I wonder how very low-gravity dust could make bind together and make hard and dense stones that later on will merge together and make some of the planets? I read some meteorites as hard as a hard stone are remnants of the early phase of solar system's formation. If this is the case, it shows that they are formed using their own gravity attracting other dust; however, I thought making such a hard stone will require a high pressure.

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    $\begingroup$ Does this answer your question? How do rocky objects between 1cm and 1m accrete to form planetesimals? $\endgroup$
    – user438383
    Commented Apr 18, 2022 at 14:06
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    $\begingroup$ Actually keeping this question is helpful, since if someone asks some question like this, the search engines will route her here. The other one is a specific question and not a general one. $\endgroup$
    – abbassix
    Commented Apr 19, 2022 at 19:21
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    $\begingroup$ These are different questions and have different answers, not a duplicate, voting to leave open. $\endgroup$
    – uhoh
    Commented Apr 19, 2022 at 19:36

1 Answer 1


The process you are referring to is called pebble accretion, which is very complex and not particularly well understood.

The initial premise of your question, that dust particles stick together via their own gravity, is not quite correct. As you say, gravitational interactions between individual particles are incredibly weak. Rather, dust particles grow via collisions within a protoplanetary disk. During this early stage of star/planet formation, gas and dust are spread out in a huge disk in which the material orbits the newly forming star at the centre. Particles collide and stick to each other via electrostatic forces. Growth continues all the way through from dust particles, to much larger planetary building blocks called planetesimals. In this sense, meteorites are indeed remnants of the early Solar System. Sometimes you will hear them refer to as 'failed planets', in that they failed to accrete enough material to grow to planetary size.

There are still a lot of open questions relating to this process. For example, once particles reach sizes greater than about a meter, collisions result in fragmentation rather than growth. This has led to the development of newer theories, such as streaming instability, which are able to better explain rapid growth.

Recommended further reading...

  1. Raymond & Morbidelli (2020) - Planet Formation: Key Mechanisms and Global Models
  2. The Exoplanet Handbook (chapter 10) - Michael Perryman
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    $\begingroup$ There was a really nice experiment on the ISS recently that demonstrated how strong and important electrostatic effects are on accreting small particles together. Sorry I can't find a better link skyandtelescope.org/astronomy-news/… $\endgroup$ Commented Apr 19, 2022 at 23:22
  • $\begingroup$ This is not quite the answer to what OP is asking: "Stones" i.e.rocks of the size of a fist or smaller (down to mm-sized Chondrules) cannot be formed via pebble accretion. The latter is the process of accreting small rocks (such as Chondrules) onto a parent seed body of lunar to mars mass in order to form larger planetary cores. How those pebbles form in the first place, i.e. how they are concentrated and melted, is not answered here. $\endgroup$ Commented Apr 26, 2022 at 9:57

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