets ergo Earth is a comet by your logic, Nigel? )

Just realised I missed out addressing this bit in #114.

No. In the same way that Pluto isn’t a comet, thoses similarities don’t make Earth a comet. In fact, you are stretching things a bit there, because Pluto contains many of the same volatile substances that comets contain, whereas Earth only shares some of the less volatile components (water and rock).

However, you make a very good point seemingly without realising it.

If that argument holds true about Earth and comets, i.e. the superficial resemblances not making Earth a comet, then it also holds true for some of the arguments you put forth about Pluto being a planet, i.e. the superficial resemblances to some of the planets (such as moons and weather that you often cite as planetary traits).

Yes, Pluto has some similarities with comets as do some exoplanets and other worlds in our solar system – but that doesn’t make Pluto a comet because it has those other key differences.

(Hmm .. Earth has water and ice, so do comets ergo Earth is a comet by your logic, Nigel? )

I agree that Pluto ain’t a comet. I was merely pointing out that it is easy to list similarities between Pluto and comets.

In the same way, it is easy to list similarities between Pluto and the eight planets, but the same argument works both ways. The similarities between Pluto and comets don’t make it a comet. In exactly the same way, the similarities between Pluto and the eight planets don’t make it a planet.