Celestia Forums

I hope that one of the geniuses here can help me wrap my brain around this.

You start with a protoplanetary disk (a "Bok" globule or something like it). It eventually contracts to form a star in the center, and planets form in "resonant" orbits around that star. This much I understand just fine; that's just gravity at work. My question is, how is that most of the hydrogen in the system "falls" into the central star, while a good portion of the "metallic" substances (in stellar terminology) forms the rocky and icy planets, comets and asteroids?

It would seem to me that ALL of the material would want to fall toward the center, but if that was the case, the star would have its beginning as a huge, rocky/icy planet that finally collapses and starts "burning." The next alternative would seem to be that the *heaviest* stuff would want to collect in the center, leaving most of the lighter elements in the surrounding disk.

That obviously isn't correct, I know, but this is what I'm having trouble understanding. All of the available materials online about stellar and planetary evolution just say, "The stuff contracts to form planets and star(s)." There's no discussion of how the different elements sort themselves out as they obviously do, from studying nature itself. For example, Earth's crust is largely silicon, the core is largely iron and nickel, and the atmosphere is primarily nitrogen. The sun, on the other hand, is primarily hydrogen. What causes these elements to "sort?" Am I even using the right term?

Thanks a bunch in advance.

smpoole7,

Welcome to the Celestia forum!

The evolution of planetary systems is a topic of ongoing research.

My understanding is that the distribution of elements usually is quite uniform throughout a protoplanetary system. However, once the central star ignites, it tends to blow away the uncondensed hydrogen and helium, leaving inner, low-mass planets with a deficit. High mass planets tend to keep more of the lighter elements.

For a more authoritative answer, you might want to consider posing your question on the BAUT forum. There usually are many knowledgable people available there, including professional astronomers. http://www.bautforum.com/

Also it is rather more difficult to keep gases bound to the planet than it is to keep solid material there. Light elements like hydrogen can easily escape from small planets.

Thanks for the responses. That actually makes sense -- I had completely ignored the radiation pressure from the protostar when it ignites. I can picture it now, at least well enough to proceed.