[Me: "Why should we treat dwarf planets any differently from dwarf stars?] Because that was the decision. They chose the term, it can mean whatever they want it to mean. They could call Pluto a pink snuffleupugus if they wanted, it is their job after all.

Well the IAU would be doing the job appallingly *badly* – doing it completely wrong even, if they call Pluto a pink snuffleupagus or anything but what it, in fact, is – a planet!

Yes, but those [Sun-grazing comets and asteroids] are much, much smaller than the planets. Pluto has something almost 4 orders of magnitude more massive in its orbit.

So what? The principle still applies – and Pluto is still a planet just as for instance, Earth would still be a planet if it was in a Pluto-like crossing situation with Jupiter. I don’t see how the mass of Neptune is relevant here. Neptune’s a planet so’s Pluto.

Besides however massive or otherwise sungrazing comets and asteroids are they still exist and still haven’t been cleared from the other planet’s orbits. You can’t say they dont exist just because their small and their existence means logically and by definition the IAU cannot term *any* object in our solar system a “planet”!

All the currently-defined planets in our solar system have no other bodies even an order of magnitude smaller in their orbit. There may be various smaller bodies around, but the gravity of each planet totally dominants its orbit.

Yes, but how far? Pluto too dominates its surroundings out to a certain distance – that’s why it has three moons!

Jupiter controls a larger volume of its orbital space than Earth does. Mars dominates far less space than Earth and so on. I’m not sure why this is relevant or significant. Sure, each planet has its own zone of control – and the more massive planets will have more space affected by their gravity – but why this should define whether something is a planet or non-planet is beyond me.

Example : Mercury is a planet but if it were out in the Edgeworth-Kuiper belt then it too would be unable to dominate its orbit like Neptune does. Would that make it a non-planet? Conversely, put Pluto were Mercury is and Pluto would then dominate its new Mercurian orbit. It just seems utterly silly to define planet in such a way that an object with less space to clear is a planet whilst one with the same mass yet more space to roam in suddenly isn’t a planet anymore.

I don’t think it should matter where a planet is – provided it isn’t a satellite of another body then it should be termed a planet.

Even the Earth/Moon system, the closest we have to a “double planet” in this solar system, their shard center of mass is inside the Earth. Pluto’s orbit, however, is dominated by Neptune, just as the asteroids at Earth’s Lagrange points are dominated by it. There is currently no need to define where the cutoff is, since there is no unambiguous case. When we learn more about solar systems we can come up with a more specific definition, but we do not know enough to do that yet.

You’re forgetting that Pluto *is* currently an ambiguous case. Many astronomers and members of the public have the good sense to ignore the IAU’s ludicrous anti-Plutonian decree.

Of course, we don’t know enough about solar systems besides our own to make a consistent, objective definition. When we know enough to get a general feeling for the consistent properties that solar systems share, then we can start talking about a general definition that applies to all solar systems. But we don’t know enough to do that yet, so the IAU stuck to just defining what we do know: our own solar system.

We’re constantly discovering numerous new exoplanetary systems including one such as HD 45364 the Pluto-Neptune analogue with more massive exoplanets which I mentioned before.

I do think there is enough now known to enable us to define ‘planet’ in a reasonable way.

I do not consider the IAU definition reasonable.

We also know from studying exoplanets – and our own solar system – that yes, planets do gravitationally affect each other and often form in resonances. This does NOT stop them being planets!

[Me : The IAU definition would make it impossible for any double planets to exist - even if two Jupiter-sized planets orbited their common centre of gravity around the one star together *neither* would actually count as a planet under the IAU’s discriminatory diktat!}

Correct. So what?

So the IAU definition is ridiculous that's what!

If you can't call such objects planets even double planets then what are they? "Dwarf planets?" Jupiter-sized dwarf planets? That's just plain silly!

There is no such body in our solar system. In fact I am not aware of any such body being detected anywhere (although I if I am wrong please tell me). In fact it may not even be possible for such a body to form. You could come up with a bizarre hypothetical scenario that would break any definition of planet. But unless there is some reason to think that such a system actually exists, then it is irrelevant to making a definition that is intended to be used for defining real things.

I disagree, hypotheticals such as this one are a reasonable way of trying to come up with a worjkable logical definition thatcovers awide range of potential circumstances. Plus, we do know of many binary stars and binary brown dwarfs. (Epsilon Indi B & C to name one such example, Teide something or other in the Pleiades for another.) It seems to me quite likely and reasonable that the same formation situation will work for Jovian type planets - I see no reason why such objects cannot form in the same way binary stars and brown dwarfs do. Of course being less massive and cooler they're fainter and they will be harder to find but I would fully expect - and predict - that they are out there & we'll one day find them.

[Me : Nor would Jupiter itself count as a planet if it orbited in the Edgeworth-Kuiper belt.]

Once again, what makes you think such a thing is possible?

Fomalhaut b, HR 8799 b,c, & d, 2MASS 1207 b – are all examples of exactly that. These are all very massive Jovian or superjovian planets located at very vast distances from their stars – even further than our Edgeworth-Kuiper belt is located! All these are planets that have been photographed. In at least the first two cases named there, circumstellar dust and debris disks similar to our Edgeworth-Kuiper disk are present so my scenario is not merely hypothetical it actually exists and already clearly shows reality contradicts the IAU Pluto-haters.

The whole reason that we have the “clearing its orbit” part of the definition is because our understanding of orbital mechanics (I believe) indicates that a body the IAU considers to be a planet would necessarily clear its orbit.

So the orbital mechanists need to improve their understanding – NOT exclude Pluto because its existence contradicts their theories! Clearly a planet defined properly does NOT in fact need to “clear its orbit” as Pluto and the HD 45364 system show.

Once again you are coming up with a hypothetical scenario without providing any reason to think that scenario is something that could actually be encountered outside of someone’s imagination.

See my examples above. Consider a bit of reasonable extrapolation and note that such situations do exist and perhaps in greater number than we currently know!

Remember too, Albert Einstein’s famous quote that :

“Imagination is more important than knowledge!”

Arguing against a definition meant to reflect reality because it cannot deal with imaginary situations that may very well be impossible is nonsense. First you have to establish that the situations you describe could actually occur, then you have to show they do actually occur, then we can talk about fixing the definition. I doubt anyone could come up with a definition that could deal with every situation anyone could come up with. Nor should it, it should only deal with what actually happens.

Pluto happens!

So does HD 45364, so does Eposilon Indi b & C – the binary brown dwraf which may be similar only slightly higher in mass to Jupiter-mass binaries. So do Jupiter mass – actually far more than Jupuiter mass – planets in the far reaches of exoplanetary systems like Fomalhaut & HR 8799! These are the realities!

So too the reality is that :

Pluto *is* a planet

… and the IAU and other Pluto-haters just need to get over themselves and accept it!

By that way, what is your proposed definition? What do you think would be better?

The original Pluto (& Eris & Ceres) inclduing definition that was proposed at that notorious Prague IAU meeting – without the Pluto-haters flawed and absurd “Orbital clearing” criteria which was undemocratically added in a last minute ambush without the best astronomers for the debate like Alan Stern being present simply for ideological reasons and anti-Pluto bias.

A planet is an object :

1. Orbiting its primary star (ie. not a moon.)
2. Not capable (ever) of shining through nuclear fusion as stars do
3. Massive enough to be round through its own gravity (hydrostatic equilibrium)

That’s basically it. If it circles its sun, isn’t itself a star & is kept round by its own gravity then its planet.

After which we can sub-divide planets into other classes :

Terrestrial like Earth

Gas giants like Jupiter

Ice giants like Neptune

Ice dwrafs like Pluto

Hot Jupiters like 51 Pegasi

Pulsar Planets like PSR 1257+12 b, c, d & e

Hot Ice planets like Gliese 436 b

SuperVenuses like Gliese 581 e

et cetera …

Doesn’t that sound more rational and reasonable than the current mess to y’all?

If not why not?

Why do “Black Cat” and others hate Pluto so much?

Outside intelligences, exploring the Solar System with true impartiality, would be quite likely to enter the Sun in their records thus: Star X, spectral class G0, 4 planets plus debris.

That’s right. Just like if Mercury were in orbit around Jupiter, it would be called a moon. Where a thing is tells you a lot about what that thing is.

By the way, I suspect Venetia Phair was herself named after a planet (a real one this time: Venus). Has a nice circularity to it.

I am not an astronomer, so my opinion about the classification and terminology is really irrelevant. One could even argue that the word “planet” is inappropriate even for objects such as Saturn and Mars, because it comes from an ancient Greek term that meant “wandering star.” Saturn and Mars are definitely not stars. Even so, Pluto never met the original criterion for being a planet, since it does not appear like a wandering star to the naked eye. It doesn’t even appear to the naked eye at all.

The terminology makes no difference. They could call Saturn and Mars waffles and Pluto and Charon pancakes, and it wouldn’t matter so long as the classification is clear and astronomers can talk about them to each other.

The only people this really affects are textbook publishers and travel agents who book trips to Pluto. I gather there are few people who fit in the second category. The only people who really have a stake in this matter are the Plutonians, if such creatures exist. I’m sure they couldn’t care less what we call it, since Pluto by any other name remains their home.