Why is there a difference between the composition of planets and stars?

[artis]

I've been thinking about this from time to time,
I'll try to make this short and understandable.

So we go back to the early universe within our current best model of extrapolation called the Big bang theory. In the early universe as it was expanding we have a very dense and energetic matter everywhere, and judging by the CMB radiation the differences in energy from one spot to another were very minimal (which is what I get from reading about the CMB) so in other words the universe was rather monolithic in it's structure and energy correct?
Now if so far everything is correct then , why later as the universe keeps expanding and the overall density as well as energy decreases and first stars and planets start to form , why is the chemical makeup of these planets and stars so different?

In other words why we have very hot and dense stars composed mainly of light elements such as hydrogen and helium (the by product of hydrogen fusion) and then we have smaller planets much like Earth composed mainly of elements that have very large atomic masses compared to hydrogen and much heavy metals and very little light elements.?I understand (sort of) the official theory that in the early universe as the first large stars were born they fused enough elements into heavier ones that as with time they exploded one after the other the universe now had an ever increasing amount of heavier elements that could form planets with time and Earth just so happens to be the result of this, but since none of these reactions were "guided" but instead happened randomly then shouldn't it be the case that as the planets formed they could have caught both lighter elements like hydrogen as well as heavier elements?
But if this were the case then I suppose the chemical structure of planets as we know them today should have been rather different from the one we observe should it not?

 

[artis]

Just to add , if a mass starts to form (like a planet) and it is surrounded by various elements both light and heavy , as we know the gravitational acceleration makes different masses fall towards the center of gravity with the same speed , so all these various elements should tend to be part of the forming gravitational mass equally right?

 

[256bits]

The inner planets( Mercury, Venus, Earth, Mars ) of our solar system are rocky, but the outer planets (Saturn Jupiter Uranus Neptune ) are gaseous.

 

[PeroK]

I've been thinking about this from time to time,
I'll try to make this short and understandable.

So we go back to the early universe within our current best model of extrapolation called the Big bang theory. In the early universe as it was expanding we have a very dense and energetic matter everywhere, and judging by the CMB radiation the differences in energy from one spot to another were very minimal (which is what I get from reading about the CMB) so in other words the universe was rather monolithic in it's structure and energy correct?
Now if so far everything is correct then , why later as the universe keeps expanding and the overall density as well as energy decreases and first stars and planets start to form , why is the chemical makeup of these planets and stars so different?

In other words why we have very hot and dense stars composed mainly of light elements such as hydrogen and helium (the by product of hydrogen fusion) and then we have smaller planets much like Earth composed mainly of elements that have very large atomic masses compared to hydrogen and much heavy metals and very little light elements.?I understand (sort of) the official theory that in the early universe as the first large stars were born they fused enough elements into heavier ones that as with time they exploded one after the other the universe now had an ever increasing amount of heavier elements that could form planets with time and Earth just so happens to be the result of this, but since none of these reactions were "guided" but instead happened randomly then shouldn't it be the case that as the planets formed they could have caught both lighter elements like hydrogen as well as heavier elements?
But if this were the case then I suppose the chemical structure of planets as we know them today should have been rather different from the one we observe should it not?

Good question! I think you should be able to find the answer already online. It didn't take me long to find this, for example:

https://forum.cosmoquest.org/archive/index.php/t-4063.html
The gist of it is:

1) The solar system is mostly hygrogen and helium, plus some heavier elements.

2) The Sun is the expected mixture of these.

3) The rest of the solar system was the same mixture.

4) Once the Sun got to a critical size it ignited and started to produce the solar wind.

5) The solar wind blew lighter elements outward, making the outer planets predominantly gas and the inner planets largely gas-less.

In other words, it's the solar wind that caused the inhomogenity in the rest of the solar system.

I wonder if that's correct? Sounds plausible.

You'll probably get a better answer from someone who actually knows something about this.

 

[artis]

Ok, I can see your points but as also one poster noted in the thread @PeroK you gave is that the sun is still a main sequence star which means it's rather young and isn't in the phase where it would produce the most heavy of metals and do that abundantly.
In fact we know that Earth and other solar system planets are quite old themselves compared to the sun so at the time when Earth was forming our sun was very young and being such a young star it couldn't contribute any of the heavier elements that Earth has at all so where did earth, Mars etc got their heavier elements from then?

 

[PeroK]

Ok, I can see your points but as also one poster noted in the thread @PeroK you gave is that the sun is still a main sequence star which means it's rather young and isn't in the phase where it would produce the most heavy of metals and do that abundantly.
In fact we know that Earth and other solar system planets are quite old themselves compared to the sun so at the time when Earth was forming our sun was very young and being such a young star it couldn't contribute any of the heavier elements that Earth has at all so where did earth, Mars etc got their heavier elements from then?

The heavier elements in the solar system are in the same proportion that they are in the Sun itself. The Sun has 99.8% of the mass of the solar system. The heavier elements outside the Sun represent a tiny fraction of the mass the solar system.

Your initial analysis, I believe, is based on the false assumption that there is a "lot" of heavier elements outside the Sun and very little in the Sun. The analysis I linked to suggests that:

99.8% of the hydrogen and helium are in the Sun
99.8% of the heavier elements are in the Sun

The 0.2% of the mass of the system outside the Sun has been split into:

A very small amount of heavier elements in the inner rocky planets.
A larger (but still very small) amount of lighter elements in the outer planets.

 

[symbolipoint]

Interesting question
Not my area at all

One thing noticeable is the arrangement is much like higher density material collecting toward the "bottom" and lower density material collecting away from the "bottom" much like we see in mixtures in a vessel here on earth.

 

[Dragrath]

What is important to note is that the differences observed are both controlled by chemical processes of the elements and their natural abundances as a result of various forms of nucleosynthesis. Initially the accretion disk around the sun as others mentioned started out with a composition like that of the Sun as it was part of the in falling gas that formed the Sun in the first place.

The reason planets and planetesmals are predominantly heavier elements is that they largely formed from particle grains that precipitated out from the Solar Nebula as material fell onto the forming Sun via collisional transfer of angular momentum. as such the planets are built out of what particles could precipitate out from the solar Nebula at a given distance from the star. For instance larger planets are formed out farther since astrophysical "Ices" could precipitate out and coalesce to form larger bodies in other words because more chemical species could precipitate out, water among the most important of those due to the abundance of its constituent elements. The distribution of this matter largely has to do with the elemental abundances, Oxygen in particular is the third most abundant element largely due to its role as one of the main products of Helium fusion in the cores of stars and the inability of all but some of the most massive stars to undergo oxygen fusion. (Carbon the other major product of Helium fusion fuses at far lower temperatures and pressures than oxygen) Because of this water seems to be highly abundant in our universe and thus the ice line or the distance away where water ice can precipitate out of the solar nebula plays a major role in planet evolution enabling the formation of giant planets to grow from more material. Within the ice line only substances with high precipitation temperatures like silica (Silicon dioxide) can precipitate out as what we call "rock". The details of how this process occurs is not yet settled since many mechanisms for growth remain uncertain in the scientific comunity but somehow planetesmals eventually form and depending on circumstances they can eventually grow and as they reach certain mass thresholds to hold onto different materials depending on their densities and time of formation they can become various types of planets.

For instance Ice Giants are predominately composed of Ices likely having formed close to the Ice line and only briefly were able to acquire material from the solar nebula and so only have a relatively small envelop of hydrogen and helium where as their cousins gas giants formed earlier and became massive enough to attract gas from the nebula itself and thus the bulk of their mass is hydrogen and helium which were the predominate materials in the nebula. In reality planets can migrate around and interact due to gas drag and gravitational effects which adds additional complexities to the process but the general types of materials seem to be governed by where the planetesmal formed in the solar nebula and its history thereafter. I hope this gives you a small taste of the basics and some key words to research on your own.

I recommend looking up
accretion and in particular look into the role of conservation of angular momentum
planetary migration
planetesmal formation
pebble accretion
collisonal growth
solar system formation
direct collapse instabilitiesGood luck! (If I have time I might go back and add specific sources if needed)

 

[Vanadium 50]

In fact we know that Earth and other solar system planets are quite old themselves compared to the sun

We know no such thing.

where did earth, Mars etc got their heavier elements from then?

Same place the sun did. From the original molecular cloud that formed the solar system.

 

[artis]

@Vanadium 50 , what exactly was wrong about my statement that Earth is almost as old as the sun, are you saying that is incorrect?

Simple googling gives the sun 4.6billion years while Earth about 4.5
Also one other thing, I think @Dragrath or someone else here brought up the formation of the solar system which is said to have formed from a previous large cloud of matter shall I say, now with regards to this I did some reading and if I am correct then we came to this conclusion through models and some observations of other galaxies/parts of the universe but we haven't got direct evidence by which we could say with 100% certainty of how this unfolded?
pardon my vague terminology since I am not an expert in cosmology, hopefully it can be understood.

 

[artis]

@Dragrath I see there are still some inconsistency between the planet forming models , where for example core accretion model takes a bit too long to produce planets made largely of light elements, but I definitely have to read more to be able to go through all of this.

Maybe this is bit offtopic but I have thought from time to time has anyone calculated from a probability theory viewpoint what is the chance of an Earth like planet (by this I mean a planet that can sustain life as we know it and in the forms/parameters that we know it) to develop through solar system formation according to the best models and evidence that we have so far.

 

[PeroK]

Maybe this is bit offtopic but I have thought from time to time has anyone calculated from a probability theory viewpoint what is the chance of an Earth like planet (by this I mean a planet that can sustain life as we know it and in the forms/parameters that we know it) to develop through solar system formation according to the best models and evidence that we have so far.

Probability theory generally applies in cases where you have a reasonable understanding of all the parameters. Otherwise, the numbers you put in and the numbers you get out are guesses.

In terms of Earth-like planets, the probability is more likely to be gained from a study of how many Earth like planets there are out there.

In other words, there are too many variables to estimate the probability accurately from "first principles". It's more of a data analysis exercise. Such as:

https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets

 

[Vanadium 50]

what exactly was wrong about my statement that Earth is almost as old as the sun

That wasn't the statement. The statement was that the Earth is old compared to the sun.

 

[artis]

@Vanadium 50 yes it seems my wording was misleading, my bad

 

[pinball1970]

The heavier elements in the solar system are in the same proportion that they are in the Sun itself. The Sun has 99.8% of the mass of the solar system. The heavier elements outside the Sun represent a tiny fraction of the mass the solar system.

Your initial analysis, I believe, is based on the false assumption that there is a "lot" of heavier elements outside the Sun and very little in the Sun. The analysis I linked to suggests that:

99.8% of the hydrogen and helium are in the Sun
99.8% of the heavier elements are in the Sun

The 0.2% of the mass of the system outside the Sun has been split into:

A very small amount of heavier elements in the inner rocky planets.
A larger (but still very small) amount of lighter elements in the outer planets.

This was a misconception I had also.
I just assumed the planets had the heavier elements and the sun had H.
Something to read up on more.
Thanks Perok

 

[artis]

Yes this was also a fact I hadn't thought about in detail that simply because the sun has such a large mass compared to the rest of the planets/bodies in our solar system it also holds most of the elements in terms of percentage.

But here I need to ask a bit more. So as you @PeroK said that the sun has both almost all of the total light element mass in the whole system as well as almost all heavy element mass within our solar system, but since Earth formed just a bit later than the sun it means that as Earth had formed sun was still young so where di Earth got the majority of it's heavy elements?
My reasoning would be that sun being so young hadn't yet produced enough of those elements to share with Earth and they got here from the original dust/matter cloud that formed the solar system in the first place or did the sun manage to make them already back then and leak out and into Earth as everything formed?

 

[Bandersnatch]

My reasoning would be that sun being so young hadn't yet produced enough of those elements to share with Earth and they got here from the original dust/matter cloud that formed the solar system in the first place or did the sun manage to make them already back then and leak out and into Earth as everything formed?

No, you're right - it's all from the primordial cloud. Whatever the star synthesises doesn't get to leave it until the final stages of its life, so it can't enrich its own system. All elements have to be supplied up-front.

 

[sysprog]

As far as we know, metals heavier than iron are not in significant quantities formed in stars of the magnitude of the sun.

As far as we know, the heavier elements are primarily formed only in events of no lesser magnitudes than those of supernovae or interstellar collisions.

In perhaps 4 billion years, when Andromeda collides/merges with the Milky Way, we might know much more than we know now.

 

[kimbyd]

Good question! I think you should be able to find the answer already online. It didn't take me long to find this, for example:

https://forum.cosmoquest.org/archive/index.php/t-4063.html
The gist of it is:

1) The solar system is mostly hygrogen and helium, plus some heavier elements.

2) The Sun is the expected mixture of these.

3) The rest of the solar system was the same mixture.

4) Once the Sun got to a critical size it ignited and started to produce the solar wind.

5) The solar wind blew lighter elements outward, making the outer planets predominantly gas and the inner planets largely gas-less.

In other words, it's the solar wind that caused the inhomogenity in the rest of the solar system.

I wonder if that's correct? Sounds plausible.

You'll probably get a better answer from someone who actually knows something about this.

This is largely correct. The Sun blew the lighter elements away from the inner planets. We know that this doesn't always happen. Presumably if an inner planet gets big enough before the Star gets really active, it can survive this (we've observed lots of "hot Jupiter" planets around other Stars that are basically huge gas giants at roughly the orbit of Mercury compared to their host star).

Basically, normal matter has lots of incredibly complicated interactions that cause elements to have different concentrations in different places depending upon their history.

 

[rootone]

Because stars are actively fusing heavier elements, and planets are not.

 

[Vanadium 50]

My reasoning would be that sun being so young hadn't yet produced enough of those elements to share with Earth and they got here from the original dust/matter cloud that formed the solar system in the first place or did the sun manage to make them already back then and leak out and into Earth as everything formed?

so where did earth, Mars etc got their heavier elements from then?

Same place the sun did. From the original molecular cloud that formed the solar system.

The answer hasn't changed.

 

[kimbyd]

The answer hasn't changed.

Yup. And to expand upon it a little bit, a more full answer without going into detail would be:

1) The early universe was almost nothing but Hydrogen and Helium.
2) Massive stars produced the heavier elements up to iron in their cores.
3) When large stars eventually go supernova, they both produce a number of elements heavier than iron, and disperse these heavier elements across light-years of space.
4) New stars form from gas clouds which collapse under their own weight.
5) As they collapse, very complicated physics causes the heavier and lighter elements from the originating cloud to wind up in different places.

 

[artis]

No objection just to add that as a summary this all sounds very deterministic and specific although from what I'm reading I'm getting the impression that this whole "planet formation" is a very undetermined and random event. Now don't get me wrong here I'm not saying that gravity is random etc, sure the laws of physics are what they are but the result of all this matter interacting and producing new "emergent properties" like solar systems and planets seems very random, well due to gravity all planets and stars are round that's a given but their size and how far apart they are and the conditions on these planets do seem very random,

not to mention that there is still gaps and unclear parts in the best models so far that try to explain all of this.

 

[kimbyd]

No objection just to add that as a summary this all sounds very deterministic and specific although from what I'm reading I'm getting the impression that this whole "planet formation" is a very undetermined and random event. Now don't get me wrong here I'm not saying that gravity is random etc, sure the laws of physics are what they are but the result of all this matter interacting and producing new "emergent properties" like solar systems and planets seems very random, well due to gravity all planets and stars are round that's a given but their size and how far apart they are and the conditions on these planets do seem very random,

not to mention that there is still gaps and unclear parts in the best models so far that try to explain all of this.

Bear in mind that the descriptions in this thread are very superficial. Far, far more is known about the process than has been stated here. There most definitely is a random component, in the sense that the initial shape, composition, and size of the gas cloud will be different for different star systems. But once that initial gas cloud stats forming a star system, the result becomes highly deterministic.

To state it another way, the initial conditions change a lot from star system to star system. But once those initial conditions are there, the rest plays out in a highly consistent way. That is to say, if you had a really good computer for simulating the system and knew the initial gas cloud composition, you'd be able to calculate with a high degree of certainty what the final configuration of the system would be.

 

[artis]

Well I wasn't implying that each time you start with a gas cloud instead of a solar system you end up with a random asteroid belt all alone or something, sure not like that. My idea was a bit different I was trying to say that you can start out with say a million different gas clouds, in almost all cases a central star will form and some planets around it, bigger or smaller more or less in count would differ from system to system but there would be resemblances.
I was rather thinking what would be the chances to make everything so "right" that you essentially have a solar system with at least one planet that resembles Earth in it's capability to sustain life.

PS. This is not me arguing a fine tuning argument although I must admit it could be labeled that way but rather my curiosity to understand the uniqueness of our place in the universe and as I said earlier , if this would be possible to calculate it would be really interesting to know what are the chances of the cosmic gas cloud "dice" to roll up an Earth vs a million other "lifeless" solar systems.

I would like to come up with some more much deeper questions but right now I'm caught in the middle of stuff and my brain isn't a multicore unit exactly.

 

[BWV]

Curious about a relationship between low or non-existent metallicity in the first stars and the size and lifespan and found this FWIW, apparently the first all-H/He stars would have been much larger and shorter-lived, generating a greater number of supernovae that provided metals to the early universe:

A team of astronomers has found the best evidence yet for the very first generation of stars, ones made only from ingredients provided directly by the big bang. Made of essentially only hydrogen and helium, these so-called population III stars are predicted to be enormous in size and to live fast and die young. Until recently, many astronomers had thought they would never be able to see such stars, because they would have all burned and died in the universe’s early history—too far for us to see. But using new instruments on the world’s top telescopes, the team found a uniquely bright galaxy that seems to bear all the hallmarks of containing population III stars.

“The evidence is strong. They did a careful job,” says Avi Loeb, chair of Harvard University’s astronomy department.

Theorists predict that the clouds of gas in the early universe would have remained relatively warm from the big bang and so would resist condensing down to form stars. Mixing in a small amount of heavier elements helps gas clouds cool, because those elements are easier to ionize and so shed heat as radiation. But those heavy elements hadn’t yet formed in the early universe, so stars grew to enormous sizes—hundreds or even a thousand times as big as our sun—before their cores were dense enough to spark fusion. Once they did get started, they burned fast and hot, emitting lots of ultraviolet light and burning out in a few million years.

https://www.sciencemag.org/news/2015/06/astronomers-spot-first-generation-stars-made-big-bang

 

[Dragrath]

@artis our knowledge on solar system formation is a bit patchy but it has improved considerably with us now being able to observe the process of planet formation around other stars as well as detect other solar systems around their stars in addition to more in situ observations from our solar system and better more comprehensive models. The limitations mostly are related to the Early phase of these events which occur at a size scale that is beyond our current ability to resolve but we can at the very least observe forming gas giants clearing gaps in their stellar disks and in some cases the planets themselves such as Fomalhaut b which appears to have some sort of reflective orbiting disk, (as we can observe it orbiting its star) allowing us to observe it models suggest around 0.5 Jupiter Masses@artis our knowledge on solar system formation is a bit patchy but it has improved considerably with us now being able to observe the process of planet formation around other stars as well as detect other solar systems around their stars in addition to more in situ observations from our solar system and better more comprehensive models. The limitations mostly are related to the Early phase of these events which occur at a size scale that is beyond our current ability to resolve but we can at the very least observe forming gas giants clearing gaps in their stellar disks and in some cases the planets themselves such as Fomalhaut b.

Answering whether Earth analogs are rare or common is currently at or beyond our detection limits at least for massive stars and will likely require the development of new generations of space based telescopes beyond our current technology limits perhaps radio Interferometry will enable us to observe the formation events of natal star systems in enough details to answer these questions but thus far we don't have those answers or the means to easily acquire them. Future work is needed and things are developing quickly in exoplanet studies.

 

[kimbyd]

I was rather thinking what would be the chances to make everything so "right" that you essentially have a solar system with at least one planet that resembles Earth in it's capability to sustain life.

Based upon our current knowledge of exoplanets, there seem to be a lot of Earth-like planets out there.

 

[Dragrath]

Based upon our current knowledge of exoplanets, there seem to be a lot of Earth-like planets out there.

Quite likely, but keep in mind that thus far we lack the ability to resolve such planets despite popular overblown claims of "Earth twins" most of which are "Super Earths" to "Sub Neptunes" I expect such analogs to be common but despite popular claims we don't really have the ability to detect real analogs yet at last around sunlike stars, as we have detected a moon mass planet around a pulsar :), but since less massive planet types seem to be more common so far they probably exist in higher abundances. To detect such planets in their habitable zone we will likely need direct imaging as the less massive a planet the smaller the radial velocity shifts meaning Earthlike planets shifts are hidden by stellar variability and or more massive planets and similary the brightness drops of Earthlike planets are at the detection limits of the transit method. Perhaps we will find out how to account for stellar magnetic activity enabling us to reduce the signal noise.