Sorry to ruin a great ongoing debate, but does it really matter what we call, or do not call, planets anyway? To believe so is really human – centric. I too was initially upset that Pluto was redefined, but it has not made much of a difference in my personal or scientific life. There will always be layman-term planets and scientific, what-ever-we care- to call-them bodies circling stars. So, lets call Pluto whatever we want, write papers using a common scientific language, – and congratulate HARPS on their work.

I agree.

Any chance you could persuade MTU to stop bashing the IAU?

3. Attempts to justify Pluto as a planet according to its characteristics such as possessing moons, despite the fact that you have found no characteristic that is possessed only by the eight IAU planets and not by any other solar system body apart from Pluto. In fact the eight IAU planets have only one characteristic that is unique to planets – they gravitationally dominate their orbital region.

Er, doesn’t our Sun do that?

Also that gravitational dominence is :

I) Relative and dependent on other factors such as the closeness of other large planets.

II) Temporary – because in the past there were other planets that collided & otherwise interacted with the current ones eg. the gas giant we think was ejected from our early solar system and the mars-sized planet that hit the Earth creating our Moon – plus in future Mercury’s orbit may get pulled into a more elliptical state leading to chaos and possibly collisions in the inner solar system.

III) Not sufficient basis to determine what aplanet is because of the points above and because we know of exoplanets that are clearly planets but not so gravitationally dominant eg. HD 45364 which is analogous to the Neptune-Pluto orbital relationship but with two gas giants. (Click on my name for source & info.)

4. Emotional appeals based on unjustified criticism of the IAU (to whit, see your employment of rhetoric above).

My criticism of the IAU on this is completely justified and indeed has been justified repeatedly in almost every post I’ve made here on this. Your refusal to accept this reality doesn’t make it go away.

5. “Dwarf planets are still planets, aren’t they?” – this is a flaw with the term “dwarf planet”, not a flaw in the definition of “planet”.

A definition of “planet” that excludes dwarf planets is as flawed and as blatantly erronous as a definition of “star” that excludes dwarf stars or a definition of dwarf animals, plants or people that excludes the dwarf varieties of those would be!

How can you really, *really* NOT get that, Nigel Depledge!?! *Headdesk.*

In other words – stop whining and live with it.

No. What the IAU decided was wrong, W-R-O-N-G, wrong. It is bleedingly obvious they got it wrong and this isn’t something that I think should be ignored but instead needs correction. ASAP.

If, say, the US Congress had decreed that pi = 3 or 1+ 1 = 5 would you be happy to let that stand because the Congress had so ruled and they’re the boss or would your view on that be the same as mine regarding Pluto’s planethood?

I don’t think you can call Pluto a non-planet any more than you can say pi =3 or 1 +1 = 5.

@85. Nigel Depledge : Very clearly we disagree on that and on this issue.

@86. Nigel Depledge :

Your arguments boil down into five main categories:
1. Hypothetical scenarios that break the IAU definition. Yet you fail even to attempt to show why these scenarios are relevant to a definition tailored specifically to our solar system as it exists now.

Our solar system as it exists now is NOT how it was before or will be in the future and is NOT typcial of most of the exoplanetary systems we have disciovered since.

Therefore defining planets narrowly in view of our current solar system is misleading and false and excessively exclusive just as it would be to say that because bats fly all mammals fly. It overlooks the reality that most planets are not like those in our currrent solar system – eg. Hot Jupiters, pulsar planets, Super-Venuses, etc ..

Are you really rejecting the Copernican “mediocrity” principle of science as much as you appear tobe doing here? I am baffled and frustrated and exasperated by just how much you are NOT seeming to get this & the other points I keep raising.

I thought you were smarter than that Mr Depledge.

<blockquote.2. Attacks on the orbital clearance criterion – or the “how clear is clear?” question – while ignoring both (a) the obvious and natural discontinuity between Mercury, Venus, Mars, Earth, Jupiter, Saturn, Ouranos and Neptune and the other solar-system objects, and (b) the same necessary pragmatism required to apply the gravitational roundness criterion.

See comnmet #96. (MTU – April 15th, 2012 at 6:34 am.) Yes, ice dwarfs are quite different sorts of worlds from gas giants and rock dwarfs just as those planetary types are equally different from each other. That doesn’t make them non-planets.

This is hollow rhetoric. Pluto has been reclassified, not “demoted”.

Reclassifying Pluto as an asteroid – effectively – a non-planet that doesn’t count as a planet is very obviously a demotion and a reclassification can certainly also be a demotion. You & the IAU you seeminglyworship just can’t get away from this reality.

Once more unto the comment #80 of Nigel Depledge -

c. Why we should ignore the obvious and natural discontinuity that exists in our solar system (whereby Mercury, Venus, Earth, Mars, Jupiter, Saturn, Ouranos and Neptune all stand out as distinct objects whereas Pluto and Ceres seem to be only the largest objects in classes of many similar objects).

Jupiter and Saturn, Earth and Venus are the largest examples of their classes of similar objects too. Does that disqualify them from planethod? No. Why then should we disqualify Pluto and Eris from planethood?

How about the natural discontinuity between supermassive Jupiter which emits more radiation than it recieves from our Sun Vs all the other planets? In many respects, Jupiter is far less “worldly” than Pluto is and far more different to what most people think of as planets.

If we have to say that some planets are so alien, such different things from what we usually think of by the word “planet” then the gas giants are that and more deserving of separate non-planetary status than Pluto and the ice dwarfs are, methinks. In many respects Pluto resembles Earth more than any world in our solar system except perhaps for Mars and maybe Titan.

d. Why we should have a definition that – despite our current ignorance of exoplanets – is universal, rather than a definition that applies specifically to our solar system.

Yeesh! You really seriously mean that question?

Because – d’uh! – astronomy applies to the whole universe, the entire cosmos about us not just our minuscule corner of it, that’s why!

e. Why the IAU should care about the opinion of people who are not working in a relevant field.

Because they are part of the wider human culture and society and planet. Because amateur astronomers & science communicators like myself are worjking invery relevantfieklds. ‘k?

f. Why anyone feels that the IAU is trying to impose anything on people who are not professional astronomers.

What? That;s an awfullybadly phrased question /assertion there Nigel. Maybe because that’s exactly what the IAU *are* doing in telling everyone in the world that they think Pluto doesn’t count as a planet justas inmy fictional example of the Riyal Zoological Society bizarrely and absurdly and wrongly claiming that they no longer accept that mice are animals!

Mind you, I’m sure the biological equivalent of the IAU have far more sense than to make any such ridiculous statements. I expected much better from the IAU too.

7d. Titan also has weather, while Mercury has pretty much no weather to speak of.

Pluto also has weather. Titan is a remarkable moon – moon because it orbits Saturn. If Mercury or Pluto orbited Saturn -or another larger planet then they’d be moons. They don’t therefore they ain’t.

You clearly accept Mercury as a “proper” planet (as do I) why then will you not accept Pluto as one given Pluto boasts more typically planetary features than Mercury does?

7e. At least two planets have no moons, and it has been noted that some asteroids have moons. Moons are not a defining planetary characteristic.

I never said they were a *defining* characteristic but I do think they are an *indicative* or IOW *suggestive* characteristic of something being a planet – if it is round and not orbiting another planet or ever shone by core nuclear fusion.

I have yet to see any cogent argument explaining any of the following:
a. Why we should classify Pluto as a planet rather than as a KBO, when its characteristics seem to resemble other KBOs more than the eight planets.

Pluto unlike other Edgeworth-Kuiper Cometary Belt Objects (EKCBOs) is round, is relatively massive, has seasons and much more that usual non ice dwarf EKCBOs lack.

Pluto is NOT just an asteroid or cometary nuclei but something that is very obviously different and more planet-like than them.

b. Why we should ignore the possible logical issues with the “how round is round?” question, while focussing on possible logical issues with “how clear is clear?”

Because gravitational self-roundness is a much better and clearer and mor eeasilydetrmined and more useful criteria than orbital clearence is.

You can see at a glance of something is round and it isan intrinsic quality not one dependent on toomany other more complex factors like having a clear orbit is.

A planet is round where-ever it is in a planetary system but whetheror not it has a clear orbit depends on what its surrounding environment is like, how far from its star it is, how stable or chaotic the orbits of other planets in its system are, etc ..

That’s why.