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A proposal to change the definition of a planet

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Or, use the proposed new definition, but add that it must have an atmosphere of at least X. Then planets become the interesting places with atmospheres. Dwarf planets have no atmospheres that meet the minimum.

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8 minutes ago, Bill Phil said:

Speaking of moons...

We're in a similar situation, but kind of weirder. We have a moon in orbit over Jupiter that's bigger than a planet, but we also have asteroid moons that are just as much moons as Ganymede.

I suppose most already recognized "major -" and "minor -" for that purpose, I think.

Also, dwarf planets, asteroids, comets, and small moons are catalogued by Minor Planet Center - coincidence ?

EDIT :

2 minutes ago, munlander1 said:

I think Mercury would no longer be a planet then.

Pluto may count again if Mercury were included in that way... Probably most comets as well ?

Edited by YNM

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Just now, munlander1 said:

I think Mercury would no longer be a planet then.

Yep :D . So Pluto comes back, and Mercury goes away... can't please everyone.

I don't have a dog in the fight, but I would prefer a definition that works for any solar system, anywhere. 

It's not impossible, and indeed was proposed and rejected in at least one exoplanet case, that planets might be trojans. That would possibly not make them planets assuming a discriminant was used to define clearing.

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My personal preferred definition of a planet does not include if it is interesting. I don't think Ganymede should (or could, I suppose. I don't feel strongly enough to actually say "should" here) be a planet because it looks cool or will feel bad. I think it should be a planet because its big enough for its shape to be defined in a significantly large part by the gravitational forces being placed on it.

Edited by 5thHorseman

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36 minutes ago, 5thHorseman said:

I think it should be a planet because its big enough for its shape to be defined in a significantly large part by the gravitational forces being placed on it.

Ceres and Pluto fills that definition as well.

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A nice picture

Spoiler

5iAwJ7v.png

 

Maybe just group all bodies in classes?
https://en.wikipedia.org/wiki/Cluster_analysis

If somebody has something like Wolfram Mathematica or so at his place, he can make a table with several columns (say, mass, radius, period, reference body mass, etc), fill it with tens (hundreds?) of known bodies and run the cluster analysis.

This obviously will give several clusters of objects and show the significance of every listed factor in this clusterizing.

We get, say, 5 statistical clusters grouping the bodies.
Then run componential analysis to find out general components which should be used as criteria for a body class selection.

And at last you can use discriminant analysis to find a class for any particular body.

As I-II clusters will probably contain just 2-4 bodies (biggest planets), we can join them.

So we'll get something like:

Substellar body Class A - Jup, Sat, Ura, Nep, Ear, Ven
Substellar body Class B - Mar, Mer, Gan, bird, Tri. Moo (don't know why Titan gets "bird", but let it be so)
Substellar body Class C - Plu, Cer, etc
Substellar body Class D - misc trash

Now the discussion would be just: would the C-class be mentioned as "planets".

 

Quote

Or, in other words, says Runyon, "Anything round in space that's smaller than a star is a planet."

Spoiler

261584021304.jpg

 

5 hours ago, mikegarrison said:

But I've come to enjoy that when I watch the birds in my backyard, I'm watching living, breathing dinosaurs.

You have your personal Jurassic park.

Edited by kerbiloid

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2 hours ago, YNM said:

Ceres and Pluto fills that definition as well.

Which is why I think they should both be planets. :)

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The thing is, "planet" had a very specific meaning for thousands of years. It meant "star that wanders around the sky instead of staying in one place like all the other stars". And they were Mercury, Venus, Mars, Jupiter, and Saturn.

So while saying Pluto is not a planet would deny a few decades of common practice, saying Mercury is not a planet would deny millennia of common practice.

I guess that would be OK if we found out it was really something totally different, like a Death Star ("That's no dwarf planet; that's a space station!"). But that doesn't seem to be the case. So it's a lot harder to break with thousands of years of history where Mercury was listed as a planet.

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13 minutes ago, mikegarrison said:

The thing is, "planet" had a very specific meaning for thousands of years. It meant "star that wanders around the sky instead of staying in one place like all the other stars". And they were Mercury, Venus, Mars, Jupiter, and Saturn.

And the Moon. And the Sun. And not Earth.

13 minutes ago, mikegarrison said:

So while saying Pluto is not a planet would deny a few decades of common practice, saying Mercury is not a planet would deny millennia of common practice.

But it's okay to put Earth in there? And remove the Moon and Sun? (Hint: I think it is)

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@Kerbaloid, the 3 letter abbreviation for Titan... also happens to be a type of bird, in addition to the slang meaning. :wink: 

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I highly support this new system (As long as it has a minimum size limit of 500 miles in diameter.) It would draw much greater attention to the many valuable and promising moons that are currently being ignored.

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7 hours ago, 5thHorseman said:

Which is why I think they should both be planets. :)

So would these bodies :

640px-EightTNOs.png

Now, there are a lot more unlisted and/or undiscovered out there. (I assume any satellite of planets can't be a planet unless CoM is waay above anyone's surface. If you drop that as well, then... good luck on having such a long, useless, insensible category.)

EDIT :

2 minutes ago, daniel l. said:

I highly support this new system (As long as it has a minimum size limit of 500 miles in diameter.) It would draw much greater attention to the many valuable and promising moons that are currently being ignored.

Even at that limit, TNOs are screaming at you to be listed. And god knows how many more things are out there...

Edited by YNM

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I think we also need an official distinction between principal moons and moonlets (or other name, that's not the problem). Something like Jupiter has 4 moons and at least 63 moonlets

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Regarding the definition problem :

Let's see... If anyone haven't been able to make it yet, Planet bodies are meant to be stable over very long timescales. Millions, billions of revolutions around the parent star (let's... gloss over brown dwarf- and/or planemo-orbting bodies yet). Can't have a challenger (or it'd head butt each other, making a new body). Can't really be in resonance with larger bodies (means it's not in control, and will once stray off). Have a low eccentricity orbit (not a stray). And preferably close to a one common plane, if many are found.

TBH I see a point in all that : you can depend your life on it (at least for counting the days).

@kunok I think moonlets are acceptable, but too often it's only when within a ring system.

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23 minutes ago, YNM said:

Now, there are a lot more unlisted and/or undiscovered out there. (I assume any satellite of planets can't be a planet unless CoM is waay above anyone's surface. If you drop that as well, then... good luck on having such a long, useless, insensible category.)

Like "Insects"? Good luck memorizing all of those. Why is the word "Planet" so special that we feel each must be a unique perfect little thing? Let there be a thousand planets. Let there be a billion. If something logically fits a reasonable definition then so be it.

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6 minutes ago, 5thHorseman said:

Like "Insects"? Good luck memorizing all of those. Why is the word "Planet" so special that we feel each must be a unique perfect little thing? Let there be a thousand planets. Let there be a billion. If something logically fits a reasonable definition then so be it.

But you can tell, when you see one, that it is an insect.

Same goes to planets. And everything else (moons, asteroids, dwarf planets, stars...)

Oh, and there already are billions of planets. Have some faith.

Edited by YNM

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1 hour ago, 5thHorseman said:

Let there be a thousand planets. Let there be a billion. If something logically fits a reasonable definition then so be it.

I don't disagree, but I perceive a potential downside: Children will no longer memorize the names of the planets; they might no longer even have a unit on the Solar System.  This might well make space exploration seem even more irrelevant to your average human than it already does.

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16 minutes ago, Nikolai said:

I don't disagree, but I perceive a potential downside: Children will no longer memorize the names of the planets; they might no longer even have a unit on the Solar System.  This might well make space exploration seem even more irrelevant to your average human than it already does.

They don't need to memorize planets to learn the solar system any more than they need to memorize star names.

The only reason to bother with this sort of "zoology of worlds" is if it helps people actually working with the information. If the definition suits the purpose, it's fine, like all other language definitions, they are in fact arbitrary anyway. The only question is what sort of pigeonholing is actually useful when thinking about large numbers of worlds. To clarify, I am interested in such sorting for all worlds, not just those that happen to be around our star, and hence I think such definitions should be able to unambiguously deal with even bizarre edge cases clearly.

Think of different ways to sort worlds:

1. Size.

2. Composition.

3. Orbital position/zone around a star.

4. Is it a moon, or does it have moons?

5. Presence of atmosphere (maybe a few broad types as separate categories based upon density).

6. Presence of magnetic field.

 

Edited by tater

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If you asked somebody in 1900 to name the planets, they would have said "Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune". This angst over Pluto is temporary. Kind of how I still tend to think of Czechoslovakia as being a country, even though I have a friend from Slovakia and I know that Czechoslovakia is not a country anymore. But people about 30 years old or younger grew up in a world where the Czech Republic and Slovakia are two different countries, and so for them that's just normal.

In a decade or so, kids are going to be making fun of their grandparents for muttering, "Back in my day, Pluto was a planet." "Right, Grandpa, and you also walked to school in the snow, uphill both ways...."

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I don't agree with including moons in the definition, just for clarity. Call me traditional, but I think a planet should be something that orbits a star.

For me, the best definition would be:

-Orbits a star.

-Not itself a star (no fusion)

-In hydrostatic equilibrium (Rigorously defined as some maximum deviation from perfectly spherical)

It's simple, precise, sure it would make lots of KBOs and TNOs, and Ceres into planets, but I don't really mind that.

And it would finally put to bed that stupid "Dear NASA, your mom thought I was big enough" meme. Yes, my mum does have a smaller mass than Neptune. Pluto probably would be big enough to eject her from its orbit!

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8 hours ago, YNM said:

But you can tell, when you see one, that it is an insect.

Same goes to planets. And everything else (moons, asteroids, dwarf planets, stars...)

Oh, and there already are billions of planets. Have some faith.

Yeah. But you can't tell immediately when you see a planet, that it IS a planet. You have to do an analysis on the space around it. Which is part of the issue some folks are having. If the definition is dependent on a factor outside the object, why even have it? In this case, it's like an insect no longer being called an insect because there are other bugs of a certain size near it. Which is an almost irrelevant property when actually studying the insect (except for certain things like swarms and such, but those don't apply to planets) itself.

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20 minutes ago, Bill Phil said:

Yeah. But you can't tell immediately when you see a planet, that it IS a planet. You have to do an analysis on the space around it. Which is part of the issue some folks are having. If the definition is dependent on a factor outside the object, why even have it? In this case, it's like an insect no longer being called an insect because there are other bugs of a certain size near it. Which is an almost irrelevant property when actually studying the insect (except for certain things like swarms and such, but those don't apply to planets) itself.

That's not really analogous, you already have to look at whether it's orbiting a star and not orbiting another planet, so you're already having to look at external factors anyway.

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23 minutes ago, Bill Phil said:

Yeah. But you can't tell immediately when you see a planet, that it IS a planet. You have to do an analysis on the space around it. Which is part of the issue some folks are having. If the definition is dependent on a factor outside the object, why even have it? In this case, it's like an insect no longer being called an insect because there are other bugs of a certain size near it. Which is an almost irrelevant property when actually studying the insect (except for certain things like swarms and such, but those don't apply to planets) itself.

Biological definitions don't work like that. Consider myxozoans; they're parasitic worm-like organisms composed of only a few cells each. Morphologically they're very similar to protozoans, and that's what they were assumed to bebut genetic testing shows they're descended from cnidarians (jellyfish, hydra, et.c.) which secondarily lost most of their complexity. Therefore, they're cnidarians; because biological classifications are based on ancestry and relationships. You need to look at the genetic space around an animal to properly classify it, not the animal in isolation.

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2 hours ago, Kryten said:

Biological definitions don't work like that. Consider myxozoans; they're parasitic worm-like organisms composed of only a few cells each. Morphologically they're very similar to protozoans, and that's what they were assumed to bebut genetic testing shows they're descended from cnidarians (jellyfish, hydra, et.c.) which secondarily lost most of their complexity. Therefore, they're cnidarians; because biological classifications are based on ancestry and relationships. You need to look at the genetic space around an animal to properly classify it, not the animal in isolation.

I'm mostly referring to the large question trees, used to determine various different properties and ultimately the group the species in question belongs to. These properties are almost entirely within the organism itself, as are it's genes. There are always exceptions to rules, of course.

2 hours ago, Steel said:

That's not really analogous, you already have to look at whether it's orbiting a star and not orbiting another planet, so you're already having to look at external factors anyway.

Except that there are rogue planets. Which means there are planets which orbit the galactic center. And just because you're looking at one external property doesn't mean you should just throw everything out the window.

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