The IAU currently calls Pluto a non planet - but also a dwarf planet. I think that is a strange way to use language myself. How can a celestial object at the same time not be a planet, but be a dwarf planet?

Also they use the term "dwarf" to refer to "capable of clearing its orbit". This has nothing to do with the way the word dwarf is usually used in English.

What happens if for instance we find a large object in the outer solar system, larger than Earth, maybe the size of Neptune, and it doesn't "clear its orbit"? Is it then a dwarf planet - and not a planet?

The IAU definition works after a fashion so far because it so happens that all the celestial objects in our solar system which clear their orbit are larger than all the ones that don't. And there is a clear dividing line. According to all the main measures for "clearing its orbit", then Mars (the planet that is least able to clear its orbit) is far better at clearing its orbit than Ceres (the "dwarf planet that is most able to clear its orbit).

But - as time goes on we may well find objects in the borderline between the two. Ones that almost clear their orbit and don't quite, and ones that do clear their orbit but only just, and others that clear their orbit according to some of the metrics and not according to others.

Indeed this new Planet X - which for some reason everyone has taken to calling "Planet 9" is close to this border if it exists. So close that if, for instance, there was an Earth mass planet in a similar orbit, then it would count as a dwarf planet according to some discriminants.

Margot's Π for the new planet is around 5.25.

By comparison Margot's discriminant for Mars is 54, the most weakly clearing of all the recognized planets. And of Mercury is 140.

An Earth mass planet in a similar orbit would have Margot discriminant a tenth of that, of 0.5247 and so would not be a planet according to Margot's discriminant.

Also a 10 Earth mass planet in an orbit with semi major axis 4000 au would not count as a planet because its Margot discriminant is 0.7385.

So it's a pretty close thing. Almost "too close to call" until we find out more. It might depend which discriminant you use.

I think there's a reasonable chance that we may end up in a situation where we start to find new "dwarf planets" in the Oort cloud that are larger than Mercury and possibly larger than Mars or Earth. At that point it would probably seem really strange to most to call them "dwarf planets". And the dividing line between planets and dwarf planets would become more and more arbitrary.

If we find a planet with Margot's Π of 1.1 and the other with a value of 0.9 - and the second one is, say, much larger than the first, both in hydrostatic equilibrium - do we still call the one with this discriminant less than 1 a dwarf and the one with it more than 1 a planet?

It is well possible that we may find such planets, if we continue to explore the Oort cloud. Earth and Neptune sized planets at various distances from the sun in various orbits.

And the IAU decision was contested at the time. And it's a bit strange in other ways, especially its definition of a planet in hydrostatic equilibrium which it paraphrases as "nearly round". Planets in hydrostatic equilibrium could easily be not even remotely round.

PLANETS IN HYDROSTATIC EQUILIBRIUM

I'm strongly in favour of calling anything a planet if it is in hydrostatic equilibrium under gravity. Easy thing, you can tell that it's a planet as soon as you know its shape.

This doesn't just mean "nearly round" planets as in the IAU definition but also triaxial ellipsoids like Haumea, and over contact binaries if we find those as well, rocheworld scenarios. They'd all be planets because they are in hydrostatic equilibrium.

GIANTS AND DWARFS - AND UBER PLANETS

And then our planet has giant planets like Jupiter, and Neptune, dwarf planets like Earth and Mercury and sub dwarf planets like Ceres, Pluto, Sedna etc.

And then can refer to uber and unter planets for the clearing out the orbit, as Alan Stern suggested in his original paper. Or any other names, but just use a separate dedicated name for this concept.

That way we can have dwarf planets that are either uber or unter - clear or don't clearer, and giant planets can be also like a Venn diagram.

And then we can subdivide the planets up as we like - giant, dwarf, subdwarf, sub sub dwarf, and slice them any way we like without having to worry about whether they are clearing their neighbourhood or not.

Then we can use other words for asteroids on borderline between asteroids and dwarf (or sub dwarf) planets, such as planetoids, say.

EXAMPLES

So Vesta for instance would be a planetoid.

Ceres would be a dwarf unter planet.

Mercury is a dwarf uber planet

Pluto is a dwarf unter planet. So also are Sedna and all the other dwarf planets discovered recently.

Neptune is a giant uber planet

f 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

Alan Stern's suggestion is to call Earth and Venus, Mars, Mercury - the planets to the left of the picture, dwarfs.

Then he would call 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

Then the Kuiper cliff Planet X if it exists, Mars or Earth sized, would be a dwarf uber or unter planet depending on whether it is able to clear its orbit.

SO THEN IT CAN BE A GIANT PLANET WHETHER IT IS UBER OR UNTER

And this new object will be a giant planet (not a dwarf, and definitely a planet). We'll be able to say this right away just as soon as we know its mass.

But we might not be sure for some time whether it is uber or unter.

And won't have this absurd (seems to me) situation where a planet has to be referred to as a "dwarf planet" and not a planet if it doesn't clear its neighbourhood. That has nothing to do with the way "dwarf" is usually understood and it doesn't help with understanding, its unintuitive. Surely whether a planet is a dwarf or not has to depend on its mass?

And even more so when you then go on to say that it is also not a planet. How can a dwarf planet not be a planet?

With the IAU definition you have tiny Mercury which counts as a planet - and you could have a planet the size of Jupiter that has to be called a dwarf planet if it is way out in the Oort cloud. and in that situation then the Jupiter would be a dwarf planet and not a planet, and the Mercury would be a proper planet and not a dwarf.!

It works so far mainly because the planets so far discovered that clear their neighbourhood also happen to be large and the ones that don't are all small.

It makes no difference of course to Ceres or Pluto if we call them planets or not - but it makes a difference to us. I'm trained as a mathematician myself and one of the most important things in mathematical reasoning is to start off with clear concepts. And the simplest, most straightforward concepts you have, the easier it is to do maths.

In this way - for planets, the simplest most straightforward concept to use as a basic idea for reasoning in astronomy is to say it is a planet if it is in hydrostatic equilibrium. Or at least, the word itself doesn't matter, but to have some unifying name for that concept so you don't have to keep saying "in hydrostatic equilibrium".

And when the one thing in common between so called "dwarf planets" and "planets" is that they are both in hydrostatic equilibrium, it seems a really contorted way to use language to say that, nevertheless, the dwarf planets are not planets. And that they are not necessarily small either, happen to be small so far but don't have to be small at all as as part of the definition. Even a Jupiter sized planet could be a "dwarf" according to this definition if far enough from the sun. I think this definition is going to confuse people and that's why I think it should be changed.

IAU CAN CHANGE ITS MIND

What if the IAU changes its definition again, as many astronomers are asking it to do - to call Ceres and Pluto both planets as well?

A definition changed once can change again. I don't think myself that this definition of a planet currently used by the IAU is a keeper long term. Sooner or later it will be found to be too awkward and they will have to change it, is my view.

Part of this is extracted from my science blog posts: Would New Planet X Clear Its Orbit? - And Any Better Name Than "Planet Nine"?

and see also: Pluto - When Is A Dwarf Planet Not A Planet?