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The chemical elements composition of the sun changes as its nuclear furnace changes elements into heavier ones. But it is supposed to be mostly Hydrogen at the start. The planets are too cold for such chemical composition changes, and contained initially the heavy elements composing them (up to contributions from various objects such as comets and meteorites, which may have contributed to all bodies of the system, and come from the same initial cloud).

Both the sun and the planets were created from the same cloud. Thus why is there such a difference in composition.

My only explanation is that only the sun was massive enough to retain an hydrogen atmosphere, most of what hydrogen was available, while smaller planets tend to lose their gaseous atmosphere, and that the sun does contains also heavy elements, in small quantity compared to the hydrogen it captured (to limit its effect on the sun nucleosynthesis).

Does that imply that the planets are to be expected to have roughly the same proportion of the different elements? ... of course not necessarily assembled in the same molecules.

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Why do the sun and planets differ in their chemical elements composition?

Because they formed by different processes.

Our solar system formed from an interstellar gas cloud. Our Sun formed from the gravitational collapse of that cloud. The Sun most likely is very similar in composition to that of the original gas cloud. The planets began forming only after the the protosun had gained sufficient mass so as to make the gas cloud collapse into a disk.

The temperature in that disk decreased with increasing distance from the protosun. The only substances that could aggregate in regions within a few astronomical units of the protosun were metals and rocky minerals. The paucity of those non-volatile compounds limited the growth of the terrestrial planets. Volatile compounds such as water and methane could aggregate beyond what is called the ice line. These volatiles compounds were much more common than are metals and rocky compounds. This enabled the quick growth of the gas and ice giants. Stuff that was very far from the protosun couldn't aggregate due to the sparseness of material far from the protosun.

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