Again, you seem to be missing – or just refusing to see – my point entirely that their orbits are NOT cleared thus like Pluto they aren’t planets if you apply the IAU decree. That these objects have many times Jupiter’s mass but don’t qulaify basedonthe IAU’s anti-Pluto law shows again how ridiculous and unworkable it is.

The point you seem to be missing – or just refusing to see – is that I don’t adhere to the IAU definition, nor do I consider it to be workable. As you seem so eager to point out, the IAU definition can be made to exclude all the major planets depending on how pedantically you adhere to the wording. In case you didn’t notice, HD 45364′s planets do not qualify under the IAU definition because they aren’t orbiting the Sun.

So what *is* meant by clear – how many asteroids or comets or small planets or large planets can cross or interescet or be in that orbit before it’s considered “clear”?

How far out must the orbit be cleared to?

At what point do you & the IAU acknowledge how large the space is and how that makes it impossible for an planet to clear everything from its orbit?

This is precisely the points you have missed because you are assuming I am working with the IAU definition. For a start I exclude objects in circumplanetary orbit (i.e. moons). I also exclude mean motion resonances, as these follow rather different stability rules from orbits which are located close to, but not in the resonance. Basically I am saying that members of asteroid belts should not be regarded as planets. Mercury, Venus, the Earth and Mars are clearly NOT members of an asteroid belt – they each exceed by several orders of magnitude the total mass of all the other objects that cross their orbits. Ceres and Pluto on the other hand are clearly members of “asteroid belts”. This is not a difficult concept to grasp, even if it may be somewhat more qualitative than quantitative. And just to spell it out in case it isn’t obvious, HD 45364′s configuration is nothing like the asteroid belt, or Neptune and the Plutinos, because there is no large population of similar-mass objects in one of the orbits.

Honestly, doesn’t this strike you as a far better and more scientifically and logically approproiate idea than the IAU’s definition with its illogical and unworkable “Orbital clearning” nonsense which was thrown inat the last minutye deliberately toexclude Pluto in an undemocratic and highlyu dubious manner by people whose motivations are also dubious?

No. Point 1 of your definition leads to extremely nonintuitive descriptions of several extrasolar systems, which contain objects exceeding the deuterium fusion limit in orbital configurations that look like planetary systems. The configuration of the systems around BD+20 2457, HW Virginis and HAT-P-13 (among others) make sense when you consider the companions to be planets, less so when you designate one (or both) of the companions as brown dwarfs.

Incidentally the politics of how the definition was arrived at are irrelevant to the question of how well the definition applies to nature: I don’t particularly care if it was arrived at democratically or not. The definition stands or falls on its own merits… in fact I believe it is pretty badly flawed, even if it is based on dynamical principles I agree with. And could you specify exactly what you mean by “dubious” motivations… care to point out some individuals and specifically state what their “dubious” behaviour was?

A moon’s worth was ejected and formed the Moon. Some of the mass undoubtedly ended up in the sun, and some of it went out into space – perhaps to land on other planetary bodies. Some is certainly ‘out there’ somewhere. The rest was incorporated into the planet, Earth and by differentiation spread throughout the planet. Lighter elements in the crust, heavier to the core.

HD 45364 has absolutely nothing to do with the Pluto/Neptune case apart from being in a 3:2 resonance.

And the 3:2 resonance means … ?

Let nme help you out here – it means that these planets have NOT cleared their orbits -& thus by IAU diktat they are NOT planets – which lets face it is just absurd – like the IAU definition.

The Fomalhaut and HR 8799 planets are orders of magnitude more massive than the protoplanetary discs, and are in fact NOT embedded within the main parts of the discs, … Not to say there isn’t a population of objects that intersects the orbits of these planets (rather like the centaurs in our solar system, or Jupiter’s Trojan asteroids), but again the planets are orders of magnitude more massive than the population.

Again, you seem to be missing – or just refusing to see – my point entirely that their orbits are NOT cleared thus like Pluto they aren’t planets if you apply the IAU decree. That these objects have many times Jupiter’s mass but don’t qulaify basedonthe IAU’s anti-Pluto law shows again how ridiculous and unworkable it is.

Your piece of sophistry there over what constititutes a “cleared orbit” just adds tothis revealing how problematic the vague and hard to define “cleared orbit” malarkey is.

So what *is* meant by clear – how many asteroids or comets or small planets or large planets can cross or interescet or be in that orbit before it’s considered “clear”?

How far out must the orbit be cleared to?

At what point do you & the IAU acknowledge how large the space is and how that makes it impossible for an planet to clear everything from its orbit?

Why include such a complicating, unnecesary and time and space dependent criterion in adefinition which should be basic and easy to apply?

What do we calla gas giant planet -or even an Earth mass one – that orbits in such a non-cleared orbit – an asteroid? A dwarf planet? An Edgeworth-Kuiperbelt obect? Yeah right ..! That would just be plain dumb -but then that’s the IAU ruling for you.

The outermost object in the pulsar planet system has a complex discovery history – it .. has not been described as a planet when the mass was constrained to be less than 1/5 Pluto

Factually incorrect. That’s how I’ve seen it described in ‘Astronomy’ magazine among other places.

This also raises the issue of what if a tiny planet with say the mass of Pluto or less ha sacleared orbit. It then *is* a planet depending on where it is -which again is unfair and inconsistent.

It all adds up to the logical and rational scientific conclusion that whether an orbit is clear or not is very much a *secondary* minor trait and NOT a defining feature.

Me : “If Earth was located where Pluto is and behaving the same as Pluto would it cease being a planet in your eyes? Really?”

Andy : Yes actually. If it were in a belt of roughly Earth-sized objects at similar orbital distances then yes I would not call this a swarm of planets at this distance, but some kind of belt.

Well that’s just ridiculous.

BTW. Would you describe our inner solar system as a “belt” making Earth, Venus, Mars and Mercury all dwarf (non)planets? Your argument leads to that conclusion. It is just that absurd.

Would you consider Earth not to be a planet if it were in orbit around a massive gas giant planet?

In that case it would be a Moon.

The three defining charcateristics I’d use for planet are pretty clear – and easily determined & defined :

a) If an object doesn’t & has never been able to shine by its own natural nuclear fusion.

(Thus Not a star or brown dwarf.)

b) If an object is in hydrostatic equilibrium ie. if its shape is controlled by its own gravity into a sphere or for fats spinning objects oblate spheroids.

(Thus not an asteroid or comet.)

&

c) If an object is NOT directly orbiting a planet or brown dwarf

(Thus not a Moon.)

Then its a planet – whether it has the mass of Ceres or the mass of Jupiter.

Simple. Easy to tell. And inclusive.

We can then divide the planetary category down into many types based on other factors – mass &composition (gas giant, rocky terrestrial planet or ice dwarf), orbital position (Hot Jupiter, Cold Jupiter, Cometary Eccentric orbiter) and even atmospheric conditions (Earth-like, Mars-like, Venus-like, etc ..)

Honestly, doesn’t this strike you as a far better and more scientifically and logically approproiate idea than the IAU’s definition with its illogical and unworkable “Orbital clearning” nonsense which was thrown inat the last minutye deliberately toexclude Pluto in an undemocratic and highlyu dubious manner by people whose motivations are also dubious?

Pluto is indeed a planet – and I think the IAU and you are one day just going to have to accept that reality. You may as well start now!

Relying on intrinsic properties of the object leads to mess and confusion. Considerations of the architecture of the host system lead to far fewer problems and a clearer picture of what is going on. (134340) Pluto is not a planet. ;-p

Ideally, I think we ought to have a classification system that can accommodate multiple subtypes to truly account for the great diversity of planetary types that are doubtless out there, but we don’t yet know enough about the potential varieties of planets to do this.

Yes, and even then it will be a lot of conflict, just look at the biologists that have upwards of 26 (!) different suggestions for definitions of species.

Btw, funny thing is that there are already several systems, and AFAIU the exoplanet definition doesn’t even have a clearing condition! It’s all misdirection from the malcontents.

What they should be arguing about is how the current definition of brown dwarf for practical purposes mislabels planets as dwarfs (and perhaps vice versa). At least, that is what I think is happening with the IIRC 13 solar mass limit. That is a real theft, … , um, loan for future adjustment!

I like that.

The distance and timing of a singular event don’t allow for any conclusion about the genereal probability for such an event.

a sample of one.

Not that I am especially good at statistics, but I don’t think either of these are true.

Besides already mentioned bayesian expectations, which can be used in testable hypotheses, frequentist probabilities apply as well. As Markk notes, the sample space of solar systems is huge, it is not “a sample of one”. An event in a finite sample space is just any subset of it, and there is no such thing as a “singular event”. (Unless you count the empty set, and then again you have a perfectly definable probability.)

Now there can be problems with probabilities if your event is sparse in all of sample space. If something is guaranteed to happen only once or twice, is the frequency definable (verifiable, testable)? But this is not such a case, since we have no theoretical or empirical reason to believe this is such a rare event. (And if we had, we would already know what we are trying to determine…)

JPL (who made the animation) published an HD version here:
http://www.nasa.gov/mission_pages/spitzer/multimedia/spitzer-20090810.html
http://www.nasa.gov/mov/377528main_V-Animation_Sorenson3.mov

It may not be bad astronomy, but that seems like bad statistics and a circular argument. To say that it’s not rare because it happened nearby is no different than to say it happened nearby because it’s not rare. It’s a sample of one.

The Fomalhaut and HR 8799 planets are orders of magnitude more massive than the protoplanetary discs, and are in fact NOT embedded within the main parts of the discs, which lie beyond the orbits of the outermost planets in these systems – the planets orbit within the cleared region interior to the disc. Not to say there isn’t a population of objects that intersects the orbits of these planets (rather like the centaurs in our solar system, or Jupiter’s Trojan asteroids), but again the planets are orders of magnitude more massive than the population. This is unlike the case of Pluto and Ceres which share their location in the solar system with objects that have similar masses.

The outermost object in the pulsar planet system has a complex discovery history – it was described as a possible fourth planet at the time when the evidence suggested a mass range which got up to 100 times that of Earth. Subsequent evidence revised this mass estimate downwards. It has not been described as a planet when the mass was constrained to be less than 1/5 Pluto – it was described as a possible asteroid or comet, not a planet. (Besides, it is not entirely clear whether the object exists at all or is just the effect of interstellar absorption)

If Earth was located where Pluto is and behaving the same as Pluto would it cease being a planet in your eyes? Really?

Yes actually. If it were in a belt of roughly Earth-sized objects at similar orbital distances then yes I would not call this a swarm of planets at this distance, but some kind of belt (though such a massive belt might be better regarded as an oligarch-stage planet-forming region). Would you consider Earth not to be a planet if it were in orbit around a massive gas giant planet? If you want to pursue this line of reasoning, please try and convince us that the major satellites of the gas giant systems should be regarded as planets.

What is your problem with our solar systems Ninth Planet?

That it’s a member of an “asteroid” belt, which has historically been a criterion for removing planetary status. Really you can split hairs over the IAU’s decision, which is poorly worded, but you can’t escape that fact.

And are all these complaints about Pluto’s status driven by the fact it was discovered by an American?

No. I, for one, am an Aussie.

OTOH, more than a few people have a sneaking suspicion that an anti-American bias was one factor influencing the Pluto-bashers.

Really, who cares.

Actually lots of people do!
Again, for one, me.

Scientifically it makes more sense for it to not be considered a planet.

Wrong, wrong, wrong & wrong. For a large number of reasons which I’ve already gone into a number of times.

How we define “planet” does matter – and the IAU definition which was drawn up specifically to exclude Pluto from planethood is an appallingly bad one from a scientific and logical viewpoint as you can see from my posts 67- 69 earlier.

Pluto is not alive and does not have feelings so the concept of “unfairness” is surely somewhat silly.

Pluto may be an inanimate object but people are not and this matters to people. Fairness and justice is a fundamental human principle and this also matters. Doing science too is a human activity when it is unfair it reflects badly on science and scientists.

Treating Pluto fairly is thus anything but silly – and your question there makes me wonder why do you seem to want Pluto treated unfairly. What is your problem with our solar systems Ninth Planet?

I think there is more for the IAU to do than waste their time reopening this dead can of worms.

As I’m sure you won’t be surprised to hear by now, I couldn’t DISAGREE more.

The anti-Pluto decison by the IAU makes them – quite rightly – look stupid and ridiculous. This tars astronomers and astronomy by association – astronomers should be able to say what a planet is and isn’t and have it make logical sense. At the moment they can’t do that. The Pluto debate has caused confusion and controversy and reflected poorly on the astronomical community.

Thus it *does* need to be addressed properly and the IAU’s disgraceful error in defining planet really must be corrected as soon as possible. I’d even go as far as saying the people who made the anti-Pluto decison should be made accountable and penalised in some way for bringing astronomy into disrepute. Their expulsion from the IAU and a ban from any further IAU votes could be considered appropriate.

But the main thing is that the IAU really do need to admit and correct their mistake. The longer they leave it the worse they – and, speaking here as an amateur astronomer myself , we – look and suffer.

On the other hand there is a clear qualitative and quantitative difference, easily observed, between objects that have cleared their orbital neighborhood, and those that haven’t. To develop a definition that does not acknowledge this plain fact in some way would in my mind be equally unsatisfactory.

Regarding your first sentence there: Is it? Really?

See the case of the exoplanetary system HD 45364 b & c which are analogous to the Neptune-Pluto planetary pairing in intersecting orbits in a 3:2 resonance. See Ken Croswell’s article : http://kencroswell.com/HD45364.html for more.

Or for that matter the imaged exoplanets of Fomalhautb & Hr 8799 which are still located in their protoplanetary disks. Seems to me this distinction is a *lot* less clear than you imply.

If Earth was located where Pluto is and behaving the same as Pluto would it cease being a planet in your eyes? Really?

Regarding your second sentence there : Why would you think it unsatisfactory?

Having a “Cleared orbit” is a function of a given planets time of existence and distance from its sun and thus is dependent on a whole number of factors extrinsic to its basic nature. (Eg. age, distance, the amount of other material present originally, gravity, how much a material is tossed its way by other planets in the system, etc .. ) I do not see why it does or should matter all that much.

IMHO “orbital clearing” is very much a secondary and contigent trait rather than a defining one.

@ 58. Ken Says:

I love the video, but have a small nit – maybe. The surfaces of the bodies look a little old for a system still in the equivalent of the Hadean. The smaller body even shows Moon-like craters including one with distinct impact rays. Surely the crusts would not be thick enough at this stage in the system to support such features?

Back on topic here – I agree & thought that too.

I also wondered why the planets wasn’t already molten at least in patches from the pounding they’d been getting as they formed – and why the gravity didn’t stretch both these exo-Mercury & exo-Lunar planets out more as they approached.

Besides, to me anyhow, the term “classical planet” would be far better used to describe the planets originally known to the ancient civilisations and earliest astronomers – the five “wandering stars” easily visible to the unaided eye of Venus, Jupiter, Mars, Mercury & Saturn.

Also not included in the letter are a few other points like the fact that Pluto has three moons, rings and an atmosphere which is more than other planets such as Mercury can boast – so if Pluto isn’t a planet why should we consider Mercury one? Of course, once we start arbitrarily ruling out planets w/o good reason* as happened to Pluto then where do we stop?

Plus there’s the whole incredibly anti-democratic and ideologically driven Prague IAU conference “vote” which excluded relevant experts like Alan Stern and was essentially an un-scientific dictator-style coup by the Pluto-bashers with only one side being given a chance. August 24th 2006 was, I think history will one day state as fact, the IAU’s most shameful and darkest day which hurt the reputation of astronomy as collateral damage along with disgracing that of the IAU.

—

* No, Pluto being located in the Edgeworth-Kuiper Belt and small is NOT in my view a good reason. At all.

I mourn for Triton. (weeps)

Bit early for that! Don’t mourn too much too soon – Triton still has millennia left to its life – plus it will be transformed into a great set of Neptunean rings! Oh,wait you meant as a planet not a moon?

@ 51. Peder Says:

Except if you put Earth where Pluto is it surely would clear the orbital path and hence be a planet again…

Bzzzt! Wrong – as Amphiox pointed out that’s just not the case.

@ 59. amphiox Says:

#47 StevoR: I assume that when you say “earth” you mean “earth and moon”, because if you swapped earth and pluto and left the moon in place, the moon would become the planet, and, in all likelihood, pluto would become the moon! (or be ejected from the orbit)

Well maybe … Or we could have two planets for the price of one : Pluto & our Moon. Three counting Charon as half of a double planet!

Or Pluto could be at the Lagrangrian L1 point in astable orbit with Earth & Moon – making for a double double planet! (Earth-Moon, Pluto-Charon) Wonder if a sort of three body suituation there would be stable? Might make things interesting for Pluto’s other small moons Nix and Hydra and its likely rings too!

@ 63 Laurel Kornfeld :

(BTW. Love your Pluto blog & thanks again for posting my “12 reasons why Pluto is a planet” there! Much appreciated. )

I have just sent the IAU* the following letter by email – everybody who agrees with this please feel free to copy and send as well:

***
Dear IAU administration

I am writing to request and strongly urge you to please revise the current IAU definition of planet which is logically flawed and inconsistent. I find the third “orbital clearing” criterion in the current IAU definition fails the ‘reductio ad absurdum’ logic test having ridiculous consequences.

Among these flaws is the bias towards planets orbiting closer to their primary. For instance, if the Earth orbited where Pluto does, it would no longer be considered a (“classical”) planet and were Pluto to orbit where Earth is it then would be considered a planet rather than merely a “dwarf planet”. Moreover, all planets in our solar system would be rendered dwarf planets by a strict use of the orbital clearing criterion since sun-grazing comets and asteroids cross the orbits of all planets making them not strictly clear and thus not strictly “planets”!

The current IAU definition is also absurdly limiting in excluding objects that clearly would be termed planets for instance were exoplanets found in the ‘double planet’ configuration of two planets of even superJovian mass then neither would be classified as a planet nor – by current IAU definition – could we ever have colliding or intersecting planets. There are already examples of exoplanets that are accepted as being planets yet breech the strict IAU definition – notably:

1.Fomalhaut b which hasn’t “cleared its orbit” but circles within a protoplanetary disk. (The same situation also applies to the planets around HR 8799.)

2. HD 45364 b & c which are analogous to the Neptune-Pluto planetary pairing in intersecting orbits in a 3:2 resonance.

3. The outermost and smallest “pulsar planet” PSR B1257+12 e which has a mass only 1/5th of Pluto’s and yet is termed a planet.

The orbital clearing criterion is also vague, arbitrary and poorly defined – it adds an unnecessary and confusing level of complexity to a definition which should be basic and easily determined.

There is also the inconsistency of having dwarf stars accepted as “proper” stars and yet dwarf planets rejected by the IAU as “proper” planets. It seems ludicrous to suggest that dwarf stars – which include our Sun and 90% of all stars are NOT “proper” (‘classical?’) stars yet this is the logical extension of the “dwarf planets are not “proper planets” current definition and exactly analogous.

I think these logical flaws and inconsistencies make the current IAU definition unscientific and, in the long term, unworkable. I consequently strongly urge and request that the current definition of the term “planet” be reconsidered and changed to a more rational, more inclusive and scientifically justifiable definition. I suggest something like the original 2006 Prague definition which excluded the illogical and absurd “orbital clearing” criteria, eg. defining a planet as an object which:

a) Has enough gravity to be in hydrostatic equilibrium (i.e. is round or if rapidly spinning a rotationally flattened spheroid.)

b) Is incapable of ever being self-luminous shining by nuclear fusion. (I.e. is not a star or brown dwarf.)

c) Is not directly orbiting another planet. (I.e. is not a moon.)

The classifications of planets so defined could then be further sub-divided into other specific types based on composition (rocky or terrestrial, gas giant, ice dwarf) and orbit (Hot Jupiter, Hot Neptune, Pulsar planet) and so forth.

I think that this simple, clear, logical and scientifically useful inclusive definition of the term “planet” which would include the current dwarf planets is vastly superior to the current scientifically and logically flawed, highly controversial and publicly unpopular and definition.

I beg you to you to consider this definition and above all to change the current one to a more rational and reasonable one which I strongly believe should count Pluto and Eris as full planets.

Sincerely:

Steven C. Raine
(w my email, phone & address)

***

* Sent to their general email address & to IAU General Secretary, Dr. Karel A. van der Hucht & IAU Executive Assistant, Vivien A. Reuter. Hope those are the right people – were the main ones listed on the contact addresses anyway. I am also going to post (snail mail) a copy too.

(weeps)

As for those who keep harping on about how the orbital location is absurd, who mourns for Triton? Getting booted out of potential (dwarf) planethood just because it got captured into Neptune orbit. So unfair…

The IAU GA is going on right now. Anyone who wants to see the planet definition discussion reopened should email the IAU executive committee and Working Group on Planetary System Nomenclaure. Email addresses are available at http://www.iau.org

“because we’re pretty sure the Moon formed when a Mars-sized object smacked into us over 4 billion years ago”

I thought this was basically accepted as fact these days.

Unquote

Actually, There are a lot of theories of how the moon was formed. they agree that that is the most probable theory.