Can a carbon-rich planet form in a solar system where all the other planets have a lower carbon-to-oxygen ratio (like Earth has silicates and oxides, or oxygen and silica)? (Let’s say the planet formed locally — not a rogue planet that was captured.)

Can the primordial accretion disk have differentiated composition ring? Or are carbon-type planets only formed with carbon-rich star systems?

  • $\begingroup$ Earth is also not a carbon-type planet, there is very little carbon down here. its just that carbon tends to form chemical compounds that are volatile and float to the surface, while silicon makes compounds that are, well, rocks. And rocks don't float. not even tiny ones. mostly Earth is an Iron world, with tiny amounts of silicon-oxygen slag floating on it, and miniscule amounts of carbon and hydrogen sludge floating on top of the slag. $\endgroup$
    – PcMan
    Dec 23, 2020 at 6:11
  • $\begingroup$ @PcMan: Well, MOST rocks don't float. Meet pumice: universityofcalifornia.edu/news/… And Earth is mostly a silicate world, with a relatively small iron/nickel core. (V = 4/3πr^3, core radius 3483 km. Earth radius 6370 km, so the core is about 1/6 of the total volume.) $\endgroup$
    – jamesqf
    Dec 23, 2020 at 18:20
  • $\begingroup$ @jamesqf Earth: 5.98e24kg.. Iron=32%, oxygen=30.1%,silicon-15.1%.. I'm not sure I would consider 32% of the whole thing as "a relatively small iron core". and I definitely would not call it by its third-most abundant element. That would be like saying the ocean is composed of pure chlorine. $\endgroup$
    – PcMan
    Dec 23, 2020 at 21:07
  • $\begingroup$ @PcMan: We're talking about two different things, I think, as your figures seem to be for mass, while I was calculating volume. The mostly-iron core is relatively small, since iron is quite a bit denser than the other major constituents. The "silicon-oxygen slag" also contains quite a bit of iron (and other stuff), and occupies a far larger volume. $\endgroup$
    – jamesqf
    Dec 24, 2020 at 17:25
  • $\begingroup$ @jamesqf Defining an object's composition by volume is a daft idea! Using that rule, a boeing 747 is made out of "air". $\endgroup$
    – PcMan
    Dec 24, 2020 at 17:30

1 Answer 1


I think it is possible that the composition of the original accretion disk is not homogeneous with respect to species types.

Once the star begins blowing off its radiation and stellar wind, this will tend to blow away the atoms in the accretion disk. Because of their different masses, lighter atoms will be blown away faster than heavier ones.

However the difference in atomic weight between carbon (12) and silicon (14) is rather low, so this separation mechanism won't be very effective in separating them, as much as it can be with respect to separating for example helium and hydrogen from iron.

  • $\begingroup$ My interpretation of this is that carbon planets would need to form farther out, and if they're needed closer to the star, migrate inward somehow. Is that on the right track? $\endgroup$
    – CAE Jones
    Dec 23, 2020 at 7:29
  • $\begingroup$ Thanks for all your input! It really help! And Merry Christmas! $\endgroup$
    – Veknor
    Dec 25, 2020 at 2:22

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