Planetary Formation: Could Collisions Explain Our Solar System?

[guesses3]

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?

 

[Vanadium 50]

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.

 

[256bits]

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.

 

[Drakkith]

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.

 

[sophiecentaur]

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?

 

[guesses3]

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?

a. So the Sun and the planets all formed from the same gas cloud, but the [core of] the Sun is not the same as the [core of] the planets?
b. My understanding is that Theia did not survive the collision with the proto-Earth, but the orbiting ejecta from the collision subsequently accreted to form the Moon. But I'm not allowed to theorise that something similar would happen in a giant impact collision with the proto-Sun, so I can't answer your question.

 

[guesses3]

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.

Again, without wishing to theorise, I would observe that most systems consist of 2 or more stars.

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.

No, I am quite happy to accept that stars form from the gravitational collapse of a gas cloud. I guess what I am questioning is the theorized formation of planets by accretion as part of that collapse, rather than as a consequence of the accretion of (some of) the ejecta from a collision involving the star/proto-star.

 

[guesses3]

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?

Thanks, that was a very helpful post (ditto to 256bits and Drakkith), especially the bit about angular momentum. I would just observe what I said in my reply to Drakkith about most systems consisting of 2 or more stars.

 

[sophiecentaur]

a. So the Sun and the planets all formed from the same gas cloud, but the [core of] the Sun is not the same as the [core of] the planets?

I think there is a gap in your view of the way things went. The early stars were formed in nebulae which consisted of virtually nothing but Hydrogen and Helium. There would have ben no planetary disc here - just a disc, consisting of H and He. Possibly there were balls of gas at appropriate distances and those would be 'real' gas giants.
The early stars, with enough mass, would have gone nova (like the ideal stars that are described in introductory lessons) and produced other elements. These elements would have been thrown out to form nebulae with various combinations of all the elements we find today. The timescales for the huge versions of early stars would not have lasted long (nothing like the billions of years for out Sun) so there would have been plenty of the nebulae we see today with all sorts of interesting spectra.

Formation of a 'second hand' star would be different with all the heavy elements now available. The fact that the early Sun contained mostly H and He is not surprising because the heavy elements are a very small proportion of what's still flying about out there. The basic processes in the Sun would be dominated by the overwhelming presence of H and He. It is no surprise to me that the planetary disc caused a selection of different chemistry in different orbits; dense planets would be expected to turn up close in and the less dense planets further out. Why would different kinds of stuff tend to separate out in 'clumps'? I don't know except that the same thing happens on Earth; we don't have a uniform soup of all the substances mixed together but gold mines and diamond mines etc. have great concentrations of useful chemicals for us.

The H and He in the Solar system would presumably have been driven out by radiation pressure and solar wind which would 'beat' the Sun's gravity.

 

[guesses3]

Formation of a 'second hand' star would be different with all the heavy elements now available. The fact that the early Sun contained mostly H and He is not surprising because the heavy elements are a very small proportion of what's still flying about out there. The basic processes in the Sun would be dominated by the overwhelming presence of H and He. It is no surprise to me that the planetary disc caused a selection of different chemistry in different orbits; dense planets would be expected to turn up close in and the less dense planets further out. Why would different kinds of stuff tend to separate out in 'clumps'? I don't know except that the same thing happens on Earth; we don't have a uniform soup of all the substances mixed together but gold mines and diamond mines etc. have great concentrations of useful chemicals for us.

OK, lots of questions!

1. I'm sure in the latest Sky at Night they discussed the Sun being "at least a 3rd generation star", ie its composition is such that it is not 1st or 2nd generation. But I'm sure they also said that this would have been contributed to by many novae, which I took to mean that a 2nd generation star doesn't go supernova, form a gas cloud, and then collapse to form a 3rd generation star. You seem to be implying that that is what happens?

2. The solar wind will strip planets of their atmospheres, hence why the inner planets have so little relatively. This has allowed the outer planets to accrete much larger atmospheres thereby growing the planets but reducing their overall density. But surely the cores of all the planets (and the Sun) are essentially the same, no? And then it just comes down to the physical phenomenon that denser substances sink?

3. I thought gold mines were the result of meteor/asteroid impacts? And diamond mines are just a product of Earth being seismically active?

 

[sophiecentaur]

You seem to be implying that that is what happens?

That's the problem with trying to get a complicated message in just one PF post without the help of a script writing team. The fact is that the distances between the particles in a nebula are (unbelievably) vast and come from more than one source. In the same way, a whole group of stars can be formed within a single nebula. Look at the Plaiedes, which is a group of young stars, formed in the same nebula and all of the same age and similar sizes, so they're all the same colour. (My favourite star cluster.)

But surely the cores of all the planets (and the Sun) are essentially the same, no?

In the simple pictures, people ignore this sort of thing but why would it not be the case? It wouldn't surprise me if there were fission of very heavy elements inside the core of a star, leaving a weaker and weaker mix in there with the H and He before the later stages of the star's life.

Afaik, it's accepted that the gas giants probably have rocky cores. After all, some of their planets are rocky so the stuff gets out there. Problem is that there is such a huge range of characteristics of the stars and planets that must be caused by the actual conditions on each (which vary over an unbelievable range).

And diamond mines are just a product of Earth being seismically active?

Yes - but local regions of the Earth have high densities of specific materials. Are you suggesting that multiple (say gold-bearing) asteroids could have landed in groups of nearby sites? The numbers and the explanations are all hard to grasp.

This thread is trying to cope with too many concurrent ideas I think.

 

[Motore]

But surely the cores of all the planets (and the Sun) are essentially the same, no?

Surely not.

Sun:

The core is made of hot, dense plasma (ions and electrons), at a pressure estimated at 265 billion bar (3.84 trillion psi or 26.5 petapascals (PPa)) at the center.[3] Due to fusion, the composition of the solar plasma drops from 68–70% hydrogen by mass at the outer core, to 34% hydrogen at the core/Sun center.

https://web.njit.edu/~gary/320/Lecture22.html

Planets:

Cores may be entirely solid or entirely liquid, or a mixture of solid and liquid layers as is the case in the Earth.

Gas giants also have cores, though the composition of these are still a matter of debate and range in possible composition from traditional stony/iron, to ice or to fluid metallic hydrogen.

https://pubs.geoscienceworld.org/gs...ian-core-Implications?redirectedFrom=fulltext
https://www.sciencedirect.com/science/article/abs/pii/0032063382901088?via=ihub

In the simple pictures, people ignore this sort of thing but why would it not be the case?

Because it is not. See above.

It wouldn't surprise me if there were fission of very heavy elements inside the core of a star, leaving a weaker and weaker mix in there with the H and He before the later stages of the star's life.

That would surprise me, as fusion is the primary nuclear reaction inside the core of a star.

This thread is trying to cope with too many concurrent ideas I think.

This thread is an idea of the OP that everything (including stars and planets) is the result of colliding giant objects (stars and planets are ejecta or debris of these collisions). When prompted to think where these colliding objects came from, his answer was that every object was made by a different big bang.

We have a good theory of planetary formation which is consistent with observations:

The nebular hypothesis says that the Solar System formed from the gravitational collapse of a fragment of a giant molecular cloud.

Did the planets form through collisions? Sure, but not of two large objects or two stars as the OP seems to think:

The currently accepted method by which the planets formed is accretion, in which the planets began as dust grains in orbit around the central protostar. Through direct contact and self-organization, these grains formed into clumps up to 200 m (660 ft) in diameter, which in turn collided to form larger bodies (planetesimals) of ~10 km (6.2 mi) in size. These gradually increased through further collisions, growing at the rate of centimetres per year over the course of the next few million years.

 

[Drakkith]

1. I'm sure in the latest Sky at Night they discussed the Sun being "at least a 3rd generation star", ie its composition is such that it is not 1st or 2nd generation. But I'm sure they also said that this would have been contributed to by many novae, which I took to mean that a 2nd generation star doesn't go supernova, form a gas cloud, and then collapse to form a 3rd generation star. You seem to be implying that that is what happens?

That's exactly what happens. Many 1st and 2nd generation stars went supernova, seeding the interstellar medium with heavier elements, which then ended up collapsing into future stars.

2. The solar wind will strip planets of their atmospheres, hence why the inner planets have so little relatively.

Also because the smaller planets don't have enough mass, and thus not enough gravity, to hold on to hydrogen and helium gas in an atmosphere.

But surely the cores of all the planets (and the Sun) are essentially the same, no?

No, not at all. I don't think the Sun every had a chance to differentiate or stratify like the planets did due to its sheer size, temperature, and the existence of convection currents that can mix material together.

 

[sophiecentaur]

Because it is not. See above

As @Drakkith says, inside stars, the material is more mobile so perhaps it's not as likely that heavy elements would find their way into a central 'core'. Nonetheless, I can't think of any mechanism that would select just H and He from the constituents of a nebula to eliminate heavy elements. I already made the comment about the relative abundances of elements in the universe. In a 'representative' nebula, you would expect very few heavy element nuclei to end up in the Sun just through aggregation of the 'slower' particles. Also the data seems to be in terms of mass (?) and, if you translate that into numbers of nuclei, the distribution becomes even more in favour of H and He.

as fusion is the primary nuclear reaction inside the core of a star.

Fusion of H is, as already discussed, by far the primary reaction but that doesn't exclude other reactions. Why wouldn't heavy nuclei be split under the conditions in the core? Fusion of heavy elements requires conditions that you find in a supernova. The proportions are different between Sun and Universe (see the table in the link) so you have three possible explanations:1. Unrepresentative nebula, 2. Some selection process during formation, 3. Fission. Can you dismiss 3??

Did the planets form through collisions? Sure, but not of two large objects or two stars as the OP seems to think:

agreed - but the fact that the disc remains such a tight disc excludes the possibility of a significant number of colliding bodies coming from random directions outside the SS. The OP seems to have ignored a few aspects of basic physics.

 

[MikeeMiracle]

I can't find the link but I recall recently seeing a picture of a proto-planetary disk which appeared to show planets starting to form prior to the star igniting fusion so it does not seem like the planets are formed purely from material the star blasts out after it ignites.

I think your also not appreciating the size of the Sun in comprison to the other planets. Any planet getting too close will just become part of the sun as it swallows it up.

OK, lots of questions!

1. I'm sure in the latest Sky at Night they discussed the Sun being "at least a 3rd generation star", ie its composition is such that it is not 1st or 2nd generation. But I'm sure they also said that this would have been contributed to by many novae, which I took to mean that a 2nd generation star doesn't go supernova, form a gas cloud, and then collapse to form a 3rd generation star. You seem to be implying that that is what happens?

2. The solar wind will strip planets of their atmospheres, hence why the inner planets have so little relatively. This has allowed the outer planets to accrete much larger atmospheres thereby growing the planets but reducing their overall density. But surely the cores of all the planets (and the Sun) are essentially the same, no? And then it just comes down to the physical phenomenon that denser substances sink?

3. I thought gold mines were the result of meteor/asteroid impacts? And diamond mines are just a product of Earth being seismically active?

1. Not all stars go supernova, only the biggest do creating all the elements heavier than Iron in the process. All the 1st generation stars were huge so they all went supernova. You also seem to imply that there is a fixed amount of gas/dust which formed a 1st gen star, and after it went nova that same material from the 1st gen star collapsed to form a 2nd gen star. 2nd generation stars are formed from the material of multiple 1st generation stars. 3rd generation stars formed from the material of multiple 2nd generation stars and likely left over material from the 1st generation also.

2. The size and pressure in the core of the sun is so great than Fusion occurs in the core, no other planets come close to this size and pressure so planetary cores are nothing like the core's of the sun.

3. While Gold and Diamond can be created this way the vast majority was already present in the collapsing dust cloud that formed the solar system after being created by previous generation stars.

 

[sophiecentaur]

I can't find the link but I recall recently seeing a picture of a proto-planetary disk which appeared to show planets starting to form prior to the star igniting fusion so it does not seem like the planets are formed purely from material the star blasts out after it ignites.

The only effects of a star on the planetary disc would be gravity - to keep the stuff there and the radiant heat, which would account for the loss of lighter elements from the inner parts of the disc, over time. So why would the disc wait for ignition before starting to sort itself out?

All the 1st generation stars were huge so they all went supernova.

How would you account for that when the only difference between original H and He clouds and the later nebulae from supernovae would be little more than traces of heavier elements. I'd be interested in a. source for that idea. But why would they all need to have gone nova. One important point about big stars is their short lifetime so you don't need many tens of millions of years for one to form and to explode - unlike main sequence stars like the Sun. That sort of statistic implies that, in a given space of time, there would be many more novae than the number of large stars that we see would suggest.

While Gold and Diamond can be created this way

Hmm; it would be the Carbon (nuclei)and not the diamond that's created in a star. Chemical combination would have taken place within its destination planet.

 

[russ_watters]

OK, but we haven't be able to observe what the subsequent result is - that will take millions of years.

This is the same argument creationists make against the fossil record. The way it works in reality is the same: you look at lots of examples at different stages of the evolution and then you connect the dots.

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?

We can be pretty sure the answer is no, since it hasn't been observed and more to the point it makes no sense. As others noted, since planets and stars are made of different stuff, there is nothing in such a collision of stars to create planets.

 

[sophiecentaur]

more to the point it makes no sense.

and it fails to satisfy all the evidence in planetary discs everywhere. How could they have formed in the same planes as random incoming objects?

People seem to want to transform astronomy into astrology.

 

[256bits]

since planets and stars are made of different stuff

The sun and planets are made from the same stellar nebula, which should have pretty much a homogenous composition. of which the sun would be representative. And pretty much Jupiter also. The surrounding gas and dust cloud early in star formation would be of the same material.

Differentiation is a two fold process for planet composition and formation.
1. Planet differentiation due to heating and melting of the rocky planetesimals allowing denser material such as iron and nickel to sink to, and form a dense core.
2. Planetesimal nebubula differentiation forms from magnetic interactions, viscous friction between regions and between dust and clumps and gas.

The differentiation process produced different compositions between planets, not that they formed from separate stuff from the nebulus cloud. Most of the research in this area is how the differentiation occurred, such as O isotope ratios, or chrondite composition.

it is pretty much accepted that the collision theory of planet formation is rejected for that which produced the vast majority of planetary systems, not because it is not possible by this process, but that collisions between stellar objects are rare. While a collision between two stellar objects in the denser globular cluster of the galaxy is at a rate of once per 10000 years, the chance of anyone such collisions for a particular object is 250 billion years. For the sun it is estimated that such a collision is 10 to the 28 years.
as russ stated.
We can be pretty sure the answer is no, since it hasn't been observed

Stellar collisions would be a research project not necessarily for the production of planetary systems but for how the dynamics of stellar objects can have outcomes observable - or not . see https://arxiv.org/abs/1102.3336 for example

 

[256bits]

How could they have formed in the same planes as random incoming objects?

A collision between two large proto stellar objects would disrupt the dynamics of the forming disks. A new disk would/ could form from the remnants, and possibly ensure the progression to planetary objects. Or it could totally disrupt the whole timetable and no planetary system is born.

 

[ibootrc]

I see great answers here. But i wanted to add this, i have watched Mr David Butler video on YouTube about the solar system formation, age & the planetary formation theory. It was informative in easy clear way. if you want to check it:

 

[berkeman]

I see great answers here. But i wanted to add this, i have watched Mr David Butler video on YouTube about the solar system formation, age & the planetary formation theory. It was informative in easy clear way. if you want to check it:

Welcome to PF and thanks. At first I was worried seeing a YouTube video posted as a reference in the PF technical forums, but that one is very informative and scientific.

 

[sophiecentaur]

I see great answers here. But i wanted to add this, i have watched Mr David Butler video on YouTube about the solar system formation, age & the planetary formation theory. It was informative in easy clear way. if you want to check it:

A useful source of good information. It's the sort of video that opens up a lot of potential topics for PF threads. All that credible data from NASA justifies a lot of the statements that we tend to take on board without too much questioning - like the timescales involved.
I watched it all the way through without any attention span problems!

I liked the explanation for the existence of very heavy nuclei, involving colliding neutron stars, on the grounds that your average supernova is just not powerful enough to get over the Coulomb force.