Minutiae of Planetary Formation

The Devil is in the Details

I’ve been a bit scarce lately, got my head into a couple of very intriguing projects. One of them involves planetary formation, which is a topic always of interest to me. The grand, epic picture of planetary formation goes something like this:  you have this nebula, swirling around in space.  Eventually it starts to contract due to gravitational fluctuations  and maybe even an outside influence (like a passing star).  Material collects in the center, which gains more mass and more gravitational influence, sucking stuff in from the immediate environment.  If the center gets hot enough, and the mass is great enough, a star “turns on”.  It immediately begins blasting out radiation, maybe a pair of jets, and scoops out a cavern around itself in the nebula.

The circumstellar disk around the star Fomalhaut; theres a planet hidden in there! Courtesy Hubble Space Telescope.  Click to embiggen.
The circumstellar disk around the star Fomalhaut; there’s a planet hidden in there! Courtesy Hubble Space Telescope. Click to embiggen.

If there’s enough stuff left over in the nebula, that starts to coalesce too as the nebula material (gas and dust) rotates in a sort of flattened disk around the newborn star.  Let that process go on long enough and eventually more little condensations form — accretions of dust and gas from the cloud.  If the accretions are close to the star, they become rocky planets (like Mercury, Venus, Earth, and Mars).  If they’re farther out, where gases (and ices) can still exist), then gas-giant and ice-giant planets can form.

That’s the Big Picture. The minutiae of planet formation involve some fairly complex interactions between gases in the nebula, between those gases and the dust, and the involvement of magnetic fields to the extent that the magnetic fields of larger bodies in the nebula (like the star, for example) affect how the particles are distributed throughout the system.  It really starts down at the particle level — where particles of dust perhaps the size of a speck of household dust that are coated with ice can be affected by heat from the star.  If they’re too close, the ice sublimates and all you’re left with is a dust particle that then has a different future ahead of it — it could accrete to other particles (depending on what it’s made of (silicates, for example), to form larger chunks of material.  Those chunks would accrete, forming boulders; the boulders would accrete to form mountain-sized chunks of rock, and so on until you get to planetesimals.

If the nucleated ice particle (the dust in the ice) lie far enough away from the star, they can start to accrete too, and through much the same process, stick together in larger quanties until you get the chunklets that bash together to form the icy moons.  That doesn’t exactly explain how the gas and ice giants get started, but that’s still a work in progress at the ‘devil is in the details’ level. On the macro-level, the leftover gas and ice chunks in the outer regions of the star system do start to coagulate and coalesce, with the ultimate result a gaseous or ice-gas planet containing a small rocky core.

All of these details come from studying the chemical makeup of planets in our own s0lar system, plus a lot of lab work by scientists who model and simulate the accretion of dust and ice particles together using the same kinds of materials they know were present in the early solar system. It is actually very painstaking work, with groups of scientists taking the time to look at the chemical and physical reactions between materials to understand exactly how they work in the interstellar environment.

Now, I’ve simplified the explanation quite a bit here. Perhaps as I get more time after this project goes to press, I can go into some of the “devilier” details for the planetary geeks among my readers. It’s fascinating work and I can’t wait to see what the planetary science modelers come up with as they apply their work to extrasolar systems that are now forming.


  1. Is it so much that the dust form together at the center of the nebulae or could there be a greater concentration of gas and dust off to one side and stars/planets start to form there? Also, what is ice made of? Every time I read about it I keep visualizing water-ice but I know that can’t be correct.

    Anyways, I really like this post.

  2. ccp

    Well, to look at it simply (the whole this way more complex than we have room for here), for the star itself, the central region is where material starts to collect — mostly the hydrogen gas, which is what the star is formed from. The dusty material is pushed along by various forces (radiation pressure, etc.) out into the nebula. The action of star birth sweeps the immediate environment of the star free from hydrogen, leaving behind the dust which can coagulate/accrete as I suggest in the main article. I imagine that this causes mass accretions and instabilities in the cloud that make it more likely for other bodies to form out of whatever’s available in that region — dust and larger “asteroid” type particles for rocky bodies, and in the outer regions you have gas and icy material and dust to form the outer worlds. In the outermost regions, you get the cometary stuff.

    The ices are not just water. In our own solar system, we see ices of methane, ammonia, carbon dioxide — stuff like that. It can exist in the icy form in the colder regions far from the Sun, so there’s no reason to think that wouldn’t happen elsewhere in other solar systems.

    Ice particles aren’t just little flakes of ice. Often enough they are bits of dust that have had ice crystals freeze around them (hailstones often form this way — with dust particles that get swept up by strong winds and then form hailstones at super high altitudes. (Google hail stones, you can find a wealth of info about how they form). In space, those little bits of ice make contact with each other in the nebula and begin sticking together as they do. Hence the growth of icy bodies. Rocky material coagualates too, but it takes a bit more force for that to happen. The pieces have to clunk together hard enough to stick together but not so hard they just break apart. That is a hot area of study right now.

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