So many images, so little time!

While true in the general sense, and something to always be aware of, in the case of our solar system it’s actually ridiculously easy to apply a discrete definition. Unlike situations where there’s a nearly continuous spectrum, with our planets there’s a 5 order of magnitude gap in terms of orbit-clearing ability between the least of the things the IAU terms a planet, and the greatest of things termed a dwarf planet.

Big deal.

How is that important unless you regard orbital clearing as important in the first place which many (incl. me & Alan Stern who called the IAU definition “idiotic”) don’t?

Ever looked at the order of magnitude change in diameter, luminosity and mass for the smallest red dwarf stars up to the most massive red supergiants? Stars differ vastly and come in huge range over many diferent aspects. So do asteroids and comets. So do the great biological classes plants range from algae and single celled forminifera all the way through to giant redwoods bigger than blue whales. Animals range from the smallest fleas to the largest dinosuars in size. We still call ‘em animals -these are very broad categories – and planets should be similar. Just as we have major genera of animals (eg. mammels, reptiles , insects, molluscs) with the smaller classes generally being more numerous in population numbers, it make sense that planets will be similar with broad classes – gas giants, rocky earth-like worlds and ice dwarfs and there’s likely far more smaller planets than larger ones. If you count the dwarf planets as planets then Pluto is in fact above average in size.

Is OC something that should define whether an object is a planet or not. No, I don’t think it is. Why do you think otherwise?

So any concerns about drawing an infinitely precise, arbitrary line through objects in the solar system is moot, because no such line is necessary.

[Nitpick] Is infinite precision ever possible? Surely not! Itwould take an infinite time to arrive at, discuss and double check> Will we ever get to the last digit of Pi?

Clearly objects in our solar system and planets generally do range in a continuum of size from Jupiter down to Ceres. Just as there is a range of mountains down to hills in geography.

It’s like, distinguishing exactly where Europe ends and Asia begins is difficult Distinguishing between Europe and the Americas is easy, because there’s an entire ocean between them.

FWIW; The Caucasus mountains I gather marks the spot Europe-Asia~wise. Oh and the start of Turkey -the dardanelles channel between the Greek side and the Asia minor one.

@ MTU: Your grammatical objection is noted and accepted. The wording is not really something I care about.

So why are you arguing here then?

Call the “planets” “Primary planets” and the dwarfs “dwarf planets” so all can be a subset of “planets”, or hey, rename dwarf planets into something other than planet.

The first option makes sense, the latter is what the IAU is effectively doing and does not.

Either way. The key thing is, the IAU definition does lock in on a significant distinction between these objects, and that distinction should be recognized. The exact terminology for doing so is not my concern.

Significant? To a degree, yes but *how significant* is OC and should it mark the division between whether something is a planet or just a division between *types* of planets? Clearly I think the latter.

And as usual, the rest of your argument for how the IAU definition is illogical or inconsistent are really just errors or misrepresentations on your part. Jupiter most likely would clear its orbit were it in Pluto’s place, as Neptune is rather close and has amply succeeded in doing so.

Not necessarily. Pluto does go a bit further than Neptune and Neptune itself hasn’t exactly cleared its orbit since Pluto and other objects cross it. Then there are the Neptune trojans which are hardly cleared – dominated perhaps but still there.

And comets passing through orbits would not count as they aren’t part of the objects neighborhood, just occasional visitors.

Depends on the comet – some are in stable or unstable inner solar system orbits well inside Jupiter’s. Some asteroids are just extinct comets. Most pass through – but some do stay.

If they were counted their mass would not significantly change the Planetary Discriminant for any of the objects in question.

Yes and I’m not arguing that comets are planets because they are too small – too small for being in hydrostatic equilibrium. Pluto is different as are Eris, Ceres and others.

It has never been the case that “cleared the orbit” implied “completely free of any other object”; that is pure misrepresentation on your part, and evidence that your objections to Pluto’s classification are not rational, but rather a rationalized emotional response.

So how clear is clear and who draws the line and who needs it to be clear anyhow?

Plus aren’t the Pluto-bashers equally being swayed by tehir anti-Pluto sentiment as thepro-Pluto people are?

I can understand that, but get over it. I don’t care if you want to be called a planet. Fine, it’s a planet. But it is not the same as the other 8 bodies called planets. There is a 5-orders-of-magnitude difference between Pluto, Ceres, and objects that have cleared their orbit. This distinction is not subtle or small, and must be recognized somehow. I don’t care what you call that, either. But the point is that whatever you call these categories, Pluto does not belong in the same one as the 8 orbit-dominating objects in our solar system. IAU correctly observed this. Protests ignore this. Grammatical issues aside, they were more correct.

I disagree. The panets are all digfernt and while pLuto is very different from the gas giants and rock dwarfs , it and its fellow ice drafs ar estill best described as planets.

toasterhead: This would still kill Pluto, though, since Pluto and Charon orbit a common barycenter. Wouldn’t it? What planet/moon system doesn’t orbit a common barycenter? (Larger point: Rules like this tend to be fuzzy.)

Ah, hence my deliberate wording there of *Directly* orbiting another planet.

I would therefore count Pluto-Charon as a double planet since neither is directly orbiting the other.

The IAU definition does NOT allow for the existence of double planets because neither has – or could clear its orbit. That I think is too limiting because I fully expect (& hope) we may one day discover (more) double planet systems.

In fact had our Moon been more massive our Earth-Moon system could’ve ended up as such!

@26. CB : Orbital clearing, however, is not fuzzy at all

But, that’s wrong – “orbital clearance” is fuzzy – very. It raises a whole set of question s of how do you define what is meant by “cleared” how long does it have to clear it, how far outrt does it have to clear it and so forth. By raising these unnecessary superflous questions and issues that can easily be dispensed with by dropping the whole “orbital clearence” (OC) criterion that OC criterion violates Occams razor. The definition of planet should be as simple and as striaghtforward as possible, round, never fusing like a star, not orbiting another planet directly pass the Occam’s Razor test, OC does NOT.

toasterhead: This would still kill Pluto, though, since Pluto and Charon orbit a common barycenter. Wouldn’t it?

What planet/moon system doesn’t orbit a common barycenter?

(Larger point: Rules like this tend to be fuzzy.)

I think their point was that the barycenter for Pluto/Charon is between the two objects, while all planet/moon combinations have their barycenter in the planet.

I’m not sure that would make a good criterion though, for the ‘fuzziness’ reason you mention. Minor changes in object mass could move the barycenter above/below the planet-candidates surface.

Orbital clearing, however, is not fuzzy at all — no minor tweaking, counting comets or not, etc would come anywhere close to changing the current categorization — and so, like hydrostatic equilibrium, makes for a good criterion.

About the planet topic: It’s hard to apply discrete definitions to a continuum. It is always a matter of conventions, slippery slopes, and so… Almost impossible to reach a perfect definition.

While true in the general sense, and something to always be aware of, in the case of our solar system it’s actually ridiculously easy to apply a discrete definition. Unlike situations where there’s a nearly continuous spectrum, with our planets there’s a 5 order of magnitude gap in terms of orbit-clearing ability between the least of the things the IAU terms a planet, and the greatest of things termed a dwarf planet. So any concerns about drawing an infinitely precise, arbitrary line through objects in the solar system is moot, because no such line is necessary.

It’s like, distinguishing exactly where Europe ends and Asia begins is difficult Distinguishing between Europe and the Americas is easy, because there’s an entire ocean between them.

@ MTU: Your grammatical objection is noted and accepted. The wording is not really something I care about. Call the “planets” “Primary planets” and the dwarfs “dwarf planets” so all can be a subset of “planets”, or hey, rename dwarf planets into something other than planet. Either way. The key thing is, the IAU definition does lock in on a significant distinction between these objects, and that distinction should be recognized. The exact terminology for doing so is not my concern.

And as usual, the rest of your argument for how the IAU definition is illogical or inconsistent are really just errors or misrepresentations on your part. Jupiter most likely would clear its orbit were it in Pluto’s place, as Neptune is rather close and has amply succeeded in doing so. And comets passing through orbits would not count as they aren’t part of the objects neighborhood, just occasional visitors. If they were counted their mass would not significantly change the Planetary Discriminant for any of the objects in question.

It has never been the case that “cleared the orbit” implied “completely free of any other object”; that is pure misrepresentation on your part, and evidence that your objections to Pluto’s classification are not rational, but rather a rationalized emotional response.

I can understand that, but get over it. I don’t care if you want to be called a planet. Fine, it’s a planet. But it is not the same as the other 8 bodies called planets. There is a 5-orders-of-magnitude difference between Pluto, Ceres, and objects that have cleared their orbit. This distinction is not subtle or small, and must be recognized somehow. I don’t care what you call that, either. But the point is that whatever you call these categories, Pluto does not belong in the same one as the 8 orbit-dominating objects in our solar system. IAU correctly observed this. Protests ignore this. Grammatical issues aside, they were more correct.

That makes sense!

What planet/moon system doesn’t orbit a common barycenter?

(Larger point: Rules like this tend to be fuzzy.)

My eyes are burning….

As it is, subtracting pictures A-B, a moving body becomes a white clear dot (its position in picture A) and a black dot (its position in picture B). The rest, theoritically, should be zero, but in fact you can get all kind of noise.

Still, I love these things. I, too, spent far too much time poring over the Galaxy Zoo images.

http://kencroswell.com/HD45364.html

that we already know of gas giant exoplanets having the same orbital resonance that the IAU and Pluto-bashers claim rules Pluto out of contention. They wouldn’t call HD 45364 b & c “dwarfs” surely? Just as they wouldn’t call Earth or Jupiter ‘non-planets’ if they happened to orbit in the Oort cloud and thus be “unclear” and surrounded by other bodies. Doing so would be being rather ridiculous wouldn’t it? Thus using the same idea to deny Pluto’s planethood is simply logically inconsistent and hypocritical.

My own prefered definition of planet would be this one based on three criteria :

1) Roundness or otherwise – if its large enough to be a globe or in the case of rapidly spinning objects like Haumea* and Achernar** a flattened egg-shape then it isn’t too small for planetary status like a comet or asteroid.

2) Self-luminous by nuclear fusion or otherwise – if its ever shone by its own core nuclear fusion then its a star, an ex-star or a brown dwarf rather than a planet. This takes care of the upper end of the planet division.

3) Directly orbiting another planet or otherwise – if it directly orbits another planet then regardless of size its a moon rather than a planet. If it orbits in a asteroid or cometary belt but still orbits the star directly it still counts as a planet because it isn’t orbiting another planet.

So if a world is round and not orbiting another planet or ever nuclear fusing then it is classified a planet.

Planets then would come in a range of size (Pluto is actually in the middle size-wise in our solar system with most planets being smaller ice dwarfs) and considerable numbers – the more the merrier – and leaving room for some stranger worlds that may yet be found.

Exoplanets have been surprising us and teaching us that planets can come in some very different varieties – Hot Jupiters, Eccentric Orbiters, Pulsar Planets, Super-Earths, Hot Ice Worlds, Carbon Planets and so forth.

Doesn’t that definition make good sense and allow easy logical classification?

Isn’t that a better option than the mess the IAU now has in place – hopefully almost certainly temporarily?

————–

* See : http://apod.nasa.gov/apod/ap080923.html

** See : http://kencroswell.com/RegulusIsOblate.html

I think They Might Be Giants addressed the planet issue nicely in their song “How Many Planets?” Four little ones, a bunch of other stuff, then four big ones, and a bunch of other stuff.

Way I think of it, the planets come in three zones or flavours – the rocky ones in the inner zone, the gassy ones in the middle zone and the icy ones in the outer zone!

@7. Orlando Says:

About the planet topic: It’s hard to apply discrete definitions to a continuum. It is always a matter of conventions, slippery slopes, and so… Almost impossible to reach a perfect definition.

Perhaps so but I think it’s easy (& been done ) to come up with something that’s far better and make smore logical sense than the current IAU definition.

In fact their original proposal that stated a planet is a planet if its roughly round (hydrostatic equilibrium) and not fusing and orbits the Sun rather than another planet works pretty well for me.

If a dwarf star is still a star – and a dwarf person still a person – why the blazes should a dwarf planet NOT equally count as a planet? The current dreadful IAU “planet” definition is plain size-ist! It is also logically inconsistent*, raises more problems than it solves and isn’t sufficently inclusive. I strongly think that ice dwarfs incl. Pluto, Eris & Sedna should count and are types of planets.

* If Earth or even Jupiter orbited where Pluto is, they couldn’t clear that orbit either. Being far from the Sun should not rule a planet out of planetary status if that’s what it is but that’s what the IAU definition effectively does. Besides no planet has a clear orbit as comets cross all planetary paths!

http://www.space.com/3398-neptune-thousands-escorts.html

which notes the idea that Neptune may boats more trojans than Jupiter &

http://www.space.com/8938-asteroid-neptune-gravitational-dead-zone.html

& as this link :

http://blogs.discovermagazine.com/badastronomy/2011/06/09/rosettas-cometary-goal-now-in-sight/

to a recent post here shows the cameras on these spaceprobes can be pretty effective as Rosetta was able to observe its target from three years travel time and over an AU away. Course they knew where to look and Churyumov-Gerasimenko may be a bit bigger and brighter than the Neptune trojans but still.

Hopefully it will find many more ice dwarf type planets one or even two of which can be visited briefly by the NewHorizons spaceprobe.

@ 2. toasterhead :

Has the New Horizons mission decided yet if they’re going to try and catch a Neptunian trojan while on the way to 134340 Pluto?

I second that question – I don’t know (will have to see what I can find) & I hope they do as that would be pretty interesting and there may be lots of those objects. Recall reading that Neptune may have more trojans even than Jupiter!

Last I heard about it a few months ago, this was still up in the air, ..

Oh I think it’s a bit higher up and further away than the air!

.. since that part of Neptune’s orbit is intersecting the galactic plane and nearly impossible to image.

Well if the NewHorizons spacecraft is heading into that area it will get an increasingly better view of that region as it flies ever closer using its cameras which might help. If there are that many Neptunean trojans out there then it occurs to me that the odds that one or more of them might be in or about the right place can’t be too bad.