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The gas giants


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Sort of similar - Jupiter probably has a rocky core. However they formed further out (probably) so can accrete ices as well as rocks which were too volatile close in.

All that said, the recent influx of exoplanets has rather put the cat amongst the pigeons as far as planet formation goes. There was a nice model worked out when we only had one solar system, its now in a little bit of a mess trying to account for the weird collection that has been found.

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I was watching a hangout recently they were saying that the 'rocky' cores often mentioned about are not 'rock' as we think about rock being, because the material at the center of these gas giants is under such immense pressure it becomes a very different material undrr those conditions.

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Oh boy thats got my head thumping. Now im a little lost, i thought that the large gas giants would have to have a metalic hydrogen at their core as they are just a few steps down from fusion and well stardom. Even if the early gas giants did start much the same way as the inner rockies and collected rock (and yes ice) as their cores wouldnt the immense pressure of the huge gaseous bulk they have since aquired change the very nature of the core?. Would the rocky core under such immense pressure and subjected to such heat be reduced to maybe a metalic substance anyway. If gas giants accumalate rocky cores during formation surely we would have to assume that stars would also form in this manner. I cant get my head around how nuclear fusion would take hold in a rocky core?. Not by any means saying your wrong of course Juliano, just that im now a little lost.

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Jupiter is a long way off of achieving fusion as present in a star.

Reading Wiki: Until about 0.08 solar masses fusion will not occur and Jupiter is 0.001 solar masses so 80 times too small. Even if all the planets formed into one mass there would still be a lot more mass required. The idea that Jupiter is a failed star is quite a bit wrong.

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There is lots of junk in the sun. This is measured as it's metallicity. To astronomers anything heavier than Helium is a metal! The sun has about 1.8% metals, which is stuff other than Hydrogen and Helium. Some metals take part in nuclear reactions, some just sit there getting in the way. They have subtle effects on how the star evolves and burns.

With a planet like Jupiter, anything heavy (and the same is true of the Earth) tends to sink to the centre so all the rocks, iron and nickel etc are in the middle sinking down through the hydrogen during the hotter phase of their lives.

Jupiter is or was far enough from the Sun to collect frozen ices and cold gases in its orbit, making it a gas giant. Far enough away to be cold and not stripped by solar wind. Stars often have outbursts during their early life, stripping atmospheres and volatiles from close in planets. So Jupiter hoovers up gas, rocks, ices and anything else that happens to be around. It is close enough to the sun to be somewhere where there is a reasonable density of material to pick up, and far enough to not be ravaged by the Sun. The equation is different for the other planets of course, and then there is the chance that the orbits have changed since formation too.

It's all quite complicated!

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So did the gas giants just go through the process of accretion with gas instead of dust?

Sort of - they just accrete anything and everything because its colder and less subject to loss. As most stuff in the universe is Hydrogen, they manage to keep that and so grow much bigger. Earth can't hold H due to size and nearness to the sun.

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