Our moon would be a planet under new definition of planethood

[Simon_Jester]

The barycenter definition is problematic in the extreme.

If that many systems would qualify, it would make sense revising our definition and acknowledging that binary systems of either two stars or a star and a brown dwarf are the norm. I don't see why it would make the definition less useful.

The problem is that "a brown dwarf" refers to celestial bodies massive enough that fusion can almost happen, or happens but not on a self-sustaining level.

Jupiter doesn't qualify as one. It's not massive enough for fusion. Not even close. There's nothing particularly special about Jupiter that makes it a fundamentally different class of object than Saturn, Uranus, or Neptune. It's just bigger than other objects in the same class, in the same sense that the Earth is a bigger 'rocky planet' than Venus, Mars, or Mercury (or Luna, if we go down that road).

So saying "the Sun-Jupiter system is a binary containing a G2 star and a brown dwarf" defines 'brown dwarf' downward to include objects that share virtually nothing in common with normal, main-sequence stars.

Basically, you're deciding that ANY gas giant is a "brown dwarf," as long as it orbits close enough to its primary that the barycenter of the joint system is outside the star. A gas giant that orbits a red dwarf at a distance of a few AU might well be a "brown dwarf" in spite of being smaller than Uranus and never having showed so much as a glimmer of fusion. Conversely, a massive gas giant on the border of becoming a main sequence star might NOT be a "brown dwarf" because it orbits far enough from a massive primary that it can't pull the shared barycenter outside the star.

In which case "brown dwarf" stops being a useful category, because it no longer refers to the physical properties of the object in question.

This is a serious negative consequence of formally adopting a barycenter definition of what constitutes a planet and what constitutes a star.

A similar objection applies to planet-moon systems: a moon which is big enough to move Mercury's barycenter outside of Mercury would not be capable of moving Jupiter's barycenter outside of Jupiter. This is in addition to the problem that celestial bodies such as Luna could easily be 'demoted' from double planet to moon by natural processes of orbital mechanics, which subverts the intent of a classification system.
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Basically, every word we use to describe celestial bodies in a planetary system should refer to some specific, functional, relevant category. And every object in that category should be special in a way objects outside the category aren't.

Stars are already defined as objects capable of supporting a self-sustaining fusion reaction. "Brown dwarfs" are already defined as objects nearly capable of supporting such a reaction. Revising either of these terms would needlessly disrupt the practice of astrophysics.

Planets are currently defined in a problematic way, as are moons, because there is no physical distinction that separates all planets from all moons. Certainly there's nothing about the composition of a planet that makes it different from a moon (Mercury and Luna are so similar that from certain camera angles a casual observer couldn't tell them apart). And at the moment there's no clear consensus definition about what a planet 'does' in functional terms.

This proposal is good precisely because it creates a functional definition: a planet is a body massive enough to clear its own orbital track of debris, to prevent the accumulation of asteroid belts or debris fields around itself.

So you're suggesting we use different criteria for the term binary depending on the objects involved in the pairing, how does that make a lick of sense? Either the barycenter does affect how we name the pairing, in which case we live in a binary system or it doesn't and we get used to seeing the Moon as a satellite planet.

It makes as much sense as trying to force the same the opposite. Different terms can and should mean different things in different contexts. And the context of stars and planets is in fact different. Trying to force things into compliance as you would simply leads to a lot of messed up logic tied in knots once you start using it.

On the contrary, bad logic and knotted thinking usually happens because someone didn't create a clear, simple system. Because then you start getting internal contradictions and having to resort to special pleading.

Personally, I think matters are fairly simple- "planet" should be defined as a body massive enough to clear its orbital track in a short amount of time on astronomical timescales. Luna might not be massive enough to do this quickly (in which case it is a satellite massive enough that we might class it as a 'planetoid'), or it might be (in which case it is a dwarf planet orbiting the Earth.

Let the chips fall where they may.

[LaCroix]

You're misinterpreting me - I know Jupiter is no brown dwarf. I never intended to blur the brown dwarf category, I only forgot to add "or a star and a big gas planet" into the list I made.

Currently, 60-70 % of the systens we know are binary or more complicated. So we should start seeing solitary systems as an outlier and regard binaries as the norm. And keep the terms straight, while we do it, or else we'll run into more and more special cases taht blur them.

And to me, saying that Sol is a G2+Gasgiant binary system or that Alpha Centauri is a G2+K1 binary is nothing that would make the term binary less useful. Saying that a system isn't binary just because the gasgiant wasn't massive enough to go brown is special pleading. If it is massive enough to move the barycenter outside of the other object, then they form a binary. Period. Having a (relatively to the solar mass) huge gas giant as a companion doesn't make that star a slut or is in any way shameful. Let's accept stellar relationships as they are.

The same for a planet and a "satellite" - if they are binary, then they are binary, may they be planet+planet, or planet+dwarfplanet, or dwarf planet plus dwarf planet or whatever stellar bodies are present. If they aren't, then they are a planet with an either dwarf planet or a moon/satellite(whatever term you prefer) sized companion/satellite/moon (whatever term you like best). Simple & factual.

[Simon_Jester]

The other problem is...

A binary star system will experience strong, significant perturbations on all bodies orbiting in the system due to the mass of the secondary.

Jupiter doesn't. Jupiter's gravity has shaped and influenced our solar system, but it doesn't randomly destabilize planets in circular orbits in the inner system or anything like that.

So a lot of analysis of the dynamics of 'binary' systems becomes totally irrelevant and academic once you're talking about a 'binary' whose lesser partner is as comparatively small as Jupiter is to the Sun, or as Luna is to Earth.

Thus, it becomes counterproductive to call such a system binary, because one of the main reasons we even bother using the word 'binary' is as a shorthand for 'the perturbations caused by interaction between the two bodies in this system are too large to ignore.'

[Terralthra]

Well, while that may be true (arguably) for our solar system, it's hypothetically not true for other small star/big planet systems. I was never intending to argue that Jupiter is a brown dwarf. It isn't, by an order of magnitude of mass. All I was arguing was that if you start taking barycenter of planet/moon systems into account when determining what orbits what, and thus whether a particular pair is a planet with a moon or a double [dwarf] planet, then you almost have to take barycenters into account when designating the primary of the next larger of systems, which I'll call here "star systems" for purposes of this post. The label, I think, is slightly misleading.

Say our solar system was 1-2 orders of magnitude smaller when it formed. Sol never ignites fusion and remains a plump, highly metallic gas giant. Scale everything else down accordingly, and you still end up with a Venus, a Terra, a Mars, and 3, maybe 4 gas giants (all much smaller, but still quite recognizable). The Oort cloud is smaller, maybe the asteroid field doesn't form. Mercury may never end up captured, but other than the star not being a star, it's still very much directly allegorical to the star system Sol has today. What do you call it? A planetary system?

This is, essentially, the reverse of what I'm saying in terms of talking about our system being a binary. No, Jupiter isn't a brown dwarf, but in terms of composition, there is very little distinguishing Jupiter from a brown dwarf. Jupiter could be ten times as big (and ten times as perturbing), and still be Not-A-Brown-Dwarf, despite being externally identical (its radius wouldn't increase as its mass does due to gravitational compression). Twelve times as big, and now it's significantly perturbing all the other orbits, the Sol/Jupiter barycenter is way out near Venus's orbit, but it's still Not-A-Brown-Dwarf, hence is technically a planet, but I'd say the planet and star are at least as significant to the system as a red dwarf/red giant system are, and they get the "binary" label. Where do you draw the line? Get it just shy of self-sustaining fusion and it's a planet, by that definition, but I'm not sure that's the most useful definition, because there's no compositional difference or sharp line between "big gas giant" and "small brown dwarf", and both will affect the system they're in a lot.

On the flip side, consider that Ganymede is an order of magnitude smaller than Mars, but even Mars has some captured asteroids we call "moons." It'd take a hell of an accurate shot, but it's certainly conceivable that Ganymede could capture an asteroid and thus have its own moon. Given the right orbital path, it could stay there indefinitely. What do we call the moon of a moon? A moon-moon?

I think we're in agreement on the larger point, which is that we don't have clear and obvious distinctions between "moon" and "planet", and uh, really, at the top end, there's no sharp distinction between planet and star, either. I can point at Sol and Jupiter and say, "yes, there's obviously a difference there," but I can't when they're much closer in size.

[LaCroix]

Why does that make studies irrelevant? It's not more or less valid by aknowleding that fact that binary systems can have a relatively small minor and look very similar to a monostar system. All calculations concerning the interactions of stellar bodies are unaffected. We can even use that knowledge to fine-tune these formulas.

You are just used to ignore the binary nature of some systems simply because they aren't exiting enough. This is special pleading, and not scientific. They still are orbiting a point outside of both bodies. Thus, binary, even though they look boring and plain compared with other binaries playing boccia with their planets. Insinsting that they aren't would be similar to claiming that pluto is still a true planet, because it's more interesting than the other dwarf planets by virtue of having cleared more of it's orbit and having more moons.

In this case, you shouldn't redefine binary as "interesting binary", but add a new qualifier to the system depicting its behavior.
You could instead call such systems like Sol, for example, an "unpertubed" binary system, if you want to state that the binary minor is too small to make the system behave significantly unlike a single sun system, while a 'true' binary gets and additional description defining the orbits of the binary partners.

And Terra-Luna would not be a binary under the barycenter definition. Terra would a planet with a dwarf-planet sized satellite(Luna) in the Sol (G2 Sol +Gasgiant minor Jupiter) "unpertubed" binary star system. Lot's of clear information.

[SpottedKitty]

Stars are already defined as objects capable of supporting a self-sustaining fusion reaction. "Brown dwarfs" are already defined as objects nearly capable of supporting such a reaction.

I thought Brown Dwarfs were defined as having net output of energy through gravitational collapse as opposed to core fusion. I know, that introduces some fuzziness between small brown dwarfs and big super-Jovian gas giants — but maybe that is the way it works and there's a bit of overlap in the middle?

[jwl]

Basically, you're deciding that ANY gas giant is a "brown dwarf," as long as it orbits close enough to its primary that the barycenter of the joint system is outside the star. A gas giant that orbits a red dwarf at a distance of a few AU might well be a "brown dwarf" in spite of being smaller than Uranus and never having showed so much as a glimmer of fusion. Conversely, a massive gas giant on the border of becoming a main sequence star might NOT be a "brown dwarf" because it orbits far enough from a massive primary that it can't pull the shared barycenter outside the star.

Don't you mean "orbits far away enough", not "orbits close enough"? Pull a planet outwards and the barycentre moves outward with it.

A similar objection applies to planet-moon systems: a moon which is big enough to move Mercury's barycenter outside of Mercury would not be capable of moving Jupiter's barycenter outside of Jupiter. This is in addition to the problem that celestial bodies such as Luna could easily be 'demoted' from double planet to moon by natural processes of orbital mechanics, which subverts the intent of a classification system.

Of course, it is possible that planets could be demoted to dwarf planets by orbital mechanics under the definition proposed in this paper, since the critical mass is dependant on the semi-major axis around the sun. For example, he calculated that the earth would cease being a planet if its semi-major axis was increased to 400au or higher, like the orbit of Sedna (which has a semi major axis of 520 au).

[jwl]

And Terra-Luna would not be a binary under the barycenter definition. Terra would a planet with a dwarf-planet sized satellite(Luna) in the Sol (G2 Sol +Gasgiant minor Jupiter) "unpertubed" binary star system. Lot's of clear information.

Well, under this proposition the Moon would be planet-sized.

Also why do you call the Earth "Terra", the Sun "Sol" and the Moon "Luna"?

[LaCroix]

OK, then he'd be planet sized... (I'll freely admit don't exactly know where we draw the border now, apart from "Pluto is below")
But calling it planet sized is fine with me, as well, if it's factual. I'll keep that in mind for the future.

I simply prefer the latin names. Since all other planets are called their Latin names, I use them for all. Also, easier to talk about Luna than having to clarify if I mean THE Moon or A moon.

[jwl]

OK, then he'd be planet sized... (I'll freely admit don't exactly know where we draw the border now, apart from "Pluto is below")
But calling it planet sized is fine with me, as well, if it's factual. I'll keep that in mind for the future.

I simply prefer the latin names. Since all other planets are called their Latin names, I use them for all. Also, easier to talk about Luna than having to clarify if I mean THE Moon or A moon.

This is the graph that appeared in his paper, with the solid line being the line for his proposal: .
If you were to think of the Moon as having the same semi-major axis as earth, it would go over the "planet" line. (That would also put is right in the middle of a large gap between the planets and planetiods, mind).

I'm not so fond of the Latin namings because it appears to me to be a thing science fiction does to appear "different", although I am more sympathetic to the "Luna" naming for the reasons you outlined.

Although BTW, the other planets are not given Latin names, they are named after Roman gods who are called the same thing in English, which is subtly different. Also, this not always the case with stars (Betelgeuse is in Arabic), moons (Titan is named after a class of Greek gods) or exoplanets (none are named yet, but the proposed names are quite a variety).

[LaCroix]

Interesting chart. That makes stuff a lot clearer.

Actually, the people giving them the names we use now didn't speak any English (mostly), but were writing in Latin, back then, and used the Latin names for Roman Gods, who coincidentally are exactly spelled the same in English for the reason English just copied the word. In science, it is tradition that the one writing first about it has the right to name it, so I just keep with the traditional names they got - Sol, Terra and Luna. When those people used a different language, well, than I use that, I don't care.

[jwl]

Interesting chart. That makes stuff a lot clearer.

Actually, the people giving them the names we use now didn't speak any English (mostly), but were writing in Latin, back then, and used the Latin names for Roman Gods, who coincidentally are exactly spelled the same in English for the reason English just copied the word. In science, it is tradition that the one writing first about it has the right to name it, so I just keep with the traditional names they got - Sol, Terra and Luna. When those people used a different language, well, than I use that, I don't care.

I don't think the Romans were the first people to write about the sun (or indeed, the Moon or any of the planets either).

[Simon_Jester]

Why does that make studies irrelevant? It's not more or less valid by aknowleding that fact that binary systems can have a relatively small minor and look very similar to a monostar system. All calculations concerning the interactions of stellar bodies are unaffected. We can even use that knowledge to fine-tune these formulas.

The problem is that a "binary star system" implicitly contains multiple stars; either we're left with a weird definition of "star" or we have to expand "binary star system" to include things that aren't stars.

You are just used to ignore the binary nature of some systems simply because they aren't exiting enough. This is special pleading, and not scientific. They still are orbiting a point outside of both bodies. Thus, binary, even though they look boring and plain compared with other binaries playing boccia with their planets...

No, I'm arguing that there is something fundamentally wrong with the barycenter definition. Because it leads to irrational results (like the Sol-Jupiter system being a binary, when it wouldn't be a binary if we replaced Jupiter with a clone of Saturn).

The difference between a 'binary' star system and a solitary star system should be fundamental, like the difference between one and two. It should not be subject to happenstance, coincidence, or depend on the details of exact astrometric calculations.

So I argue that the barycenter definition is flawed by nature, for reasons that have nothing to do with the Jupiter-Sun system as such.

Whereas you are taking the barycenter definition as an axiom.

The thing is, I strongly support the idea that every celestial body should have at least one specific, clearly defined name that is based on its own physical characteristics. A name which has little to do with the characteristics of the body it orbits, or the radius of the orbit. I propose the following:
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A "star" is a collection of hydrogen and other elements, characterized by its ability to sustain ongoing fusion reactions. "Brown dwarfs" are a borderline category of almost-stars that lack this ability but may be able to sustain low-level fusion in their cores. And "stellar remnants" being burned-out aftermaths of the main sequence fusion reaction.

A "gas giant" is an accretion of gaseous mass, with or without a solid core, that falls far short of the mass requirement for sustained fusion (i.e. an order of magnitude short).

A "planetoid" is an accretion of solid mass (rocky, icy, metallic, or for that matter big giant rubber planetoids made by the Magratheans), large enough to compact itself into a roughly spherical form.

An "asteroid" is of similar composition to a planetoid, but is too small for its own gravity to exert meaningful effects on its shape.
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Once you have this set of orderly definitions for the basic nouns, then you can construct definitions of things like "systems," and of things whose status depends on what they're orbiting such as "planets," and so on.

A "star system" is an array of celestial bodies dominated by the gravitational and electromagnetic effects of one or more star-like bodies. A "binary" star system contains two stars, and so on.

A "star-like body" is here defined to include stellar remnants, and may or may not include brown dwarfs- a good way to define what is or is not a brown dwarf would be by deciding what the cutoff is below which there's no point in treating the brown dwarf as 'star-like.'

A "planet" is an object in orbit around one of the star-like bodies of a star system, which is sufficiently massive to clear its own orbital track. The "planet" may or may not be in orbit around another celestial body; gas giants in particular could easily support planetoid-sized satellites large enough to clear the gas giant's orbital track even if the gas giant weren't there.
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The reason we have a problem here is because some of us are trying to define terms like "star" in terms of a relation with another body. Which means that you can often change a body's designation just by moving it a relatively short distance or changing its mass slightly.

Using a system of terms that works that way is a bad idea in science, because it means that you need to come up with complex, cumbersome terms for simple, common things. Something like saying "main-sequence quasi-stellar body" when you used to say "star," or "bistelliferous system" when you used to say "binary system."

Insinsting that they aren't would be similar to claiming that pluto is still a true planet, because it's more interesting than the other dwarf planets by virtue of having cleared more of it's orbit and having more moons.

No, that would be different because those insistences aren't necessarily true. We have no way of knowing whether some of the other dwarf planets have more or less moons than Pluto, or whether they have cleared more or less of their orbital tracks. And we may not know that answer firmly for centuries, if ever.

And Terra-Luna would not be a binary under the barycenter definition. Terra would a planet with a dwarf-planet sized satellite(Luna) in the Sol (G2 Sol +Gasgiant minor Jupiter) "unpertubed" binary star system. Lot's of clear information.

Note that I was aware that the Earth-Moon/Terra-Luna system would not be a binary under the barycenter definition. My point was simply that the barycenter definition is rather arbitrary. And in the extreme limiting case you can get a lot of alleged "binaries" that satisfy the binary condition... without satisfying any other relevant criterion of what constitutes a binary.

It's like, our definition of "life" is ambiguous but we can all agree that life forms exhibit many or all of the properties on a short list (reproduction, metabolism, response to stimulus, maintenance of internal homeostasis, and so on).

Now suppose we suddenly develop a new definition of "life," based on some single criterion like "contains carbon." The problem is that under this definition, random hunks of limestone and decaying bodies are "life," as are many other things that lack any of the things about living organisms that make them of scientific interest (like the ability to maintain homeostasis and to reproduce).

Adopting such a definition of "life" would not be to the benefit of biology as a science.

Likewise, I do not think the barycenter definition is useful for astronomy as a science.

Stars are already defined as objects capable of supporting a self-sustaining fusion reaction. "Brown dwarfs" are already defined as objects nearly capable of supporting such a reaction.

I thought Brown Dwarfs were defined as having net output of energy through gravitational collapse as opposed to core fusion. I know, that introduces some fuzziness between small brown dwarfs and big super-Jovian gas giants — but maybe that is the way it works and there's a bit of overlap in the middle?

Hm. I did not know this, and do not know if it is true. I fully accept and embrace this definition of "brown dwarf" if it is the mainstream definition used by science. Among other things because it lets me say compact, simple things like "Lord Kelvin expected the sun to go out in thirty million years- because he thought it was a brown dwarf and you can analyze the physics of a brown dwarf using 19th century science."
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Basically, you're deciding that ANY gas giant is a "brown dwarf," as long as it orbits close enough to its primary that the barycenter of the joint system is outside the star. A gas giant that orbits a red dwarf at a distance of a few AU might well be a "brown dwarf" in spite of being smaller than Uranus and never having showed so much as a glimmer of fusion. Conversely, a massive gas giant on the border of becoming a main sequence star might NOT be a "brown dwarf" because it orbits far enough from a massive primary that it can't pull the shared barycenter outside the star.

Don't you mean "orbits far away enough", not "orbits close enough"? Pull a planet outwards and the barycentre moves outward with it.

Hm, sorry, was getting confused by applying inverse square law calculations where I should not have. You are right and I was wrong.

A similar objection applies to planet-moon systems: a moon which is big enough to move Mercury's barycenter outside of Mercury would not be capable of moving Jupiter's barycenter outside of Jupiter. This is in addition to the problem that celestial bodies such as Luna could easily be 'demoted' from double planet to moon by natural processes of orbital mechanics, which subverts the intent of a classification system.

Of course, it is possible that planets could be demoted to dwarf planets by orbital mechanics under the definition proposed in this paper, since the critical mass is dependant on the semi-major axis around the sun. For example, he calculated that the earth would cease being a planet if its semi-major axis was increased to 400au or higher, like the orbit of Sedna (which has a semi major axis of 520 au).

Well, under my classification, they're still a subtype of 'planetoid.' It might be reasonable to introduce an extra term to our definition of 'planet' to include large Earth-sized bodies out in the Kuiper Belt, simply because it's practically impossible for a body of ANY mass to clear its orbital track if it only orbits the star every ten thousand years.
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Also why do you call the Earth "Terra", the Sun "Sol" and the Moon "Luna"?

I was doing it first- at least with Sol and Luna. I do it because those Latin terms have a historical basis and allow us to clearly distinguish between "the moon" or "a moon" and the specific body of rock that orbits our planet.

As to why use the Latin terms, well... because the scientific community of the modern world traces its intellectual ancestry to European astronomers who used Latin and read ancient Latin sources. Modern science is largely an English-speaking discipline for historical reasons, and those same historical reasons support it using Latin roots for basic terminology. If you want to dispute the use of Latin nouns to refer to celestial bodies, go ahead- but have the argument in Quechua or Chinese.

[LaCroix]

Interesting chart. That makes stuff a lot clearer.

Actually, the people giving them the names we use now didn't speak any English (mostly), but were writing in Latin, back then, and used the Latin names for Roman Gods, who coincidentally are exactly spelled the same in English for the reason English just copied the word. In science, it is tradition that the one writing first about it has the right to name it, so I just keep with the traditional names they got - Sol, Terra and Luna. When those people used a different language, well, than I use that, I don't care.

I don't think the Romans were the first people to write about the sun (or indeed, the Moon or any of the planets either).

I never said it were the Romans ( although a lot of them were Italians...). I spoke of the people that fathered the western concept of scientific Astronomy. And back then, the language of scolars was Latin. See Simon's post.

[madd0ct0r]

Oh for God's sake.the paper quoted above uses earth. Stop being pretentious the lot of you.

[LaCroix]

@Simon
I understand you, but frankly, the barycenter definition is an axiom for a binary system. I never heard any system (which I admit doesn't mean anything) called a binary without meeting that criterium.

To use an image - Sol and Jupiter behave like this (not to scale), just with Jupiter far out and Sol baically always almost touching the barycenter:


Sol does have a lot of P-type planets, and Jupiter has its own P-type objects, its many moons. And some objects far out in the belt may even be S-type objects.

I accept that you want to define that Stars should only be counted as binary if they have at least another star as a minor, but in this case it walks and quacks like a duck, if only very quiet. Increase the mass of Jupiter 5times, and it would still be very much below brown dwarf threshold, but definitely have more influence on the system. You can go all the way up to very massive non-stellar objects and you will get something that definitely is behaving like a "proper" binary.

Take a red dwarf M sun (10% of solar mass) and give it a Jupiter(0.1% of solar mass) 50 times larger - and we have a gas giant half the mass of the red dwarf. That definitely would behave like a binary. You can now wiggle and call that gas giant a brown dwarf, but it doesn't make him a star.
I bet they will find one of those down the line, and then you'd need to shift the definitions, again, because you can't ignore that one. For that reason I actually do not like the concept of a brown dwarf, for it's just an almost arbitrary point (between 13 to 75 times the mass of Jupiter, that's a lot of wiggle room) on the gasgiant-solar body scale that is considered the tipping point between gasgiant and star, depending on composition.

My point is that if you are forced to draw an ambigous line just to keep an unwanted definition (that binaries could be formed with a gas giant as a minor) out, you are not creating a stable system. I think it's better to accept that our system is actually not a proper single star one, but due to this and that fact, it mostly behaves like one (as far as we are able to measure with our instruments).

[LaCroix]

Oh for God's sake.the paper quoted above uses earth. Stop being pretentious the lot of you.

So what - if it were written in German, it would use Erde, and Hungarian it would be called Föld. Doesn't make the name Terra any less valid. Remind yourself that we never called out anybody that their use of Earth was wrong, we just didn't use it.

[madd0ct0r]

I didn't say Sol, Terra or luna was wrong either, just pretentious.

[Terralthra]

You told people to stop.

[jwl]

Insinsting that they aren't would be similar to claiming that pluto is still a true planet, because it's more interesting than the other dwarf planets by virtue of having cleared more of it's orbit and having more moons.

No, that would be different because those insistences aren't necessarily true. We have no way of knowing whether some of the other dwarf planets have more or less moons than Pluto, or whether they have cleared more or less of their orbital tracks. And we may not know that answer firmly for centuries, if ever.

We can make a pretty good guess, based on the kind of calculations in the paper this thread is talking about. The answer is that it should take Pluto ~1.2 trillion years to clear it's orbit ((2.8e-2)^-(4/3)*10 billion). So it its orbit is to an extent "cleared" it is more down to Neptune or the fact that it happened to be in an area which didn't have much stuff there in in the first place.

I thought Brown Dwarfs were defined as having net output of energy through gravitational collapse as opposed to core fusion. I know, that introduces some fuzziness between small brown dwarfs and big super-Jovian gas giants — but maybe that is the way it works and there's a bit of overlap in the middle?

Hm. I did not know this, and do not know if it is true. I fully accept and embrace this definition of "brown dwarf" if it is the mainstream definition used by science. Among other things because it lets me say compact, simple things like "Lord Kelvin expected the sun to go out in thirty million years- because he thought it was a brown dwarf and you can analyze the physics of a brown dwarf using 19th century science."

No, this isn't right. A brown dwarf, using the standards outlined in this paper at least, can undergo deuterium fusion but not hydrogen-1 fusion, and it gets it's energy from that.

Of course, it is possible that planets could be demoted to dwarf planets by orbital mechanics under the definition proposed in this paper, since the critical mass is dependant on the semi-major axis around the sun. For example, he calculated that the earth would cease being a planet if its semi-major axis was increased to 400au or higher, like the orbit of Sedna (which has a semi major axis of 520 au).

Well, under my classification, they're still a subtype of 'planetoid.' It might be reasonable to introduce an extra term to our definition of 'planet' to include large Earth-sized bodies out in the Kuiper Belt, simply because it's practically impossible for a body of ANY mass to clear its orbital track if it only orbits the star every ten thousand years.

Sedna's orbit (or most of it, anyway) is way outside the Kuiper belt. According to the calculations in this paper, Earth would clear the kuiper belt over time if it was put there, look at the graph and imagine the Earth was moved on top of Pluto or Eris. Also, doing a calculation based on Jupiter's given Π value, it should stay a planet way further out than that, up to 64000 au or a light year ((4e4)^(8/9)*5.2), which is much further than Sedna's orbit.

[Simon_Jester]

@Simon
I understand you, but frankly, the barycenter definition is an axiom for a binary system. I never heard any system (which I admit doesn't mean anything) called a binary without meeting that criterium.

Criterion?

Anyway, my entire point is that the barycenter definition is flawed, among other things because we have good reason to think that planets migrating in the early formation of a star system can play an important role in how those planets evolve. Having the same planet be part of a binary system and not part of a binary system at different times because of angular momentum transfer is WORSE than having no clear dividing line between what constitutes a planet and what constitutes a brown dwarf.

I accept that you want to define that Stars should only be counted as binary if they have at least another star as a minor, but in this case it walks and quacks like a duck, if only very quiet. Increase the mass of Jupiter 5times, and it would still be very much below brown dwarf threshold, but definitely have more influence on the system.

Again, I have heard cutoffs for the lower bound on brown dwarf mass as low as two Jupiter masses. The IAU uses a more stringent definition at the moment- but if the IAU weren't in the middle of redefining a lot of basic vocabulary we wouldn't be having this conversation.

Personally I am quite prepared to define 'brown dwarfs' as stars for purposes of whether or not a star system is a binary.

I bet they will find one of those down the line, and then you'd need to shift the definitions, again, because you can't ignore that one.

No need- we can use simulations to sort out the relevant criteria and have very clearly defined standards for what does and does not qualify as a brown dwarf, long before any such body is discovered.

My point is that if you are forced to draw an ambigous line just to keep an unwanted definition (that binaries could be formed with a gas giant as a minor) out, you are not creating a stable system.

Thing is, I'm not doing this to "keep an unwanted definition out." I'm doing this because the alternative is a definition that is both scientifically useless and deeply flawed. It'd be like if we came up with a definition of "life" that included rotting corpses and limestone. Such a definition does not serve a useful purpose, because it lumps together fundamentally different categories of thing as though they were the same.

When actual scientists DO adopt such definitions, it just results in them having to create another entirely different set of terms so that they can take the over-broad category and break it down into subcategories that are actually useful.

Oh for God's sake.the paper quoted above uses earth. Stop being pretentious the lot of you.

I've been using "Luna" and sometimes "Sol" but not "Terra..." I think.

You're misinterpreting me - I know Jupiter is no brown dwarf. I never intended to blur the brown dwarf category, I only forgot to add "or a star and a big gas planet" into the list I made.

Currently, 60-70 % of the systens we know are binary or more complicated. So we should start seeing solitary systems as an outlier and regard binaries as the norm. And keep the terms straight, while we do it, or else we'll run into more and more special cases taht blur them.

And to me, saying that Sol is a G2+Gasgiant binary system or that Alpha Centauri is a G2+K1 binary is nothing that would make the term binary less useful. Saying that a system isn't binary just because the gasgiant wasn't massive enough to go brown is special pleading. If it is massive enough to move the barycenter outside of the other object, then they form a binary. Period. Having a (relatively to the solar mass) huge gas giant as a companion doesn't make that star a slut or is in any way shameful. Let's accept stellar relationships as they are.

The catch is that referring to a "binary" system as one that contains two stars is even simpler, and has secondary advantages.

One, it means the term 'binary' always refers to a specific class of object likely to be of scientific relevance, we don't have to invent a new term like "bistelliferous systems" or some other silly term in order to convey the relatively simple idea of "this system contains two stars."

Two, it means that 'binary system' is independent of the orbital radius of the smaller body. Barycenters move. It's silly to construct a definition under which the Sun-Jupiter or Earth-Moon system is 'binary' today but was not 'binary' at some time in the past, purely because the smaller object in the system moved out to a larger orbital radius. Especially since under some recent theories of planetary formation, Jupiter had a much more significant impact on our solar system before it was a binary partner (i.e. when it was in the inner system) than it does today.

This is, essentially, the reverse of what I'm saying in terms of talking about our system being a binary. No, Jupiter isn't a brown dwarf, but in terms of composition, there is very little distinguishing Jupiter from a brown dwarf. Jupiter could be ten times as big (and ten times as perturbing), and still be Not-A-Brown-Dwarf, despite being externally identical (its radius wouldn't increase as its mass does due to gravitational compression). Twelve times as big, and now it's significantly perturbing all the other orbits, the Sol/Jupiter barycenter is way out near Venus's orbit, but it's still Not-A-Brown-Dwarf, hence is technically a planet...

As noted, I've heard cutoff thresholds for brown dwarf status as low as two Jupiter masses or so.

but I'd say the planet and star are at least as significant to the system as a red dwarf/red giant system are, and they get the "binary" label. Where do you draw the line? Get it just shy of self-sustaining fusion and it's a planet, by that definition, but I'm not sure that's the most useful definition, because there's no compositional difference or sharp line between "big gas giant" and "small brown dwarf", and both will affect the system they're in a lot.

Another issue is that almost every star system we look at, for purposes of finding exoplanets, has gas giants. They seem to be a very common, normal byproduct of stellar formation processes. It becomes almost pointless to even talk about 'binary' star systems as a separate category if ALL systems are 'binary' on account of virtually all such systems having gas giants.

On the flip side, consider that Ganymede is an order of magnitude smaller than Mars, but even Mars has some captured asteroids we call "moons." It'd take a hell of an accurate shot, but it's certainly conceivable that Ganymede could capture an asteroid and thus have its own moon. Given the right orbital path, it could stay there indefinitely. What do we call the moon of a moon? A moon-moon?

A satellite of Ganymede. Simple.

As I understand it... "Fortunately" this is unlikely to arise frequently, as it tends to be hard to form stable orbits around such bodies. Close orbits around 'airless rockball' bodies (like Luna) tend to be unstable due to gravitational anomalies, and distant orbits are so vulnerable to perturbation that they don't stay in orbit around the moon for long.

I think we're in agreement on the larger point, which is that we don't have clear and obvious distinctions between "moon" and "planet", and uh, really, at the top end, there's no sharp distinction between planet and star, either. I can point at Sol and Jupiter and say, "yes, there's obviously a difference there," but I can't when they're much closer in size.

"Moon" refers pretty specifically to bodies that are in orbit around a planet and which are much smaller than the planet. In compositional terms there's no difference but we can work around that- because to say something is a 'moon' is NOT to make a statement about its composition. It's a statement about the orbital mechanics.

Remember that my idea was to define only four categories of object in compositional terms: "star," "gas giant," "planetoid," and "asteroid." Everything else is either a statement made based on counting the number of such objects which are present, or based on the orbital mechanics of relationships between them.

Hm. I did not know this, and do not know if it is true. I fully accept and embrace this definition of "brown dwarf" if it is the mainstream definition used by science. Among other things because it lets me say compact, simple things like "Lord Kelvin expected the sun to go out in thirty million years- because he thought it was a brown dwarf and you can analyze the physics of a brown dwarf using 19th century science."

No, this isn't right. A brown dwarf, using the standards outlined in this paper at least, can undergo deuterium fusion but not hydrogen-1 fusion, and it gets it's energy from that.

To be fair this is true to a point- but a low-end brown dwarf will stop undergoing fusion within a short amount of time (on the order of millions or tens of millions of years), after which point its only source of energy is gravitational collapse.

Sedna's orbit (or most of it, anyway) is way outside the Kuiper belt. According to the calculations in this paper, Earth would clear the kuiper belt over time if it was put there, look at the graph and imagine the Earth was moved on top of Pluto or Eris. Also, doing a calculation based on Jupiter's given Π value, it should stay a planet way further out than that, up to 64000 au or a light year ((4e4)^(8/9)*5.2), which is much further than Sedna's orbit.

I overgeneralized- what I was getting at is that we might reasonably include an exception in our definition of "planet" for anomalous, large planetoids found very far from the parent star.

Based on our surveys of the Kuiper Belt, such objects would necessarily be rare, which removes the main objection to classing dwarf planets as planets in the first place. The objection being that such objects are so numerous that it subverts our attempt to classify only a limited number of outstandingly massive and significant bodies as 'planets.'

[Zeropoint]

I'm coming in a bit late, but I've read the thread.

1) I like the idea of categorizing things as a planet or not via the diagram that jwl posted. Simple, clean, and objective. I don't really care if some objects have to get recategorized, and I don't understand why anyone would make a fuss about it beyond a simple resistance to change.

2) As I understand it, the whole debate about "what IS a planet, anyway?" centers around the fact that we want the label "planet" to tell us something about the object it's applied to. Pluto got "demoted" when we realized that our working definition of "planet" had become "things on this list", and therefore saying that something was or wasn't a planet didn't convey any information about what it WAS.

3) I don't see a problem with the definition of "moon" including things that orbit other moons--or excluding it. Define "moon" as meaning a natural satellite of something other than a star, and then a satellite of Ganymede is a moon. Define it as a satellite of a planet, and then it isn't. As long as we have a consistent and meaningful definition, I'm good.

4) And finally, since I don't work in astronomy or any kind of space science, I don't really have the "standing" to have my opinion counted on this matter. The IAU is composed of people whose daily job will be complicated by definitions of astronomical terms, so to me it makes sense that they get to choose, and I'm happy to go with their opinions. Solar system bodies don't care what we call them; what matters is efficient communication.

Edit: I've used a lot of "I" language in this post (there I go again!). That's because it's intended to convey that this is all simply my opinion, with which a reasonable person could disagree, rather than statements of fact.