[chris (OP)]

Donald Piniach had this to say in reaction to a recent episode of a Naked Scientists podcast:

I am not satisfied with the cursory and non descriptive [explanation] of how the gas cloud of hydrogen and helium with the conservation of momentum and energy can ever even get two atoms to stick together even with gravity pulling them together because with an elastic collision, they would still split apart. Convince me scientists have some idea of the problem with solar formation. I think all Rocky planets came from remnants of gas giant planet collisions, like the Earth's moon from something colliding with primordial earth.

What does everyone think?

[Kryptid]

A collision between two atoms is quite different from a collision between two macroscopic objects like rubber balls. Atoms have fuzzy electron clouds with mobile electric charges inside. For example, two hydrogen atoms that approach each other at low speed will form a bonding orbital (and anti-bonding orbital, but this is usually unoccupied) due to overlap of their 1s orbitals. Since this bonding orbital is lower in energy than the individual 1s orbitals, the bond is stable and holds the atoms together. If the atoms are moving too quickly (i.e. the gas is too hot), the bond will not be strong enough to hold the atoms together.

Gas clouds in space cool off over time by radiating heat and expanding, so any cloud would eventually become cool enough to contract under the force of gravity and (if it contains the appropriate elements), condense into solid bodies. Tiny solid particles like dust were found in an experiment in micro-gravity to attract one-another due to electrostatic charges. This helps the process of forming larger solid bodies which eventually grow until they have the needed gravity to accelerate their growth into fully-sized planets.

[chiralSPO]

Atoms and molecules in a flame on Earth are moving with an average speed significantly greater than 1 km/s, and while they certainly do bounce off one another quite often, they stick often enough to predictably produce stable molecules of H₂O and CO₂ every time (when burning hydrocarbons in the presence of oxygen). That right there establishes that there is a problem with the elastic collision model.

Atoms and molecules can also aggregate without forming chemical bonds or taking gravity into effect. An example of this is condensation. Water molecules in the atmosphere have an average speed of about 500 m/s, but when there is a high enough concentration of water in the air (humidity), the water molecules will stick to each other more often than they bounce apart, and droplets of water will form, in the form of clouds, fog, dew, or frost. Under certain conditions, this can even lead to the formation of larger aggregates like snow flakes, rain drops, sleet or hail (some hailstones can be more than 5 cm in diameter!)

[jeffreyH]

While electrons can be considered as point particles, protons and neutrons can't. This view is supported by data from particle collisions. They are more like small spheres. The scattering angles are one likely reason that atoms and molecules will tend to clump together.

[chris (OP)]

Fantastic answers; thanks everyone. Made me think about this in an entirely different way and it wasn't even my question!

[Bored chemist]

...
with the conservation of momentum and energy can ever even get two atoms to stick together
...
I think that you would say it's impossible fro two hydrogen atoms to recombine to forma a molecule of H2.
So, since that's clearly wrong, you have to see look to see it really works.
One part of the answer is the emission of radiation which can carry both  energy and momentum, thus balancing the needs of the conservation laws.
The the other answer is the presence of a third particle which can do the same thing.

[yor_on]

lovely answers :)

So do you have any idea of what the necessary mass relative geometry would need to be, to become a ? metal for example, as that seems to be what our earths core (iron/nickel alloy) consist of Kryptid? Or am I wrong assuming this and the metal collecting at the core is a later process?

[Kryptid]

lovely answers

So do you have any idea of what the necessary mass relative geometry would need to be, to become a ? metal for example, as that seems to be what our earths core (iron/nickel alloy) consist of Kryptid? Or am I wrong assuming this and the metal collecting at the core is a later process?

I'm not completely sure I understand the question, although I'm pretty sure I've read that metal concentrates itself in the core of planets due to it being a higher density than other materials (such as oxide minerals) while the primitive planet is still hot and mostly liquid. For lower-gravity, faster-cooling bodies such as asteroids, this mechanism would be less effective at removing metal from the crustal rocks and concentrating it in the core. I've read that even the asteroid 4 Vesta has an iron-rich metallic core, so I'm guessing that pretty much any dwarf planet or planet larger than that should have metal in its core as well (the concentration of metal depending upon where it formed in its planetary system as well as the overall composition of the proto-planetary disk).

[evan_au]

I think all Rocky planets came from remnants of gas giant planet collisions

I think that forming a rocky planet out of Hydrogen and Helium is a slow way to do it.

It is thought the the gas cloud from which the solar system formed contained matter from stars which had gone supernova, spewing large amounts of carbon, oxygen, silicon, iron and other heavy elements into the cloud of hydrogen and helium formed in the Big Bang.

Hydrogen forms small molecules (H₂), and Helium doesn't readily form any molecules. Both remain gases at quite low temperatures.

However, other elements like silicon and oxygen readily form aggregates (silica) that remain solid up to quite high temperatures. Similar for iron and carbon. So rocky planets could condense out of a protoplanetary disk quite close to their protostar, while gases like Hydrogen and Helium would not condense unless they were quite far from their protostar. Substances like water and CO₂ with intermediate boiling points could condense at intermediate distances.

I expect that the early days of a planetary system would be a violent place. But there is no need to break rocks out of a gas giant, when dust particles of heavy elements form all by themselves in the debris of supernova explosions.

See: https://en.wikipedia.org/wiki/Protoplanetary_disk

[yor_on]

Hmm, I was thinking, or rather wondering, at what 'Gravity/pressure' this gas of particles would 'condense' into that first instance of what we call earth. Also about how different 'weights' would sort itself out as it grew, or not sort themselves out. Then we have the question of what will react/bind with what too of course. D*mn it, I shouldn't have asked. I will get confused.

[Kryptid]

Hmm, I was thinking, or rather wondering, at what 'Gravity/pressure' this gas of particles would 'condense' into that first instance of what we call earth. Also about how different 'weights' would sort itself out as it grew, or not sort themselves out. Then we have the question of what will react/bind with what too of course. D*mn it, I shouldn't have asked. I will get confused.

You might be interested in the following links: http://burro.case.edu/Academics/Astr221/SolarSys/Formation/planets.html, https://en.wikipedia.org/wiki/Planetary_differentiation, and https://en.wikipedia.org/wiki/Rain-out_model

[yor_on]

Thanks Kryptid

[PmbPhy]

Donald Piniach had this to say in reaction to a recent episode of a Naked Scientists podcast:

I am not satisfied with the cursory and non descriptive [explanation] of how the gas cloud of hydrogen and helium with the conservation of momentum and energy can ever even get two atoms to stick together even with gravity pulling them together because with an elastic collision, they would still split apart. Convince me scientists have some idea of the problem with solar formation. I think all Rocky planets came from remnants of gas giant planet collisions, like the Earth's moon from something colliding with primordial earth.

What does everyone think?

If the distribution of matter in the universe started out as perfectly uniform then planets, stars, galaxies etc would never have been able to form. But that's not what happened. It started out uneven. In areas where large regions of a collection of atoms congregated it created a gravitational field into which it attracted other atoms until a large gravitating bodies were able to form. That's how those bodies formed.

As for atoms combining it only takes a collision between atoms to form but not in the way described above. Its a quantum mechanical process. Not one that can be described by visualizing balls of clay slamming together. E.g. put two hydrogen atoms close together and they become polarized and thus start attracting each other electrically. To describe in all detail quantum mechanically in all its glory is something extremely messy and something I've never seen done,