So Pluto is a planet!?

[m4v]

You are missing the point - the difference between the planets is as big as between them and dwarf planets. The actual value is hardly interesting, it is how they relate to each other.

The difference between planets and dwarf planets is that planets have enough mass for be the dominant body in their orbit, the relative mass between them is not a defining factor.

I am indicating that all those obvious lines some people see are not as obvious as they are supposed to be. They are arbitrary, which is not necessarily a problem in itself, but the fact that they are vague is a problem. As soon as you add data or apply it to another system your classification will fall apart. Sure, defining planets by roundness like I advocate also means arbitrarily establishing a threshold value, but is actually pretty clear-cut beyond that.

A planet has to be massive enough for be in hydrostatic equilibrium, orbit the Sun, and be the dominant body in its orbit. There's nothing vague are those points.

[Camacha]

The difference between planets and dwarf planets is that planets have enough mass for be the dominant body in their orbit, the relative mass between them is not a defining factor

You are still not understanding what I am saying. Here I was talking about how these numbers that attempt to express dominance, the discriminant, relate to each other. There is a huge spread, making the term dominant rather vague. This, in turn, makes the whole requirement vague. It also starts causing problems when you go back in time or look at other systems.

A planet has to be massive enough for be in hydrostatic equilibrium, orbit the Sun, and be the dominant body in its orbit. There's nothing vague are those points.

The last one is. That is the whole point

Of course, a planet needs to orbit a star, not just the Sun

Edited October 4, 2014 by Camacha

[m4v]

You are still not understanding what I am saying. Here I was talking about how these numbers that attempted to express dominance, the discriminant, relate to each other. There is a huge spread, making the term dominant rather vague. This, in turn, makes the whole requirement vague. It also starts causing problems when you go back in time or look at other systems.

I really don't get what are you trying to say, you say is vague but I don't see anything vague about it. The discriminant (or µ, as I will call it from now on) is just a quotient between masses, is a real number taken from measurable data. The criteria is that if a potential planet has µ > 100 then is a planet, if not is a dwarf planet, but even that boundary is not really set in stone because there aren't any objects that get close to it, so there's no need to get all worked up and try to solve a problem that isn't one.

[Camacha]

I really don't get what are you trying to say, you say is vague but I don't see anything vague about it.

I can't argue about you not seeing it That changes little about what it is though, because even if you take the proposed µ > 100 as guide it does not solve any of the issues I indicated. There still is a huge spread, and totally breaks down in previous states of our solar system or in other solar systems. You also get the weird situation where planets can go back and forth between dwarf and non dwarf status without ever physically changing.

I cannot help but feel that saying its fine because we can use it now and in our own system is a rather narrow view on things.

Edited October 4, 2014 by Camacha

[metaphor]

I would say the Stern-Levison parameter is better than Soter's discriminant. It's based on quantities that are easily observed even in extrasolar planets, and is less variant over the history of a solar system. If you plot it on a log scale, you can see that there is a much bigger gap between planets and non-planets than there is between gas giant planets and rocky planets for example.

I don't think the first characteristic of the IAU planet definition (that it should be round) is needed at all, the "clearing the orbit" part pretty much excludes non-spherical objects (and it's a pretty fuzzy characteristic anyway).

Mars in a way is in fact smaller than it should be, given the usual models of planetary formation. That's the main problem that the Grand Tack model solves, and it has to do with Jupiter migrating inward then outward again in the very early solar system.

Edited October 4, 2014 by metaphor

[Camacha]

I don't think the first characteristic of the IAU planet definition (that it should be round) is needed at all, the "clearing the orbit" part pretty much excludes non-spherical objects (and it's a pretty fuzzy characteristic anyway).

That's an interesting approach. The problem I see with that is that in earlier solar systems there are no planets, even if there using your common sense are. What's your take on that?

Another problem I see is that planets can switch between dwarf and full status without ever changing a bit.

Edited October 4, 2014 by Camacha

[Albert VDS]

Please explain why you think a planet can change status without changing a bit.

[Alchemist]

Similar to what I just said in another thread, I think that the public getting upset about Pluto being a planet or not shows that they are not being taught astronomy correctly. They need to learn about all the objects in the solar system, not just the few we arbitrarily call "planets". ALL the interesting and large worlds need to be covered. More time should be spent learning about Europa or Titan than is spent learning about Mercury or Uranus, for example. And yet, more people know what Mercury and Uranus are than know what Europa and Titan are. Astronomy is not being taught correctly, because of this misguided fixation on "planets".

That's true. Focusing on some arbitrary classification may result in some strange results. Because after the "how many planets are there?" question there might come the questions like "how many moons does Saturn have?"

We really should focus more on some unique properties that can be found not on accounting similar objects over and over. Just to name some:

ice crust of Europa (and possible ocean beneath)

atmosphere of Titan (proto-Earth atmosphere?)

asteroid belt and why it never formed a planet (and the very question of rock planet formation)

Pluto/Charon as an example of double planet. (+still a viable example of ice dwarf even if there are dozens of similar-sized objects there)

[iCT]

Yes, Pluto is a planet, but dwarf one. Like Ceres.

[m4v]

I can't argue about you not seeing it That changes little about what it is though, because even if you take the proposed µ > 100 as guide it does not solve any of the issues I indicated.

What I see is that you're making out issues out of nothing.

There still is a huge spread

And that is a problem why? the solar system is that way, complain to however designed it. It actually makes sense anyway, since a planet either clears its path or it doesn't, values in between are unstable.

and totally breaks down in previous states of our solar system

I fail to see the issue here, you say it "breaks" but in reality you mean planets fail to meet the planet criteria when they were in formation? so? they are protoplanets then, or something. Astronomers will deal with that when there's the need for it.

or in other solar systems.

When we get the capability of detecting dwarf planets in other solar systems with reliability then that might be important, right now anything we can detect is going to be planet, no need of putting the cart in from of the horse. Or not, since there's already the Stern-Levison parameter, pointed by metaphor, that helps in that aspect. Of course you were ignorant of that too.

You also get the weird situation where planets can go back and forth between dwarf and non dwarf status without ever physically changing.

I think you're making the solar system to be way more dynamic and changing than what it really is, unless you mean that the IAU is going to keep on changing the definition in the future, quite possible as new findings are found.

I cannot help but feel that saying its fine because we can use it now and in our own system is a rather narrow view on things.

Maybe you should try to become an full fledged astronomer and propose a new criteria to the IAU instead of arguing about it in Internet then.

I would say the Stern-Levison parameter is better than Soter's discriminant.

Both yield the same results, so at this moment is not important whichever you pick to use, and we can't detect dwarf planets in extra solar systems in a reliable way anyway.

Edited October 4, 2014 by m4v

[SSR Kermit]

Similar to what I just said in another thread, I think that the public getting upset about Pluto being a planet or not shows that they are not being taught astronomy correctly. They need to learn about all the objects in the solar system, not just the few we arbitrarily call "planets". ALL the interesting and large worlds need to be covered. More time should be spent learning about Europa or Titan than is spent learning about Mercury or Uranus, for example. And yet, more people know what Mercury and Uranus are than know what Europa and Titan are. Astronomy is not being taught correctly, because of this misguided fixation on "planets".

I can't but agree; I've taught astronomy to 12 year-olds (sixth-grade here) and they were absolutely riveted by hearing about Eris, Sedna, Makemake and the other plutoids out there. They also saw the obvious similarities to Triton when I lined up all the similar objects in the Solar System. Titan, Ganymede, Ceres and Europa were favourites too.

Edited October 4, 2014 by SSR Kermit
spleling

[metaphor]

That's an interesting approach. The problem I see with that is that in earlier solar systems there are no planets, even if there using your common sense are. What's your take on that?

Like I said in the other thread... Early solar systems do not in fact contain planets, since there's only a disk of gas and dust. Once the planets start coalescing, there's a point at which their Stern-Levison parameter increases to much greater than 1, at which point you could say they are planets. This happens within ~100,000 years for gas giants and about 1 million years for rocky planets. Most likely there are many more bodies with Stern-Levison parameter greater than 1 (called oligarchs), but they collide with each other forming larger planets within a few million years. So you could rather have the opposite problem, in which there are too many "planets" when defined by S-L parameter >1, but common sense says that they will all collide with each other and form fewer planets.

[_Augustus_]

I don't think Pluto should be a planet. If we call it a planet, then why don't we throw in Eris, too? It is larger and very similar. And for that matter let's call all the Sednoids and Plutoids planets, because they're mostly spherical and most of them have moons, too. That would make us have something like 400 "planets" in our solar system. See my problem?

Besides, Pluto has a few weird traits that no known planets have but other dwarf planets do.

1. A weird "moon" with a 90-degree inclination and half it's size and mass that is also in a binary system with it.

2. A bunch of asteroid moons that orbit it's barycenter with Charon

3. Resonance with a planet

4. A high inclination relative to the non-dwarf planets

Edited October 4, 2014 by _Augustus_

[Camacha]

So you could rather have the opposite problem, in which there are too many "planets" when defined by S-L parameter >1, but common sense says that they will all collide with each other and form fewer planets.

I cannot see too many planets as being a problem. Somehow people have a wish of ending up with roughly ten planets, but if it turns out there are 50 of 300, sure, why not? An issue I can see with that approach is that you need to have a fairly decent picture of a system to say anything about things being planets or not. Looking back at the history of our own system that would have meant planets would only have been classified as such fairly recently - in theory - and it certainly is going to cause some trouble with exosystems, as a lot of the detected planets would not be planets any more. At the same time I recognize some similar problems when just using the solar orbit plus hydrostatic requirements, although I think those are somewhat more easily solved from a technological point of view.

Any definition needs to take exoplanets into account, as this field is hugely expanding and will continue to do so for decades to come. Our understanding of the subject has exponentially increased since the current definition was adopted.

I don't think Pluto should be a planet. If we call it a planet, then why don't we throw in Eris, too? It is larger and very similar. And for that matter let's call all the Sednoids and Plutoids planets, because they're mostly spherical and most of them have moons, too. That would make us have something like 400 "planets" in our solar system. See my problem?

What exactly is the problem with us having 400 planets? It's not like there is any obligation to teach or learn about all of them, you could limit yourself to the biggest or most relevant 15 to 25. I actually think it would be very beneficial to the understanding of people that there is a lot more out there than just the 9 biggest heaviest balls of stuff. I feel you should name Eris too when talking about the features of the solar system and their place, as well as the trans-Neptunian objects (but perhaps as a group). With all the amazing and great missions going on right now there is actually a lot to tell.

Of course you were ignorant of that too.

Since we are only dishing up arguments that have been discussed before and apparently reached the point of unpleasantness I am going to leave it at that. If anyone wishes to counter the arguments presented in ways we haven't mulled over yet I would love to discuss them.

[RuBisCO]

Personally, and this is just me, but I think the terms "Small solar system body" and "drawf Planet" are horrible, besides anglocentric, they just sound awful. Why could we not stick with terms like "asteriod" and "planetiod"? I'm fine with a category between planet and asteroid just give it a better name.

[Camacha]

I'm fine with a category between planet and asteroid just give it a better name.

If I am really going to nitpick, I feel dwarf planets should be a subgroup of full planets. All roundy floaty thingies, some are just more established than others. If you are going to place dwarf planets in a separate category you might better call it something that is not also a planet.

Edited October 4, 2014 by Camacha

[metaphor]

I cannot see too many planets as being a problem. Somehow people have a wish of ending up with roughly ten planets, but if it turns out there are 50 of 300, sure, why not?

I would think there's a limit in a solar system of something like 20 planets or so, since there's only so much gas and dust in a protoplanetary disk with which to form planets.

An issue I can see with that approach is that you need to have a fairly decent picture of a system to say anything about things being planets or not. Looking back at the history of our own system that would have meant planets would only have been classified as such fairly recently - in theory - and it certainly is going to cause some trouble with exosystems, as a lot of the detected planets would not be planets any more.

The nice thing about the Stern-Levison parameter is that it's only dependent on mass and semimajor axis, two of the easiest to observe quantities for exoplanets. All of the detected bodies in exosystems are planets or larger, and nothing else that's discovered would make them not-planets, since their mass and semimajor axis won't change with the discovery of more bodies.

Note: the S-L parameter doesn't actually say anything about a planet's orbit actually being cleared, it only says something about a planet's potential to clear its orbit, or the time it would take to clear its orbit from the time it arrived in that orbit. In mature solar systems that's going to be the same thing though.

[lajoswinkler]

Personally, and this is just me, but I think the terms "Small solar system body" and "drawf Planet" are horrible, besides anglocentric, they just sound awful. Why could we not stick with terms like "asteriod" and "planetiod"? I'm fine with a category between planet and asteroid just give it a better name.

Yeah, what ever happened with the word "planetoid"? It represents a planet-like object. Pluto, Eris, Sedna, Ceres, Makemake and others suit that name perfectly.

[Camacha]

Yeah, what ever happened with the word "planetoid"? It represents a planet-like object.

I am not sure I like the inconsistency that causes with respect to asteroids.

[ZetaX]

But asteroids are "star-like". So like small balls of fusing plasma, except that they are not

[Camacha]

So like small balls of fusing plasma, except that they are not

They are almost perfect opposites, with one being huge and hot and the other being tiny and typically icy and cold. So, almost, I guess

[worir4]

People are so passionate for and against Pluto.

[lajoswinkler]

I am not sure I like the inconsistency that causes with respect to asteroids.

Asteroids - shards and lumps, planetoids - Ceres, Pluto and alike. It fits quite nicely.

But asteroids are "star-like". So like small balls of fusing plasma, except that they are not

In case you're not joking, asteroids are named like that because once upon a time people didn't know what they were. Too tiny to be planets, they optically behaved just like stars, bumpy points in telescopes. Hence "star-like".

People are so passionate for and against Pluto.

The only passion I see is "for Pluto as a planet, without arguments and with lots of feelings" and "reason".

[Camacha]

Asteroids - shards and lumps, planetoids - Ceres, Pluto and alike. It fits quite nicely.

The nomenclature does not really line up. Small planets are planetoids (or planet-like), but even smaller rocks are called star-like. History has an explanation for that, but it makes the terms somewhat muddled. One could of course argue that science if full of contradictory terms, but I am not sure we should add more.

People are so passionate for and against Pluto.

Are we going to blow it up?

Edited October 5, 2014 by Camacha

[CDRW]

But if we blow it up, what happens to all the sapient Helium II-based lifeforms? Where they gonna live, huh? Are you prepared to deal with the flood of refugees? I thought not. We should lobby to have Pluto designated as an important and historical nature preserve.