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.

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.