Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jerrybell on 17/07/2007 15:42:26
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There have been a few podcasts that deal with forming planets, and I recently watched a show called "The Universe" on the sun. In it, they assert that our solar system resulted from a supernova - we know this because of the existance of heavy metals - uranium, etc, that could not have formed in a small star like our sun. In the aftermath of the supernova, 99% of the "debris" that hung around formed into the sun, and the rest formed the planets. I have a few problems with this:
1. The resultant material of a supernova should be primarily heavy elements - presumably it exploded after it "crunched" when it ran out of lighter fuel. Certainly, it would seem that there wouldn't be much hyrdrogen/H isotopes left. So, where did the hydrogen that fuels our sun come from?
2. if you completely negate and ignore #1 above, and assume that there was a big cloud of hydrogen, carbon, heavy metals, etc, etc, how is the distribution of heavy vs. light matter in the solar system accounted for?
- are there a whole bunch of heavy metals (representative of the composition of the rest of the solar system) in the sun?
- why did some planets end up being composed of largely heavy matter (inner planets), and some end up as gaseous - the gas giants? It seems like the distribution would be more homogenous than that.
Thanks
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This is a long and complicated question.
Firstly the supernovas (note the plural) that originated part of the cloud from which the sun formed happened a long time (many millions of years) before the sun formed and the heavy metals that were expelled by them were mixed with a lot more of the primordeal hydrogen and helium. The cloud had to expand and cool right down before it could collapse again under its initially very weak gravity to form the sun and no doubt several other stars that were formed at the same time (stars nearly always form in clusters).
It is generally assumed that the amounts of constituent elements in the surface of the sun as seen in the solar spectrum are a pretty good indication of the original proportions of elements in this cloud.
The initial formation of the sun and bones of the planetary system probably only took a few million years (quite quick on this timescale).
During the formation of the sun and planets from the cloud various lumps formed and gradually coalesced to form the planets. While the sun was forming Jupiter was probably the biggest heavy element core but the inner planets rocky cores weren't all that much smaller. Initially as it contracted and released its gravitational energy the sun was much more energetic than it is now (although not necessarily hotter) this is because it was bigger. This greater heating blew away the hydrogen and helium atmospheres of the inner planets and have them a real scorching. The sun settled down to become initially quite a bit dimmer than it is today the inner planets then suffered quite a bombardment with comets and planetesimals from the cooler outer solar system to replenish their light elements this gave resulted in them all having heavily cratered surfaces.
Life probably started under the sea in a largely glaciated planet or may even have started on Venus before it got too hot.
The sun is gradually getting brighter an within a few thousand million hears will render the earth uninhabitable