Planetary Composition: Elements Beyond Silicon, Iron & Carbon

[Yae Miteo]

So... Theoretically, planets (that are not gas giants, or water planets) could be composed of silicon, iron, or carbon. Are there any other elements that could make up most of a planet?

 

[Student100]

They could be comprised of insert pure speculation. Exoplanets haven't been around long, it's likely the majority of exoplanets planets will be comprised of the same materials we see in our own solar system, based on the ratios of material that the system formed from.

The recent diamond planet turned out to be a dud.

 

[Yae Miteo]

So where do the materials come from? The same materials that formed the star?

 

[Bandersnatch]

So where do the materials come from? The same materials that formed the star?

Yes, unless it's a rogue planet captured by the gravity of the star.

Planets and stars are currently thought to coalesce from the same collapsing gas clouds. The "metallicity"(in astronomical parlance the abundance of elements heavier than helium) of the cloud determines* the proportions of mineral abundance on the planets.

Here's a review paper on terrestial planet formation:
http://arxiv.org/abs/1208.4694

*As the paper describes, there are some processes that affect the final composition. E.g., planets too close to the parent star will have their volatile elements blown away, so the innermost planets will tend to be denser, iron rich, and rocky. The outer planets should include more gas giants and "dirty snowball"-type of planets.(but there are exceptions - cf "hot Jupiters").

 

[Yae Miteo]

So a planet could be composed of, say, copper?

 

[Bandersnatch]

Not really, no.

The primordial gas cloud is a mixture of all the elements. It is statistically impossible to expect just one element to collapse to form a planet while excluding all the rest -- unless you come up with some plausible process to sort them out.

Imagine the cloud as a pile of sand composed of various sizes and colours of grains. If you scoop a bucketfull to make a sand castle, you wouldn't expect the contents to be just 1mm-diametre black grains -- especially if these were very rare in the pile in the first place.
You can expect the average size of the grains to be larger if you scoop from the top of the pile than if you do from the bottom, since gravity will tend to separate them like that.
Similarly, you can expect inner rocky planets to be less abundant in e.g., volatile carbohydrates(so, less H and C than you'd expect just by looking at the abundance of elements in the Galaxy: http://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements), because there is a process that separates them.

Planets will always be mixtrures of various elements, and the rough abundance of them will be close to the abundance of elements in the universe. You'll have a lot of hydrogen, carbon and oxygen, and little heavy metals. You'll never have just one element.

 

[Yae Miteo]

But why are some planet types labeled as silicate, iron, or carbon? http://www.nasa.gov/vision/universe/starsgalaxies/Earth'sized_planets.html (it's in the picture that says: Predicted Sizes of Different Kinds of Planets)

 

[Bandersnatch]

But why are some planet types labeled as silicate, iron, or carbon? http://www.nasa.gov/vision/universe/starsgalaxies/Earth'sized_planets.html (it's in the picture that says: Predicted Sizes of Different Kinds of Planets)

Note that Earth is a silicate planet. It most emphatically is not composed only of silicon, or even only of silicon compounds. It just means there is an overabundance of silicates as compared to the primordial gas cloud composition(which we can infer from looking at the Sun).

Additionally, from what I can see, the article references a study that took idealised conditions of e.g., pure water or pure iron forming a planet, to determine some boundaries for possible sizes. It should not be taken to mean that there can plausibly exist a planet composed only of of H20, etc.

 

[Yae Miteo]

So, as a side thing, almost any size planet could have the same gravitational pull as earth, depending on its composition?

 

[Bandersnatch]

I think it's abit more complicated that that.

At the most basic, to keep 1g, all you need to make sure is that the planet satisfies the relation:
1=ρ*R where ρ is the average density expressed as a fraction of average Earth density, while R is the planet's radius as a fraction of Earth radius.

So if you want a planet with twice the Earth radius, you just need it to have half the density of Earth. Same the other way around.

But different materials react differently to compression, so if you try to make your planet from e.g., pure gas, it wouldn't stay at the radius you'd want it to but collapse until it reaches equilibrium. Same for different kinds of rocks etc.

I believe determining the more accurate possible sizes of planets, taking gravitational compression into account, was the purpose of the study mentioned in the article you linked to earlier.

 

[Yae Miteo]

This may sound like a silly question, but what does the asterisk operator you used mean? I only have a basic grasp of calculus so it probably is from a form of math that I do not know how to use, I'm just wondering what level math I would need to know. (edit: should I have asked this in the math section instead?)

 

[Bandersnatch]

That's just simple, regular, run-of-the-mill multiplication. It's a very simple relationship.
Getting to it is a bit more interesting, but still relatively simple. You just need Newton's gravity equation and the formula for the volume of a sphere.

And take it easy with the "thanks" button. Once is more than enough :)

 

[Yae Miteo]

Ok. Sorry about the thanks button... I'm really new here...

 

[Yae Miteo]

Oh, and once again, this may be more appropriate for the math forums, but how do I find ρ?

 

[Bandersnatch]

It's simple algebra, really.

1=Rρ
R=1/ρ
ρ=1/R

Start with one of the variables. Say, density of some material from the wikipedia, or some radius you'd like the planet to have. Express it as a fraction of Earth density(5.5 g/cm^3) or radius(6700km), and substitute to the equation.

E.g., if you want a planet to be made of some substance with density of 2g/cm^3, then in you'd substitute ρ=2(g/cm^3)/5.5(g/cm^3). The units cancel out, so you end up with R=1/(2/5)=5/2. Which means the radius would need to be 5/2 of Earth's, whic is 5/2*6700km=16750km.
And so on.

Intuitively, the relationship simply means that if you want a planet twice the size(radius) of Earth, it'll need to be half the density to still produce 1g at the surface.