Formation of rocky planets without a star

[Mechanochemist]

TL;DR Summary

Is there a good reason why it is assumed that rocky planets only form in protoplanetary discs?

After googling for many hours I am still not understanding why the formation of rocky planets shall be restricted to protoplanetary discs.

Why can't they form from dustclouds far away from any star? Agglomeration should still happen over time. Jeans criterium should not play a role since it is not relevant for the formation of rocky planets in a protoplanetary discs.

So, can somebody explain what I am missing out?

 

[Bandersnatch]

I can offer a guess: you need the star to blow away the volatiles from the vicinity of the proto-planet.
Otherwise the rocky core - which precipitates first from the cloud due to higher condensation temperatures - continues accumulating the remaining material and develops a gaseous envelope. Since there's always more volatiles in the cloud than rock-forming elements, you end up with a gaseous - or, given enough time for the post-collapse heat to dissipate and the volatiles to precipitate - an icy planet.

 

[sophiecentaur]

Otherwise the rocky core - which precipitates first from the cloud due to higher condensation temperatures - continues accumulating the remaining material and develops a gaseous envelope.

That seems a fair enough argument. But (one more step backwards) why does the OP not talk in terms of lone Planets, in general? To form planets, there probably needs to be a large local mass (photo star) to herd all the dust, gas and stuff into a protoplanetary disc in the first place. I tried a quick search about formation of planet sized objects far away from stars but I guess my search terms weren't smart enough. I just got loads about 'Exoplanets' around other stars.

And where do the occasional comets and rocks with calculated extra solar origin come from?

 

[renormalize]

I tried a quick search about formation of planet sized objects far away from stars but I guess my search terms weren't smart enough. I just got loads about 'Exoplanets' around other stars.

Check out https://en.wikipedia.org/wiki/Rogue_planet, which says:
"Rogue planets originate from planetary systems in which they are formed and later ejected. They can also form on their own, outside a planetary system."

 

[sophiecentaur]

Check out https://en.wikipedia.org/wiki/Rogue_planet, which says:
"Rogue planets originate from planetary systems in which they are formed and later ejected. They can also form on their own, outside a planetary system."

That's interesting and it implies that planets which have been formed in isolation would be gas planets because of the low temperature of where they formed. To be rocky, it seems that they would have had to be formed as part of a planetary system.
It makes me wonder what is actually meant by "Rogue Planets". Is it like rogue (lone) elephants without implying they can do damage?

 

[Vanadium 50]

Most moons in the outer solar system are icy, not rocky. I expect the same for bodies that form even farther out.

 

[sophiecentaur]

Most moons in the outer solar system are icy, not rocky. I expect the same for bodies that form even farther out.

Likewise comets are full of light molecules.

 

[Mechanochemist]

That seems a fair enough argument. But (one more step backwards) why does the OP not talk in terms of lone Planets, in general? To form planets, there probably needs to be a large local mass (photo star) to herd all the dust, gas and stuff into a protoplanetary disc in the first place.

So the density of dust is too low if there is not strong attractor like a star to have a sufficiently large number of collisions of dust particles to form a planet?
Sorry for my maybe stupid questions: do we know anything about the density of interstellar dust clouds?
Dust originates from supernovea. So a huge amount of material is pushed outwards and should disperse over time. However, the material could collide with other material from other supernovea and start things like star formation or, maybe, also planet formation? I find it quite difficult to imagine the material clouds around a supernova. The amount, the density, the size of the particles. #

 

[Mechanochemist]

I can offer a guess: you need the star to blow away the volatiles from the vicinity of the proto-planet.
Otherwise the rocky core - which precipitates first from the cloud due to higher condensation temperatures - continues accumulating the remaining material and develops a gaseous envelope. Since there's always more volatiles in the cloud than rock-forming elements, you end up with a gaseous - or, given enough time for the post-collapse heat to dissipate and the volatiles to precipitate - an icy planet.

This sounds reasonable. If the gas prevents the agglomeration of dust (I am not sure about this, though), it would be a great explanation of why rocky planets can only form in protoplanetary discs around stars.
I do not understand how this gaseous envelope is formed around the dust particles(?).
Accumulation of particles that experience no strong gravitational pull might be caused by electrostatic forces. However, collisions with atoms or molecules might neutralize the charged particles, stopping the accumulation caused by this force. So, this might hinder the formation of larger dust particles. But it might still come to agglomeration of particles by van der Waals attraction or slow collisions. Anyhow, I wonder about the size of the starting particles set free by a supernova.

Does anyone know some sources of people who worked in this field?

 

[berkeman]

Does anyone know some sources of people who worked in this field?

I'll page @Ken G although he works more with stellar issues...

 

[sophiecentaur]

Dust originates from supernovea. So a huge amount of material is pushed outwards and should disperse over time.

But new stars form when random local patches of high density start to form so it's not dispersed for ever. The same thing would have been a problem with the primordial H and He clouds and they managed to sort out star formation. Simulations that many stars are formed at a time. The numbers and sizes are just staggering so intuition won't work.

The question of 'what happened first' with the dust clouds to form stars with planetary discs or individual planets would all be down to small details about probability of things getting closer enough to accrete. We need a man with a big simulator and the interest to answer this question.

 

[Vanadium 50]

Technically, planets only orbit the sun. Part of the de-planetification of Pluto. This should be "exoplanets".

 

[sophiecentaur]

Sounds like a branding problem. It doesn’t affect the Physics tho’.
And we already use the term Planetary Disc . I won’t lose any sleep.

 

[Vanadium 50]

And "planetary nebula".

 

[Ken G]

I'm not sure why the concept of the Jeans mass was dismissed originally in the OP, it seems to me that this is pretty fundamental if you want to form pretty much anything beyond grains of dust in the ISM. The point there is, you have a very low density medium (even a giant molecular cloud is pretty low density), so you can't get enough collisions to form anything larger than a few micron dust grains. The problem is that if you have GMC gas sitting inside lower density gas (the rest of the ISM), the force balance there is perfectly stable and nothing more really happens. But if you build up the Jeans mass of material in the GMC, now you have a force balance that is starting to look like gravity balancing internal pressure, and that type of balance under the nearly isothermal conditions in a GMC (enforced by the rapid exchange of heat via optically thin light propagation) is quite unstable! Thus you tend to get loss of force balance and free fall, and that is the start of forming much larger things.

The problem for forming planets, instead of stars, is that the Jeans mass is many thousands of solar masses, so you can only build smaller things (even normal stars) via fragmentation. So that leads to what gets called "the initial mass function", which kind of peaks around a solar mass. I don't know if this initial mass function is well understood yet, so it might not be so clear how the fragmentation continues down to very small scales like ice giants or even rocky (exo)planets. (The IAU dropped the ball on that one, nobody is going to be saddled with calling these exoplanets.) But I think the point above is well taken that if you start to build up too much of a rocky or icy planet, you will start pulling in hydrogen also, if there is no star nearby to either blow the hydrogen gas away or heat it up until it can't be captured. So when the Wiki talks about lone planets forming on their own, I suspect they must mean gas giants, which are basically unglorified brown dwarfs. That would just be the tail of the initial mass function, which probably does not extend down to Earthlike planets, but who's to say it's completely impossible?

 

[sophiecentaur]

And "planetary nebula".

Afaik, they have nothing to do with actual planets. They are formed from gas ejected from a red giant (?).

 

[Vanadium 50]

That's kind of the point.

 

[sophiecentaur]

If the gas prevents the agglomeration of dust (I am not sure about this, though), it would be a great explanation of why rocky planets can only form in protoplanetary discs around stars.

It's probably all to do with the numbers; there is 750 times as much mass of H as Fe. and multiply that by 75 for atomic mass ratio! Even in the extremes, there is probably a lot more gas atoms than dust in any nebula. There can hardly be any preferential mechanism to make just the dust accumulate in a spot. All that counts will be total mass. That means much more mass of gas than solids. It's only after formation of a planet with lots of gas and rock that other selection mechanisms strip the gas away. Those conditions must involve high temperature / high local gravitational attraction and there's not a lot of those in very deep space.

 

[sophiecentaur]

The OP question is along the lines of “why don’t you find gold nuggets in space?”
There’s a necessary chain of events, starting with a nebula and ending with . . . .