This whole thing is a matter of discussion and varying opinions. Yes the IAU "settled the question" but their decision doesn't have unanimous support amongst astronomers generally. With Alan Stern of the New Horizons mission one of those who argues strongly that Pluto is a planet. Note that he is one of the two authors of the paper that introduced the idea of "clearing the neighbourhood" in the first place. Clearing the neighbourhood.
Most of the answers here explain why it is a dwarf planet and not a planet, so I think for balance may help to have an answer that shows the other POV (Point Of View), that perhaps we should re-classify it back as a planet again.
At the moment there's a division between planets that clear their neighbourhood according to their definition all happen to be big, with Mercury the smallest. And the ones that don't, like Pluto, Eris etc, are all rather small.
But in the future we will surely find more objects larger than Pluto.
The WISE spacecraft has has ruled out the possibility of a Saturn sized object out to 10,000 times the Earth - Sun distance, and a Jupiter size or larger object out to 26,000 times that distance. (I.e. 26,000 AU.)
But we might still find Earth or Mars sized objects out there. Some think that's the explanation of the Kuiper cliff.
If we find a Mars sized object, or even an Earth sized object, a bit beyond 50 au, which helps to shepherd the comets in the Kuiper belt - but is not able to clear its orbit according to the IAU definition - is that a planet?
At the moment we have two discrete populations, planets that clear their orbit and small "dwarf planets" that don't. But there may be many things in between that are large and don't clear, or only partly clear.
I think the IAU definition will begin to fall apart if we find planets like that as it will seem odd to many to say that a Mars or Earth sized dwarf planet is not a planet.
The IAU made the decision but immediately afterwards there was a fair bit of criticism of how the decision was made by other planetologists.
For instance the decision was made in a meeting that might not have been very representative of astronomers generally. Only 5% of the members of the IAU were present to vote - and perhaps 1% of all working astronomers had the opportunity to vote. Were they representative enough of the general astronomical community to make this decision? Should non attending members been involved as well, and astronomers not in the IAU?
Inside the planet definition process
Anyway - there's nothing to stop anyone calling Pluto a planet if they want to :). We have freedom of speech, and it's not harming anyone to do that.
Sometimes people do call Pluto a planet, especially in conversation, sometimes in articles also, if they are sympathetic to this idea that it should be classified as a planet.
Actually there's a petition you can sign here if you want to support the view that Pluto should be reclassified back as a planet again, in view of the new discoveries, showing a surprisingly complex world with geological processes, nitrogen glaciers etc.
International Astronomical Union: Declare Pluto a Planet #PlutoFlyby
I signed the petition myself, and this is what I said there:
There's no doubt that this idea of "clearing the neighbourhood" is a useful concept in study of planetary systems. Where there's a parameter you can calculate to tell you how effective, e.g. the Earth is at clearing its neighbourhood. The answer is, very effective, any NEAs will be cleared out within about 20 million years unless in a resonant orbit - either to hit the Earth, Moon, Sun, Jupiter, Mars etc, but they can't stay in the vicinity of the Earth in an independent non resonant orbit.
It's a continuous parameter so even Pluto does clear its orbit to some degree, and so you have to choose a "cut off" point at which the planet clears its orbit sufficiently for the definition. But Pluto is many orders of magnitude less effective at "clearing its orbit" than Earth. So if that's your criterion, to use that parameter, it is reasonable to exclude Pluto.
The only question is a terminological one - is this how we want to understand the word "planet"? Is it a convenient and good and useful way of using the word? I.e. more a question of language than science.
After a fair bit of thought, then I'm inclined to side with Alan Stern on this, for the reasons given.
So, the solar system, I'd say, has four gas giant planets: Jupiter, Saturn, Uranus and Neptune, four medium sized rocky planets Mercury, Venus, Earth and Mars, and numerous dwarf sized planets Ceres, Pluto, Eris, Haumea, ... And some of them are better at clearing out their neighbourhood than others.
But anything large enough to reach a gravitational equilibrium shape is a planet, if it orbits a star and is not a moon of another planet and if it isn't a star itself or a brown dwarf "almost star". If two such objects orbit an axis not within either of them it's a double planet, making Pluto and Charon the only known double planet so far.
Any gravitational equilibrium shape would count including rapidly spinning three axis ellipsoids like Haumea, and roche-world type double planets, or planets with three or four Roche type lobes, or even "donut shaped planets" if such exist (So You Thought You Knew What Planets Look Like? ... Shapes Of Rapidly Spinning Planets) - I'd call them all planets so long as they are big enough to self gravitate and reach gravitational equilibrium - unless, of course, they are moons, brown dwarfs or stars.
And I'd use the word "dwarf" to refer to the size of the planet, not to whether or not it clears its neighbourhood. As it happens none of the dwarf planets in our solar system clear their neighbourhood, if you use the word to refer to e.g. planets of around the size of the Earth's Moon or smaller.
But in other exoplanet solar systems around dwarf stars, perhaps some tiny planets would "clear their neighbourhood", if so I'd still call them dwarf planets if they are small. But as a flexible word so in some contexts, so long as you make it clear what you mean, you could call the Earth a dwarf planet.
If you want a more precise term, Alan Stern suggested calling Earth, Mercury, Venus and Mars dwarf planets, and Pluto, Ceres, Eris etc "sub dwarf" planets. See Page on swri.edu
It just seems rather odd to define "dwarf" as meaning "clears its neighourhood" which has nothing at all to do with the way this word is normally understood in English.
With the IAU's definition, if we found a Neptune sized object somewhere far out in the Oort cloud - there is no way it could "clear its neighbourhood" so we'd have to call it a "dwarf planet" which seems a decidedly odd use of the word "dwarf", if you can in principle have a Neptune sized dwarf, and a Mercury sized non dwarf.
If you need a word for this concept, then coin a new word or just say "clears its neighbourhood".
I understand that this seems a minor matter. But I think clarity of basic concepts may have wider reaching effects. The simpler and easier to understand the basic concepts, then the easier it makes it for everyone all the way through the subject.
In maths when you work with axiom systems, you are constantly trying to find simpler definitions and easier to state and understand axioms. So I think that's why I, as a mathematician who worked on foundations of mathematics and axiom systems, find the simpler definition of a planet particularly appealing.
While, to call it a "dwarf planet" but not a planet, if it is the size of Neptune in the Oort cloud, and a planet, and not a dwarf if it is the size of Mercury and close to the sun - this is so far from natural ways of thinking about language - and based on rather recondite ideas - that it seems more likely to lead to confused ideas rather than to help with clarity of thinking in the future.
Same also for the other rather paradoxical conclusions you end up with if you use this definition, such as - that it might change from a dwarf planet (and not a planet) to a planet (and not a dwarf) and back again if some episode sends more than usual amounts of material into its neighbourhood.
Or for that matter, once we have megatechnology we could turn Pluto into a planet simply by artificially clearing out everything in its neighbourhood. Then there would be nothing for it to clear out and so by default it would then count as a planet by the definition, and would no longer be a dwarf. Again that just seems confusing, to use the words that way.
You can't say it is wrong, this is natural language and it is often paradoxical in how it works. But given the chance of a simpler way of using language I'm inclined to go with that where possible.
And because it is an observational science - simpler also would include easy to observe - seems to make sense to use concepts that are easy to observe as your basic concepts.
So that way - you can tell just by looking at it if it is a planet or not. Just as you can do with stars or brown dwarfs, making it a similar kind of definition to the definition of a star or a brown dwarf.
If it isn't orbiting any star but wandering the space between the stars, I'd still call it a planet actually, if not a brown dwarf or star itself, but a "rogue planet" or some such. After all the word planet just means "wandering star" in Greek. So I don't see why it has to be bound to another star
That's just me stating a POV ("Point Of View") in support of Alan Stern's views - but - are plenty of answers here saying that Pluto is not a planet so may be interesting to have a answer putting forward another POV.
For an overview see This is the post where you can comment about the IAU planet definition by Emily Lakdawalla
And wikipedia article: IAU definition of planet
And: Pluto Has An Atmosphere, Moons, And Maybe Geological Activity. Can We Call It A Planet Now?
I've just done this as a science blog post here: Let's Declare Pluto As A Dwarf Or Sub Dwarf Planet AND A Planet!
Most of the answers here explain why it is a dwarf planet and not a planet, so I think for balance may help to have an answer that shows the other POV (Point Of View), that perhaps we should re-classify it back as a planet again.
At the moment there's a division between planets that clear their neighbourhood according to their definition all happen to be big, with Mercury the smallest. And the ones that don't, like Pluto, Eris etc, are all rather small.
But in the future we will surely find more objects larger than Pluto.
The WISE spacecraft has has ruled out the possibility of a Saturn sized object out to 10,000 times the Earth - Sun distance, and a Jupiter size or larger object out to 26,000 times that distance. (I.e. 26,000 AU.)
Artist's impression of the NASA Wide-field Infrared Survey Explorer. Researchers proved that Nibiru and Tyche can't exist in results published in 2014, using data that it collected in 2010 to 2011.
But we might still find Earth or Mars sized objects out there. Some think that's the explanation of the Kuiper cliff.
Has a dip at 50 au. Models showed the number of objects should double at 50 au. So some suggest there must be a planet, as large as Mars or Earth to "shepherd" the kuiper belt like Saturn's shepherding moons for its rings. What if we find an object large enough to do that, also as large as Mars or Earth, but which can't "clear its orbit" according to the IAU definition.
If we find a Mars sized object, or even an Earth sized object, a bit beyond 50 au, which helps to shepherd the comets in the Kuiper belt - but is not able to clear its orbit according to the IAU definition - is that a planet?
At the moment we have two discrete populations, planets that clear their orbit and small "dwarf planets" that don't. But there may be many things in between that are large and don't clear, or only partly clear.
I think the IAU definition will begin to fall apart if we find planets like that as it will seem odd to many to say that a Mars or Earth sized dwarf planet is not a planet.
ORIGINAL DECISION WAS MADE BY ONLY 1% OF WORKING ASTRONOMERS
The IAU made the decision but immediately afterwards there was a fair bit of criticism of how the decision was made by other planetologists.
For instance the decision was made in a meeting that might not have been very representative of astronomers generally. Only 5% of the members of the IAU were present to vote - and perhaps 1% of all working astronomers had the opportunity to vote. Were they representative enough of the general astronomical community to make this decision? Should non attending members been involved as well, and astronomers not in the IAU?
Inside the planet definition process
WE HAVE FREE SPEECH - YOU CAN CALL IT A PLANET IF YOU WANT TO :)
Anyway - there's nothing to stop anyone calling Pluto a planet if they want to :). We have freedom of speech, and it's not harming anyone to do that.
Sometimes people do call Pluto a planet, especially in conversation, sometimes in articles also, if they are sympathetic to this idea that it should be classified as a planet.
PETITION IN WAKE OF DISCOVERIES BY NEW HORIZONS
Actually there's a petition you can sign here if you want to support the view that Pluto should be reclassified back as a planet again, in view of the new discoveries, showing a surprisingly complex world with geological processes, nitrogen glaciers etc.
International Astronomical Union: Declare Pluto a Planet #PlutoFlyby
I signed the petition myself, and this is what I said there:
"The rule about not clearing neighbourhood doesn't make too much sense to me what about the Jupiter trojans (about as numerous as the asteroids of the asteroid belt) and Jupiter family comets and even Neos?
And for that matter - has Neptune cleared Pluto out of its orbit?
And anyway - could anything as far out as Pluto clear its neighbourhood given the wide variety of inclinations and eccentricities of KBOs?
- and it's a bit of a strange definition if you don't know if it is a planet until you have done a census of all the other objects in its neighbourhood - and if applied to exoplanets - would mean that you just don't know if they are planets or not until you know a whole lot more than we can expect to know by remote observation in early stages.
And does that mean that in the early solar system - e.g. in late heavy bombardment when a whole lot of objects came into the inner solar system - possibly from the outer solar system, or wherever they came from, that for a while Earth ceased to be a planet until it had cleared them all out of its orbit?
Or if you make it a kind of theoretical thing "capable of clearing its orbit, but right now it is a bit overwhelmed by new material so hasn't actually done it yet" then it just seems a rather quixotic way to define a planet.
I just don't find the idea very convincing myself, from what I've seen of it so far.
So, I'm inclined to go with Alan Stern there, it doesn't make much sense to me to not call Pluto a planet for this reason.
Though calling it the largest of the dwarf planets is also good, but a dwarf planet surely is a planet? Again it seems kind of logically odd to not call them planets. I'd call them all planets, and qualify them as "dwarf planets" to say they are small - and add in Ceres as a planet, but a dwarf planet. And leave "dwarf" as vague so that compared to the gas giants, then Earth and Venus are also dwarf planets, but in ordinary use a dwarf planet would be a planet that is smaller than Mars, around the size of the Moon, not much larger than it.
VALUE OF "CLEARING ITS NEIGHBOURHOOD" AS A SCIENTIFIC CONCEPT
There's no doubt that this idea of "clearing the neighbourhood" is a useful concept in study of planetary systems. Where there's a parameter you can calculate to tell you how effective, e.g. the Earth is at clearing its neighbourhood. The answer is, very effective, any NEAs will be cleared out within about 20 million years unless in a resonant orbit - either to hit the Earth, Moon, Sun, Jupiter, Mars etc, but they can't stay in the vicinity of the Earth in an independent non resonant orbit.
It's a continuous parameter so even Pluto does clear its orbit to some degree, and so you have to choose a "cut off" point at which the planet clears its orbit sufficiently for the definition. But Pluto is many orders of magnitude less effective at "clearing its orbit" than Earth. So if that's your criterion, to use that parameter, it is reasonable to exclude Pluto.
The only question is a terminological one - is this how we want to understand the word "planet"? Is it a convenient and good and useful way of using the word? I.e. more a question of language than science.
After a fair bit of thought, then I'm inclined to side with Alan Stern on this, for the reasons given.
FOUR GAS GIANT PLANETS, FOUR MEDIUM SIZED ROCKY PLANETS AND NUMEROUS SMALLER DWARF SIZED PLANETS - WHY NOT?
So, the solar system, I'd say, has four gas giant planets: Jupiter, Saturn, Uranus and Neptune, four medium sized rocky planets Mercury, Venus, Earth and Mars, and numerous dwarf sized planets Ceres, Pluto, Eris, Haumea, ... And some of them are better at clearing out their neighbourhood than others.
But anything large enough to reach a gravitational equilibrium shape is a planet, if it orbits a star and is not a moon of another planet and if it isn't a star itself or a brown dwarf "almost star". If two such objects orbit an axis not within either of them it's a double planet, making Pluto and Charon the only known double planet so far.
Any gravitational equilibrium shape would count including rapidly spinning three axis ellipsoids like Haumea, and roche-world type double planets, or planets with three or four Roche type lobes, or even "donut shaped planets" if such exist (So You Thought You Knew What Planets Look Like? ... Shapes Of Rapidly Spinning Planets) - I'd call them all planets so long as they are big enough to self gravitate and reach gravitational equilibrium - unless, of course, they are moons, brown dwarfs or stars.
AND USE THE WORD "DWARF" TO REFER TO SMALL PLANETS
And I'd use the word "dwarf" to refer to the size of the planet, not to whether or not it clears its neighbourhood. As it happens none of the dwarf planets in our solar system clear their neighbourhood, if you use the word to refer to e.g. planets of around the size of the Earth's Moon or smaller.
But in other exoplanet solar systems around dwarf stars, perhaps some tiny planets would "clear their neighbourhood", if so I'd still call them dwarf planets if they are small. But as a flexible word so in some contexts, so long as you make it clear what you mean, you could call the Earth a dwarf planet.
If you want a more precise term, Alan Stern suggested calling Earth, Mercury, Venus and Mars dwarf planets, and Pluto, Ceres, Eris etc "sub dwarf" planets. See Page on swri.edu
Alen Stern's suggestion is to call Earth and Venus, Mars, Mercury - the planets to the left of the picture, dwarfs, and the really tiny Pluto, Charon, KBO objects etc, to the right of the picture, sub dwarfs. But they are all planets. Image from: Illustrations - Roberto Ziche
THE ENGLISH WORD "DWARF" DOESN'T MEAN SOMEONE OR SOMETHING WHO "CLEARS OUT YOUR NEIGHBOURHOOD", IT MEANS SOMETHING SMALL
It just seems rather odd to define "dwarf" as meaning "clears its neighourhood" which has nothing at all to do with the way this word is normally understood in English.
With the IAU's definition, if we found a Neptune sized object somewhere far out in the Oort cloud - there is no way it could "clear its neighbourhood" so we'd have to call it a "dwarf planet" which seems a decidedly odd use of the word "dwarf", if you can in principle have a Neptune sized dwarf, and a Mercury sized non dwarf.
If you need a word for this concept, then coin a new word or just say "clears its neighbourhood".
VALUE OF SIMPLICITY IN LANGUAGE
I understand that this seems a minor matter. But I think clarity of basic concepts may have wider reaching effects. The simpler and easier to understand the basic concepts, then the easier it makes it for everyone all the way through the subject.
In maths when you work with axiom systems, you are constantly trying to find simpler definitions and easier to state and understand axioms. So I think that's why I, as a mathematician who worked on foundations of mathematics and axiom systems, find the simpler definition of a planet particularly appealing.
While, to call it a "dwarf planet" but not a planet, if it is the size of Neptune in the Oort cloud, and a planet, and not a dwarf if it is the size of Mercury and close to the sun - this is so far from natural ways of thinking about language - and based on rather recondite ideas - that it seems more likely to lead to confused ideas rather than to help with clarity of thinking in the future.
Same also for the other rather paradoxical conclusions you end up with if you use this definition, such as - that it might change from a dwarf planet (and not a planet) to a planet (and not a dwarf) and back again if some episode sends more than usual amounts of material into its neighbourhood.
Or for that matter, once we have megatechnology we could turn Pluto into a planet simply by artificially clearing out everything in its neighbourhood. Then there would be nothing for it to clear out and so by default it would then count as a planet by the definition, and would no longer be a dwarf. Again that just seems confusing, to use the words that way.
You can't say it is wrong, this is natural language and it is often paradoxical in how it works. But given the chance of a simpler way of using language I'm inclined to go with that where possible.
And because it is an observational science - simpler also would include easy to observe - seems to make sense to use concepts that are easy to observe as your basic concepts.
SO THAT AS FOR A STAR OR A BROWN DWARF, CAN TELL JUST BY LOOKING AT IT IF IT IS A PLANET
So that way - you can tell just by looking at it if it is a planet or not. Just as you can do with stars or brown dwarfs, making it a similar kind of definition to the definition of a star or a brown dwarf.
If it isn't orbiting any star but wandering the space between the stars, I'd still call it a planet actually, if not a brown dwarf or star itself, but a "rogue planet" or some such. After all the word planet just means "wandering star" in Greek. So I don't see why it has to be bound to another star
That's just me stating a POV ("Point Of View") in support of Alan Stern's views - but - are plenty of answers here saying that Pluto is not a planet so may be interesting to have a answer putting forward another POV.
For an overview see This is the post where you can comment about the IAU planet definition by Emily Lakdawalla
And wikipedia article: IAU definition of planet
And: Pluto Has An Atmosphere, Moons, And Maybe Geological Activity. Can We Call It A Planet Now?
I've just done this as a science blog post here: Let's Declare Pluto As A Dwarf Or Sub Dwarf Planet AND A Planet!
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Robert Walker
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