Consider a loose pile of rocks in an early solar system, say a tiny asteroid. As it accretes more and more material, the pressure at its center eventually becomes great enough to collapse the gaps between the rocks, which further increases the density, which increases the gravitational force, ... which leads to a runaway process that compacts a loose pile of rubble into a near-spherical body of almost solid rock. (And some ices and liquids.)

But where do the rocks themselves come from? Surely at least that much pressure is needed to fuse dust into macroscopic rocks? Why isn't this a chicken-and-egg problem?

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    $\begingroup$ First, there's a misconception. Compressing a body does not increase the force of gravity on outer-lying masses. That's Gauss's famous divergence theorem. Addressing your question, we've had recently a similar question about rocks in space astronomy.stackexchange.com/questions/19865/… maybe the answer there satisfied you, then I don't have to type things double ;) $\endgroup$ Feb 15, 2017 at 0:48
  • $\begingroup$ @AtmosphericPrisonEscape This is interesting! I've just asked the question What does Gauss' divergence theorem say about compression of a body under self-gravitation?. $\endgroup$
    – uhoh
    Feb 15, 2017 at 4:08
  • $\begingroup$ @AtmosphericPrisonEscape by the way your answer here is really helpful. You've taken quite a lot of Astromineralogy science and theory and concisely outlined it with references. $\endgroup$
    – uhoh
    Feb 15, 2017 at 4:17

3 Answers 3


In general, solids are made from from atoms that originate naturally in stellar nucleosynthesis,
see for example the composition of our sun's atmosphere from an older paper of
Asplund et al (2005): Solar element composition

Here you see for example, that silicon (Si), an important rock-forming element, has an abundance relative to hydrogen of $10^{-3}$, or for every 1000 hydrogen atoms there is one silicon atom. Also we find lots of oxygen, carbon, magnesium, iron,... all known rock-forming elements.

The above work has been performed using spectroscopy, and in a similar way we can detect solids in space mainly through three means:

  • Spectroscopy (again): Small particles of $\mu m$-sizes exist in vast numbers in the spectrae of red giants (I'll comment on the formation later), white Dwarves, in cold molecular clouds and even in far away Quasars. Those numbers are so huge that we can detect prominent features like the 10 $\mu m$-feature of the $Si-O$ bond that we find in many earthly minerals. See also the following example of a clear detection of silicate minerals in the envelope of a young star (the broad trough at 10 $\mu m$) (c)Uni Hawaii: enter image description here

  • As mentioned, when solids grow, their numbers diminish. Thus big solids around the sizes of mm become invisible in spectrae, but sometimes can be detected through scattered light, as they undergo Mie-scattering with visible or infrared light when being very near to a stellar source.
    Then one can take pictures like the one from Benisty et al. 2015 of the young stellar system MWC758, where we see dust spirals possibly caused by the existence of young planets:
    enter image description here

  • Reflection of solar light: As solids grow to cm, meter, kilometer-sizes they become invisible for the above methods. Only at sizes of tens to hundreds of kilometers and only in our solar system can we then detect them again through the reflection of solar light.

Now that we have established that solids exist in space, let us review the theoretical perspective of growth mechanisms:

  • dust of up to $\mu m$ condenses out in the relatively cool atmospheres of red giants (AGB stars), where the ionized environment helps to grow solids through charge separation. Details e.e. in here.
  • this dust is blown away in the strong AGB star winds into the interstellar medium, where it helps to create new star systems (see also the second image I linked).
  • All successive growth to larger sizes has to happen in the interaction with gas, thus it usually happens in the relatively dense environment around young stars
  • The dust has now to overcome the bouncing barrier in order to grow into cm to meter-sized particles that we call 'pebbles'. How particles overcome this barrier is unclear in general, but the article I've linked mentions a few solutions.
  • Pebbles are defined by their property that they interact with the orbiting gas in protoplanetary discs through friction. This interaction allows them to trigger the streaming instability, a very efficient mechanism that is able to compress clumps of pebbles directly into km-sized solids.
  • Now small comets grow through direct collisions, up to planetary sizes. Only in this stage geological mechanisms can play a role, like the igneous separation of minerals that you've mentioned.

That's it for now. I think with this description it is clear that the whole problem is not chicken-and-egg, but if you think otherwise then please comment.


Micrometer-sized dust condenses in red-giant atmospheres, then interacts with gas around young stars to grow to mm, then overcomes the bouncing barrier to reach cm to m sized "pebbles." (Answer derived from comments by A.P.E.; sadly, theirs links are dead. Edit: A.P.E. reposted an answer that I could accept.)

  • $\begingroup$ Sorry, I apparently didn't see that a while ago, and also didn't see that the question and subsequent answer was deleted. I can repost it. $\endgroup$ Dec 26, 2017 at 23:25

The initial seeding of rocks is thought to be a rapid and energetic event resulting in the formation of chondrules which are millimeter-sized spherules that form as molten (or partially molten) droplets in space before coalescing to their parent asteroids.

Recent evidence suggests that the solar system comes from the hottest stars devoid of hydrogen at the surface and the resulting wolf-rayet nebula, which would give similar aluminium and iron isotopes found nearby. That's cool if we want to know where the energy came from to condense nebula into chondrules. Magnetic flares, nebula lightning and sunstorms may give the energy to seed the first rocks of a nebula.

The Wikipeda page of cosmic accretion theory is very mysterious.

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    $\begingroup$ Does paragraph 2 mean that our solar system comes from other stars? Or that solar systems only develop around stars of that kind? Or something else? $\endgroup$ Dec 30, 2017 at 17:42
  • $\begingroup$ google.fr/… $\endgroup$ Dec 30, 2017 at 18:55

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