Is there a theoretical size limit for a planet?

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Jupiter is huge. TrES-4 is 1.8 times the size. How big can planets actually get? is there a limiting factor? cheers.
 
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  • #2
anorlunda
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Jupiter is huge. TrES-4 is 1.8 times the size. How big can planets actually get? is there a limiting factor? cheers.
If a protoplanet is large enough, it becomes a star. So the upper limit for a planet should be the same as the lower limit for a star.

https://en.wikipedia.org/wiki/Stellar_mass#Range
With a mass only 93 times that of Jupiter (MJ), or .09 M☉, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core.
Edit: Then there are brown dwarf stars.

https://en.wikipedia.org/wiki/Brown_dwarf#Low-mass_brown_dwarfs_versus_high-mass_planets

Size and fuel-burning ambiguities​

Brown dwarfs are all roughly the same radius as Jupiter.
 
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  • #3
sushi b
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Thanks!
 
  • #4
sophiecentaur
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If a protoplanet is large enough, it becomes a star.
And double stars are not uncommon - asymmetrical pairs, too.
 
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fresh_42
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Jupiter is huge. TrES-4 is 1.8 times the size. How big can planets actually get? is there a limiting factor? cheers.
This mainly depends on what it is made of. Hydrogen or helium planets become stars if they are too heavy. A giant iron ball cannot ignite fusion. It would be interesting to know how heavy these can get and if they will become neutron stars.

There is also a difference between size and weight.
 
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  • #6
sophiecentaur
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This mainly depends on what it is made of. Hydrogen or helium planets become stars if they are too heavy. A giant iron ball cannot ignite fusion. It would be interesting to know how heavy these can get and if they will become neutron stars.

There is also a difference between size and weight.
I think the important thing is the statistics of relative quantities of elements in a nebula where the star is forming. I can't imagine that you are likely to get any star / giant planet forming out of anything else but mainly H and He and a small amount of gash other elements.
 
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  • #7
fresh_42
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I think the important thing is the statistics of relative quantities of elements in a nebula where the star is forming. I can't imagine that you are likely to get any star / giant planet forming out of anything else but mainly H and He and a small amount of gash other elements.
That's why I said: it would be interesting to know. That you can't imagine is a weak argument. I could imagine that there are huge iron balls out there in that huge universe. The question is then: Is there a limit for them and will they become magnetars or similar objects? I can't imagine Canis Majoris, yet, it exists.
 
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  • #8
Vanadium 50
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It will be difficult to create a bright line between brown and red dwarfs. One possibility is the presence/absence of lithium, But even that is a continuum.
 
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  • #9
sophiecentaur
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Yes. A bit of a weak argument. But, bearing in mind that you know a bit about what’s been observed, can you imagine ( or, rather, does it seem likely) a vast amount of iron forming up in the same place? It seems that the formation of planetary systems appear to produce small rocky planets near the star and the more massive ‘gas’ planets further out. Possibly solid cores but only small.
Iron would not exist in a neutron star because the density is inconsistent with elements. So is there evidence of iron stars, whatever that could entail?
 
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  • #10
fresh_42
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Yes. A bit of a weak argument. But, bearing in mind that you know a bit about what’s been observed, can you imagine ( or, rather, does it seem likely) a vast amount of iron forming up in the same place? It seems that the formation of planetary systems appear to produce small rocky planets near the star and the more massive ‘gas’ planets further out. Possibly solid cores but only small.
This is pure speculation. I am asking for facts.

Given a certain amount of hydrogen or helium determines whether fusion ignites or not. This defines mass and size.
Iron would not exist in a neutron star because the density is inconsistent with elements. So is there evidence of iron stars, whatever that could entail?
This is again pure speculation. Of course, I can imagine giant collections of iron. To say they do not exist without any evidence is nonserious to say it politely.

A certain amount of iron defines probably a barrier when gravitation overcomes electromagnetism. It is a legit question to ask: which amount? A neutron star isn't completely homogeneous neutrons. What will iron become under more and more pressure if not a neutron core? How big can an iron ball get before we no longer call it a planet?

If I had asked you some decades ago whether you can imagine metal hydrogen you would have answered with a clear NO. I refuse to make such statements like "cannot be because I cannot imagine". Show me your calculations on why something cannot exist!
 
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  • #11
snorkack
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If a protoplanet is large enough, it becomes a star. So the upper limit for a planet should be the same as the lower limit for a star.
In terms of mass, it is not clear, because there may be a range of masses where presence/absence of fusion depends on composition and even history.
In terms of radius, it is not clear either, because in the range of maximum planet size, there is a wide range of masses where radius is nearly constant with mass, and might even decrease.
 
  • #12
sophiecentaur
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Show me your calculations on why something cannot exist!
That's the wrong way round. You are proposing the existence of something so you need to provide evidence of one being observed or at least suggest a process that could produce it. Without that, we could be proposing the existence of stars made of green cheese.

My 'non imagination' is in fact based on some pretty good evidence. If you look at this table of relative abundance of elements in our galaxy (you would accept that there is evidence of that?) you can see that there is about 0.1% of iron present. The probability that an object of, say 1030kg, consisting of just Iron could emerge from random processes is pretty small. A process would need to work very hard against that sort of statistic - but maybe you have one up your sleeve?
 
  • #13
snorkack
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That's the wrong way round. You are proposing the existence of something so you need to provide evidence of one being observed or at least suggest a process that could produce it. Without that, we could be proposing the existence of stars made of green cheese.

My 'non imagination' is in fact based on some pretty good evidence. If you look at this table of relative abundance of elements in our galaxy (you would accept that there is evidence of that?) you can see that there is about 0.1% of iron present. The probability that an object of, say 1030kg, consisting of just Iron could emerge from random processes is pretty small. A process would need to work very hard against that sort of statistic - but maybe you have one up your sleeve?
Not completely wrong. We have multiple possible constraints.
We see that Earth, Venus and Mars are big, yet have very little hydrogen. Clearly there was a process separating iron and rock from hydrogen.
What are the limits of that process? In Solar system, the biggest chunk of iron and rock is 1 Earth mass (Earth), but the biggest chunk of hydrogen here is just 1 solar mass (sun). We plainly see in sky chunks of up to 100 solar masses (some stars). We would not see quite big chunks of iron and rock. So can we derive what unusually big chunks of rocks and iron would look like, how we could see them and why we cannot? Why do they not form?
 
  • #14
sophiecentaur
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Not completely wrong.
Praise indeed, young man. :smile:
It's always worth considering the actual numbers involved when pushing the envelope.



Clearly there was a process separating iron and rock from hydrogen.
You're right - which is why I was referencing the formation of planetary systems. But the ratio of abundance within the Solar System doesn't appear to actually suggest giant rocky planets. The estimated mass of Jupiter's core is 10-40 Earth Masses but (from magnetic measurements, I assume) there doesn't appear much Iron is in there.

As the data about exoplanet systems increases, it will be easier to compare out Solar System with others. So far, the evidence seems to indicate that the lumps of Iron out there are of relatively limited size. I would think that the magnetic field round a really massive iron object would make it stand out because of its resultant interaction with charged particles at great distances - possibly severely affecting its parent star emissions. I guess the OP could usefully search for reports of that sort of thing.
 
  • #15
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Just speculating, but wouldn't it be possible to find large concentrations of iron, up to whatever the size limit before gravitational collapse. near the centers of galaxies, where the ejecta from multiple supernovae might find its way into a stable orbit around another star or black hole? the farthest explanets found are around 17K LY, about 2/3rds the distance to the center of the Milky Way, so we have no idea what weird cold objects may be floating around there?
 
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  • #16
anorlunda
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Just speculating, but wouldn't it be possible to find large concentrations of iron, up to whatever the size limit before gravitational collapse.
That is a pretty complicated question because planets are not created independently. They are created as part of a solar system. For example, regions closer to the central star are exposed to more intense solar wind than the far out regions.

A good place to start study is:

https://en.wikipedia.org/wiki/Planetary_differentiation
 
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  • #17
sophiecentaur
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so we have no idea what weird cold objects may be floating around there?
Actually, we have a fairly good idea about the conditions out there and, as I commented earlier, you would need to have some good counter evidence that the necessary conditions exist for forming iron stars. PF doesn't really go in for too much speculation.

Science never says "never" but we only consider things seriously when we either can see them or we can see suitable mechanisms at work.

PS it's pretty "cold" everywhere out there, except near stars and planetary systems. For a planet to form, enough material needs to coagulate and to pull itself together. The gravitational potential energy will cause a lot of heating which will melt the materials so that they fuse together into a spheroid. This will leave that object pretty warm inside for billions of years - even if there's no appreciable nuclear energy involved.
 
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That is a pretty complicated question because planets are not created independently.
Iron is built in Super Novas, or shortly before. Couldn't it be that all non-metal elements get blown away whereas the metals start to clump? If someone rules out that scenario then there must be a physical law that excludes such a possibility.
 
  • #19
sophiecentaur
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Couldn't it be that all non-metal elements get blown away whereas the metals start to clump?
So you're saying that the elements would spread out in concentric rings, according to atomic mass? I agree that the H and He would go further but there is a whole range of velocities so would you expect good separation (as in a mass spectrometer)?

The process of star formation is rather the other way round, though. Material draws together in clumps in an indiscriminate way, towards a local centre of mass in a sort of half-orbital motion round the CM and half other nearby masses. I saw a good animation of a computer simulation but I have lost the source URL. I have a feeling that the separation into planets and into a planetary disc is the selective mechanism for elements. (And, as @anorlunda says, the solar wind)
 
  • #20
anorlunda
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where the ejecta from multiple supernovae might find its way into a stable orbit around another star or black hole?
Couldn't it be that all non-metal elements get blown away whereas the metals start to clump?

Those statements make it sound like new stars form only in the neighborhoods of old stars. Or rocky planets only where the original gas cloud had excess heavy metals.

This Wiki article may help.

https://en.wikipedia.org/wiki/Star-forming_region#Stellar_nurseries
 
  • #21
fresh_42
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I am only objecting to the attitude that something is called impossible, only because it didn't happen here. I am not saying there is, I am saying: there can be unless it is ruled out by physical law. To pretend knowledge about planet formings is ridiculous given the small sample that we have and the fact that all other planets can't be observed directly. The reasons to rule out a certain configuration can only be given by physical laws, not observation or opinion.
 
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  • #22
fresh_42
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The OP has asked:
Is there an upper bound for the mass of a planet?

The answer he got was:
Yes, in case it is a gas planet.

One would think that it might be legit to ask about the other cases! All I read here was "I cannot imagine!". Great!
 
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  • #23
russ_watters
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The OP has asked:
Is there an upper bound for the mass of a planet?

The answer he got was:
Yes, in case it is a gas planet.

One would think that it might be legit to ask about the other cases! All I read here was "I cannot imagine!". Great!
I agree that "It would be interesting to know" >> "I cannot imagine". Unless specifically impossible (and maybe not even then), there's no good reason why such a hypothetical shouldn't be entertained.
 
  • #24
fresh_42
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I keep asking for facts because it actually interests me. How heavy can an iron core become before it turns into something else, and what? A neutron "star", a magnetar? Is there an upper bound, a physical restriction, not an imaginary one?

The most helpful comment so far has been that it is difficult to draw the line between red and brown dwarfs. For e.g. we know that there are real planets, real wanderers out there. Why shouldn't one of them become heavier on its intergalactic journey than those we know from bounded systems? Is there a critical mass for iron?
 
  • #25
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ISTM that whatever upper mass limit you set for a rocky planet during the normal formation of a solar system, somewhere in the universe n of these rocky planets collided to form an object at whatever is the gravitational limit before it turns into something else per above -if it’s possible but improbable, then it has occurred
 
  • #26
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Technically, according to the IAU during the de-Plutofication episode, the upper bound for a planet is the mass of Jupiter. A planet orbits the sun, and not any other star.

I don't like this definition, but the IAU hath spoken. We will quickly get into definitions and semantics if we all get to pick what we call a "planet". Especially here, since many non-stars would be "rogue planers" which are neither planets nor exoplanets.
 
  • #27
Drakkith
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I keep asking for facts because it actually interests me. How heavy can an iron core become before it turns into something else, and what? A neutron "star", a magnetar? Is there an upper bound, a physical restriction, not an imaginary one?
The upper bound is probably the Chandrasekhar limit, which is about 1.4 solar masses. Beyond this the iron can't hold itself up under the immense amount of pressure and it collapses into a neutron star.
 
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  • #28
Drakkith
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One thing to note is that astronomical terms and definitions mainly come in the context of how the universe looks 'today'. A 0.9 solar mass ball of iron and nickel that's NOT a stellar remnant might not be explicitly theoretically impossible, but one would be hard-pressed to come up with a widely agreed upon definition for it since it vastly outmasses every known and theoretical non-stellar-based object we've ever seen. It's far more massive than any planet or brown dwarf, and more massive than even the largest red dwarfs. Calling it a planet would be dubious at best, since it would essentially form a binary system with any star it bound with. The surface gravity would be roughly somewhere between 70,000 and 200,000 g's and it would be immensely hot.

In nearly every way it would be a white dwarf, except for the fact that a white dwarf is a stellar core remnant.
 
  • #29
Drakkith
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The OP has asked:
Is there an upper bound for the mass of a planet?

The answer he got was:
Yes, in case it is a gas planet.

One would think that it might be legit to ask about the other cases! All I read here was "I cannot imagine!". Great!
With respect, if a new member had suggested that we include hypothetical iron planets with no known formation mechanism, they probably would have been ridiculed and possibly thread banned if they had kept insisting it needed to be included.
 
  • #30
snorkack
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We actually know PSR_J1719−1438_b. 1,02 Jupiter masses but at most 40% its radius - apparently no lower bound.
Is the upper mass bound of a planet vs. a white dwarf identical with the upper mass bound of a planet vs. a brown dwarf?
 
  • #31
fresh_42
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With respect, if a new member had suggested that we include hypothetical iron planets with no known formation mechanism, they probably would have been ridiculed and possibly thread banned if they had kept insisting it needed to be included.
What do you mean by theoretical? We live on one.

Earth is an example of an iron planet. You mentioned that 1.4 sun masses would probably be too heavy. There is a wide range between them. I'm just interested in the betweens.
 
  • #32
Drakkith
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What do you mean by theoretical? We live on one.
Well what do you mean by an 'iron planet'? Earth is certainly a very, very dirty iron planet if you're going to call it an iron planet. It's less than 1/3rd iron by mass after all, and only beats out oxygen by about 2%. By number of atoms, oxygen easily beats iron by a huge margin. I think Sophie's reply in post #6 took you to mean a ball of pure or near-pure iron, not an Earth-like planet. Hence his talk about the chemical makeup of potential formation regions.
 
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  • #33
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This is pure speculation. I am asking for facts.

Given a certain amount of hydrogen or helium determines whether fusion ignites or not. This defines mass and size.
Sometimes the facts are grey. If an object is heavy enough to fuse deuterium, then it is a brown dwarf, and not a planet. But does the fusion have to be continuous? What if the fusion reaction has a tendency to stall out and then kick in again later? What if the density is such that added shock waves from meteor strikes push the planet into fusion, but only for a while? What if a brown dwarf just runs out of deuterium? Do we call that a planet?

Iron stars may come into existence someday: https://en.wikipedia.org/wiki/Iron_star. The problem with them existing now is that there is nothing to "purify" the iron and prevent it from attracting lighter elements. If a big mass of iron formed somewhere, it would be surrounded by other elements, attract them, and soon just be a conventional planet with a typical iron core.
Clearly there was a process separating iron and rock from hydrogen. [for Earth and smaller planets]
What are the limits of that process?
The process was that the objects were small enough that the accumulated hydrogen could escape over time. The larger a planet is, the more lighter gasses it will hold on to, and the less iron will dominate.
The estimated mass of Jupiter's core is 10-40 Earth Masses but (from magnetic measurements, I assume) there doesn't appear much Iron is in there.
Is this relative quantity or absolute?
 
  • #34
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The upper bound is probably the Chandrasekhar limit, which is about 1.4 solar masses. Beyond this the iron can't hold itself up under the immense amount of pressure and it collapses into a neutron star.
Hmm but what if it were made purely of Manganese instead of iron? Oh wait. That would just be a stellar core undergoing fusion to iron. Never mind 😅
 
  • #35
Drakkith
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Hmm but what if it were made purely of Manganese instead of iron? Oh wait. That would just be a stellar core undergoing fusion to iron. Never mind 😅
I'm not sure what it would do. The Silicon burning process in stars seems to skip Manganese, instead adding an alpha particle to Chromium-48 to make Iron-52.
 

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