Planetary Formation

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Summary:
Do planetary systems form as per the Giant-Impact Hypothesis (Theia/proto-Earth collision)
I'm very much an amateur with a keen interest in space/the universe and it occurred to me the other day that our solar system could have been formed in exactly the same way that it is hypothesised that the Moon/Earth system formed through the (proposed) collision of Theia and the proto-Earth.

If our planetary system were the result of one or more collisions of a sizeable body and the proto-Sun - either before or after its nuclear fission ignition - then this would explain why all the bodies in the solar system are in the same plane, why they all orbit in the same direction, the existence of the Asteroid Belt, Kuiper Belt, Oort Cloud, etc. Such a collision could even be what initiated the sun's nuclear fission and/or caused its rotation.

Any thoughts?
 

Answers and Replies

  • #2
anorlunda
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Colliding objects don't need to be large. Anyhow, your view is oversimplified.
Inelastic collisions are only part of the story.

From The Potato Radius: a Lower Minimum Size for Dwarf Planets
Gravity alone cannot make things collapse. To collapse “gravitationally”, material has to get rid of energy and angular momentum. Only when dissipative structures and/or processes (accretion disks, viscosity, friction, magnetic breaking, inelastic collisions, dynamical friction) act to export energy and angular momentum, can an object collapse
 
  • #3
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Could I just clarify: are you saying that the ignition of the sun could well have been the result of a collision with a (not necessarily large) body?
 
  • #4
Bandersnatch
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The formation and structure of the solar system is adequately explained by the collapsing molecular cloud model. There is ample observational evidence that this is how young systems form.
 
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  • #5
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OK, thanks. It just seems ipso facto wholly inconsistent that the Earth-Moon system should have formed in one way and the solar system to have formed in a completely different way.

BTW, can I ask how we have observational evidence if it takes millions of years for solar systems to form?
 
  • #6
anorlunda
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Why focus just on the solar system?
Why are Saturn's rings ring shaped?
Why is the galaxy disc shaped?
 
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  • #7
Bandersnatch
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OK, thanks. It just seems ipso facto wholly inconsistent that the Earth-Moon system should have formed in one way and the solar system to have formed in a completely different way.
It's not that these are wholly different processes. They're pretty much the same process - that of gravitational collapse - only on different scales and at different epochs in the evolution of the system.
The point is, there is no need to hypothesise additional steps along the way. From fundamental physical laws it follows that if you start with a cold cloud of gas, it will collapse under its own gravity while conserving angular momentum. This naturally forms a corotating disc of debris around the central overdenisty, explaining all the features listed in the OP. So we stick with the most parsimonious description.
With the giant impact hypothesis for the Earth-Moon system, it's there to provide an explanation for a particular observation - that of significantly lower density of the satellite as compared to the planet. It's necessary for a complete description.

BTW, can I ask how we have observational evidence if it takes millions of years for solar systems to form?
As is the case with many astronomical observations, but also with other sciences where the studied processes take too long to observe (e.g. geology, biological evolution), one relies on taking snapshots of many different objects, at different stages of evolution.
E.g. there is no single star that anyone could have observed evolving from proto-star through the main sequence and until its 'death' of choice. But by observing many individual stars, a robust model of stellar evolution has been developed.
Similarly here - observations of stellar nurseries, protoplanetary discs, and mature stellar systems all fit the picture of evolution expected from theoretical analysis.
 
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  • #8
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Why focus just on the solar system?
Why are Saturn's rings ring shaped?
Why is the galaxy disc shaped?
Well, exactly! The creation method of the Earth-Moon system can be extrapolated to other planets, our and other solar systems, and even galaxies.

I'm a big fan of Occam's Razor.
 
  • #9
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With the giant impact hypothesis for the Earth-Moon system, it's there to provide an explanation for a particular observation - that of significantly lower density of the satellite as compared to the planet. It's necessary for a complete description.
Then by a similar argument, the lower densities of Jupiter, Saturn and Uranus relative to the Sun are explained by them having been formed as the result of a giant impact?

(Of the outer planets, only Neptune is denser than the Sun I believe).

I believe you also get an even more compelling argument if you just compare densities of cores (ie excluding the sun's/planets' atmospheres).
 
  • #10
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It's not that these are wholly different processes. They're pretty much the same process - that of gravitational collapse - only on different scales and at different epochs in the evolution of the system.
The point is, there is no need to hypothesise additional steps along the way. From fundamental physical laws it follows that if you start with a cold cloud of gas, it will collapse under its own gravity while conserving angular momentum. This naturally forms a corotating disc of debris around the central overdenisty, explaining all the features listed in the OP. So we stick with the most parsimonious description.

As is the case with many astronomical observations, but also with other sciences where the studied processes take too long to observe (e.g. geology, biological evolution), one relies on taking snapshots of many different objects, at different stages of evolution.
E.g. there is no single star that anyone could have observed evolving from proto-star through the main sequence and until its 'death' of choice. But by observing many individual stars, a robust model of stellar evolution has been developed.
Similarly here - observations of stellar nurseries, protoplanetary discs, and mature stellar systems all fit the picture of evolution expected from theoretical analysis.
Sure, I appreciate that. My point is that we have not actually observed the formation of any particular solar system so we can only theorise as to how they form.

I guess the biggest problem I have is that if a cold cloud of gas collapses under its own gravity then it will ipso facto collapse to the gas cloud's centre of gravity, ie all the gas will initially act to form a single, giant body - not a system of orbiting bodies. As I said, I'm just an amateur so I have no knowledge of the high-level physics calculations that have gone into modelling such.
 
  • #11
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I guess the biggest problem I have is that if a cold cloud of gas collapses under its own gravity then it will ipso facto collapse to the gas cloud's centre of gravity, ie all the gas will initially act to form a single, giant body - not a system of orbiting bodies.
Well the mass of the sun is 99.91% the mass of the sun+planets in our solar system so in a way it's true. But because of chaotic dynamics, inhomogenious and anisotropic spread of the gas, it makes clumps -> ipso facto you get planets.
But the ultimate test is if the theory coresponds to observations and it this case your theory of collisions doesn't match observations as well as the mainstream theory does.
 
  • #12
Vanadium 50
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it will ipso facto collapse to the gas cloud's centre of gravity, ie all the gas will initially act to form a single, giant body - not a system of orbiting bodies.
Ipso facto? How about ipso wrongo?

If this is true for planets, it's true for stars. Why are there multiple stars in the galaxy? Shouldn't there just be one omnistar? And why do gaalxies have satilites? And why are there galaxy clusters...
 
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  • #13
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Well the mass of the sun is 99.91% the mass of the sun+planets in our solar system so in a way it's true. But because of chaotic dynamics, inhomogenious and anisotropic spread of the gas, it makes clumps -> ipso facto you get planets.
Surely the inhomogenious spread of the gas is irrelevant since the cloud of gas will have a single centre of gravity, and therefore all the matter will move to that point (rather than clumping together first) regardless of its spread?

EDIT: Or are you saying that the gas cloud doesn't collapse, but rather pockets of the gas cloud collapse?
 
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  • #14
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Ipso facto? How about ipso wrongo?

If this is true for planets, it's true for stars. Why are there multiple stars in the galaxy? Shouldn't there just be one omnistar? And why do gaalxies have satilites? And why are there galaxy clusters...
Well surely that all comes down to how all the matter was formed in the first place - it would be good evidence that there wasn't a single Big Bang, I guess. But that's going off at a tangent, somewhat.
 
  • #15
Vanadium 50
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So every galaxy, every star, every planet, every moon and every asteroid and comet was formed by its own big bang?

Ipso wrongo indeed!
 
  • #17
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So every galaxy, every star, every planet, every moon and every asteroid and comet was formed by its own big bang?

Ipso wrongo indeed!
No, of course not. But the universe as we know it could be the product of more than one Big Bang.

You seem really quite defensive. I'm just a curious amateur asking questions.
 
  • #18
Ibix
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But the universe as we know it could be the product of more than one Big Bang.
Not unless you want to completely rewrite general relativity it couldn't.
 
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Btw you know that personal theories are not allowed on this forum, right?
Really? It's that closed-minded?
Not unless you want to completely rewrite general relativity it couldn't.
OK, I'll add that to my to-do list :cool:

One quick question (getting completely off-topic now): if there had been just a single Big Bang then surely all of the matter in the universe would essentially describe the surface of a sphere, no?
 
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  • #20
Ibix
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Really? It's that closed-minded?
It's in the rules and the mission statement: we advance science more by helping people become informed about what is known than by entertaining uninformed speculation. That isn't closed minded - it's focussing on what twenty years of experience as a science forum tells us is productive.

if there had been just a single Big Bang then surely all of the matter in the universe would essentially describe the surface of a sphere, no?
No. The Big Bang was everywhere, so matter is everywhere.
 
  • #21
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It's in the rules and the mission statement: we advance science more by helping people become informed about what is known than by entertaining uninformed speculation.
OK, well I guess I'm just trying to understand how we know that the solar system did not form in the same way that it is theorised that the Earth-Moon system was formed.

No. The Big Bang was everywhere, so matter is everywhere.
OK, well I don't understand what that means!
 
  • #22
Ibix
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OK, well I guess I'm just trying to understand how we know that the solar system did not form in the same way that it is theorised that the Earth-Moon system was formed.
Well, if we propose that all planetary bodies are formed by collisions between planetary bodies where did the first planetary body come from? As others have noted we can see other star systems in various stages of evolution, and we assume ours developed the same way.

OK, well I don't understand what that means!
It means the universe was initially filled with hot dense matter which cooled into the structures we see today. It did not all expand at the same speed from a single point, which is what would be required for all the matter to lie on the surface of a sphere.
 
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  • #23
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Well, if we propose that all planetary bodies are formed by collisions between planetary bodies where did the first planetary body come from? As others have noted we can see other star systems in various stages of evolution, and we assume ours developed the same way.
OK, well as I understand it a star is formed when a gas cloud collapses. Have we observed what happens when two stars collide?
It means the universe was initially filled with hot dense matter which cooled into the structures we see today. It did not all expand at the same speed from a single point, which is what would be required for all the matter to lie on the surface of a sphere.
OK, I still don't really understand what that means :cool: But on a related note, the universe contains a fixed amount of matter. So does the universe therefore have a centre of mass/gravity?
 
  • #25
berkeman
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the universe contains a fixed amount of matter.
How much, exactly? :wink:
 

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