Planetary Formation

  • #1
guesses3
30
0
TL;DR 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
Staff Emeritus
Insights Author
11,184
8,577
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
guesses3
30
0
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
Science Advisor
3,326
2,628
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.
 
  • Like
Likes Klystron, Vanadium 50 and russ_watters
  • #5
guesses3
30
0
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
Staff Emeritus
Insights Author
11,184
8,577
Why focus just on the solar system?
Why are Saturn's rings ring shaped?
Why is the galaxy disc shaped?
 
  • Like
Likes Vanadium 50
  • #7
Bandersnatch
Science Advisor
3,326
2,628
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.
 
  • Like
Likes Aanta and berkeman
  • #8
guesses3
30
0
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
guesses3
30
0
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
guesses3
30
0
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
Motore
138
136
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
Staff Emeritus
Science Advisor
Education Advisor
29,533
14,962
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...
 
  • Like
Likes phinds, russ_watters and Motore
  • #13
guesses3
30
0
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?
 
Last edited:
  • #14
guesses3
30
0
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
Staff Emeritus
Science Advisor
Education Advisor
29,533
14,962
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
guesses3
30
0
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
Science Advisor
Insights Author
2022 Award
10,068
10,626
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.
 
  • Like
Likes russ_watters
  • #19
guesses3
30
0
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?
 
  • Skeptical
Likes weirdoguy and Motore
  • #20
Ibix
Science Advisor
Insights Author
2022 Award
10,068
10,626
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
guesses3
30
0
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 theorized 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
Science Advisor
Insights Author
2022 Award
10,068
10,626
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 theorized 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.
 
  • Like
Likes russ_watters
  • #23
guesses3
30
0
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
Mentor
64,122
15,323
the universe contains a fixed amount of matter.
How much, exactly? :wink:
 
  • #26
guesses3
30
0
OK, but we haven't be able to observe what the subsequent result is - that will take millions of years.

I guess what I'm asking is do we know whether or not planetary systems are formed as the consequence of the collision of two stars?

EDIT: Another way of looking at what I'm asking is how planetary systems form because I'm guessing they don't form as a consequence of a star going supernova, yet our Sun is reportedly at least a 3rd generation star.
No. It's infinite in extent as far as we can tell and with the same average density everywhere, so there is no center of any kind.
When you say that the mass of the universe is infinite, is that not simply an inference from the fact that we don't know if it has a centre of mass/gravity?
 
Last edited:
  • #27
guesses3
30
0
How much, exactly? :wink:
Ah, something else for my to-do list :cool:
 
  • #29
guesses3
30
0
Thanks. The first line of that caught my eye:

"The various planets are thought to have formed from the solar nebula, the disc-shaped cloud of gas and dust left over from the Sun's formation."

So we don't actually know the process of planetary formation. But moreover, this hypothesis is predicated on a gas cloud only partially collapsing, with the remaining gas remaining in orbit around the proto-star and going on to form its planets. Has this actually been observed? I'm guessing not since it's a process that would take millions of years (plus the technology we have can barely detect exo-planets), whereas we have observed two stars colliding. And it has been theorized that the Earth-Moon system resulted from two sizeable bodies colliding. In other words, two sizeable bodies colliding resulted in the Earth-Moon system, so what is the ultimate result of two stars colliding?

As I said, it just seems completely inconsistent to me that orbiting planets in a solar system should have formed entirely from the remnants of a gas cloud, whereas the Earth-Moon system formed from the collision of two large bodies.
 
  • #30
Motore
138
136
This is going in circles now. The Wikipedia entry is the current best theory we have of planetary formation of our solar system. It matches observations.

As I said, it just seems completely inconsistent to me that orbiting planets in a solar system should have formed entirely from the remnants of a gas cloud
It seem completely consistent to me.

whereas the Earth-Moon system formed from the collision of two large bodies.
An how did the bodies form in the first place? Before the moon there already needed to be a protoEarth.

Are you proposing a theory without a reference to a peer review paper backing it up? We already established that this is not allowed on this forum. Also by your own statement you are an amateur in this field so wouldn't be best to read a few textbooks and do a little (couple of years) of actual learning astronomy and physics before commenting?
 
  • Like
Likes russ_watters and weirdoguy
  • #31
guesses3
30
0
This is going in circles now. The Wikipedia entry is the current best theory we have of planetary formation of our solar system. It matches observations.
OK, so I guess I was trying to establish whether or not there was any reason why the planets did not form as a consequence of a body colliding with the Sun. The only answer seems to be that the current accepted theory for planetary formation is one of accretion, which doesn't really answer my question. If there is no known reason it's fine to say so.
It seem completely consistent to me.
The Moon became a satellite of the Earth through a body colliding with the proto-Earth, but the planets became satellites of the Sun WITHOUT a body colliding with the proto-Sun. How is that consistent?
An how did the bodies form in the first place? Before the moon there already needed to be a protoEarth.
In the same way that the Moon became a satellite of the Earth, ie a body (eg another star) colliding with the proto-Sun. But I'm not allowed to proffer such a theory here, so I don't understand why you are asking such a question?😟

So one further question: do we know that all the moons of the other planets were simply captured, ie without collision with the (now orbited) planet?
 
  • #32
Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
29,533
14,962
So we don't actually know the process of planetary formation.
You are setting a very high bar for other people's theories, yet..

Really? It's that closed-minded?
demand we set a low bar for yours (which are not even allowed here - see the rules).
Not cool.

any reason why the planets did not form as a consequence of a body colliding with the Sun.

a. Planets are not made of the same material as the sun.
b. The sun is a ball of gas as hot as a star. How does the projectile survive?

I would suggest that before you speculate, you learn what the current evidence is. And check the forum rules.
 
  • Like
Likes phinds and berkeman
  • #33
256bits
Gold Member
3,762
1,788
In the same way that the Moon became a satellite of the Earth,
It can be considered that earth-moon is a binary system as a result of a collision.
Stellar collisions result in a larger star or two stars revolving around each other.
No idea of what happens to all of the ejecta, but depending upon impact, either grazing or more head on, and upon velocity, other material can be assimilated into the central body or flung out of the system.

If we do consider the ejecta that has not left the system, to begin revolving around the central region, elliptical orbits should be chosen with an 'aphelion' near the central region. Whether these orbits become circular is a dynamical question - of an n-body problem if some ejecta is more compact, with energy and momentum exchange, A large part of the ejecta would be in vapourized form - due to it being that way from the start.
If we can get the vapourized ejecta to follow circular orbits through dynamical mixing than are we not back to the situation of a central region surrounded by disk of material that would have to condense to form planets.

Stellar collisions are not all that common in the grand scheme of things due to the vast distance of separation.
 
  • #34
Drakkith
Staff Emeritus
Science Advisor
2022 Award
22,246
6,321
As I said, it just seems completely inconsistent to me that orbiting planets in a solar system should have formed entirely from the remnants of a gas cloud, whereas the Earth-Moon system formed from the collision of two large bodies.

There need not be a single formation mechanism for everything. The collision hypothesis about the origin of the Moon specifically came about as a way of explaining why the Moon and the Earth are so similar in makeup and so close in size (most moons are far, far smaller than their planets). No other bodies in the solar system match each other in composition as far as I am aware, and the standard accretion and capture mechanism doesn't do a good job of predicting this. Hence the need for a new model.

OK, so I guess I was trying to establish whether or not there was any reason why the planets did not form as a consequence of a body colliding with the Sun. The only answer seems to be that the current accepted theory for planetary formation is one of accretion, which doesn't really answer my question. If there is no known reason it's fine to say so.

It's not that such a thing isn't possible (it may or may not be), it's that it doesn't actually make a difference and it adds an extra complication. The material the planets formed from had to come from somewhere. Either it was leftover from the initial formation of the Sun, or something collided with the Sun and send out a large cloud of material. Both would result in an accretion disk, but your idea has the added step of requiring that an immense object (a considerable fraction of the Sun's mass) collide with the Sun.

So if an object did collide with the Sun, what was it? Where did it go? How did it form? When did the collision occur? The questions go on and on.

Keep in mind that the formation of the Sun is a very different process from the formation of a planet, despite both essentially being a collapse of matter into a dense body. Planets form around an object that is absolutely massive relative to themselves. Their formation process is affected by the star/proto-star in the form of heat, stellar wind, magnetic fields, etc. Stars form from the collapse of a large gas and dust cloud without a single massive body to influence them. Stellar nurseries commonly produce dozens or hundreds of stars at a time, all of similar chemical makeups.

So by your logic, it isn't consistent that stars don't form around larger objects, like planets do. But we know they don't. Stars don't orbit larger stars, which orbit even larger stars, they are the largest dense objects in the universe and the only 'objects' larger than themselves are just their own loosely bound populations orbiting each other in space in the form of galaxies, galaxy clusters, etc.
 
  • #35
sophiecentaur
Science Advisor
Gold Member
27,805
6,323
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.
What justifies the models that cosmologists use is not that they can watch one solar system progress through its life. Physical 'laws' have been proved to be pretty reliable under many conditions.

Another good example is the Hertsprung Russel Diagram, on which you can plot all the stars we see. The relative populations of stars in the various regions of HR are a great indication of the times taken for stars to spend in each of their phases of development. We only see any particular star for a few decades but that doesn't invalidate the approach.
The material the planets formed from had to come from somewhere. Either it was leftover from the initial formation of the Sun, or something collided with the Sun and send out a large cloud of material.


The actual numbers involved are very strong evidence to support the theory that planetary systems are home grown.
If you take a Nebula and watch it collapse, it cannot form a single body (star) because of the conservation of angular momentum. A star cannot be stable with much more mass than it has, for a given rotation rate. The initial formation of a planetary disc will have been from material that couldn't get into the centre (travelling too fast). Within the initial disc, there will be a range of orbits and objects will collide and merge together according to their constituents, to form rocky and gas giant planets. The result would be objects in mostly near-circular orbits and all going the same way round as the star rotates - all sharing more or less the same axis.

I read somewhere that the angular momentum of the Sun itself is much less than the total angular momentum of the Solar System. 99% of the mass but the distances (squared) of the planets are huge.

The notion of a large body colliding with the star to form a planetary system is not an attractive one. The collision cross section is tiny and that mechanism wouldn't account for the fact that many / most stars we see have planetary systems. OK, the occasional extra solar visitor could cause a very rare event. There is evidence of comets from deep space entering the SS but they are very rare - and that's the ones that don't actually collide with the Sun. Where would those 'other stars' come from, to have collided with each star to form planetary systems?
 
Last edited:

Suggested for: Planetary Formation

Replies
9
Views
643
Replies
0
Views
549
Replies
9
Views
309
  • Last Post
Replies
12
Views
782
Replies
2
Views
776
Replies
6
Views
236
Replies
5
Views
1K
Replies
5
Views
1K
  • Last Post
Replies
18
Views
3K
  • Last Post
Replies
1
Views
931
Top