Extra-solar planets

The non amateur stuff. Hawking, black holes, that sort of thing

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joe
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Post by joe »

What do you think is actually been "hyped" and what purpose do you think such "hype" would serve.
Perhaps importance and funding respectively? Not that I necessarily agree. :)...the importance bit anyway. If you have to publish in order to receive funding then publish I say.
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Cliff
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Post by Cliff »

Dear Joe
Sorry but I do not understand your recent comments.
However, is it really known with dertainty that Jupiter has a solid rocky core?
Best wishes from Cliff
KendalAstronomer
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Post by KendalAstronomer »

There may be limiting factors for rocky planets, but it depends on geography. The British have a loose definition of the lower limit of a rocky planet being when the body is big enough to be fully differentiated. During the course of evolution, the theory goes, the planets are hot liquid for a time (well, liquid may be too strong a word, but fluid). During this time, heavier elements will be dragged to the centre by the gravity of the planet - this is why we have an iron core. The centre will be compressed solid. Fighting against this happening is the cooling effect of space. The crust forms as the outer part of the planet cools, how far down the crust goes depends on how hot the planet is. If the body is too small, the whole thing cools before the heavier elements sink to the core, they are termed as not being "differentiated". The larger a body is, the more chance of differentiation occuring. The process of differentiation itself releases heat, as the heavier elements release potential energy. The lighter elements they displace won't need as much energy to be forced up as the heavier elements release coming down, and the difference goes towards heating. One consequence of differentiation is that there are no heavy elements left on the crust when it cools. All iron on Earth, for example, appears in seams - the remains of meteorites colliding with the Earth that have been distorted by the collisions and by plate tectonics happening around them.

In 'our' definition, the body must be above the theoretical limit for full differentiation. Unfortunately, this definition rules out Sedna as a planet, probably Pluto too and even Mercury.

THe American definition for small rocky planets is just if it is substantially bigger than the things around it and in a stable orbit, which it doesn't share with anything except the odd moon, we'll call it a planet. Sounds fair enough for me...

The upper limit for rocky planets is when they are so massive, the heat generated by differentiation keeps them liquid, and the atmosphere around them is retained, relatively thick. They don't form a crust and the dividing lines between liquid and gas become blurred as the self gravity compresses the gas.

This brings us onto the core of Jupiter, which could be liquid or solid or whatever. Jupiter is said to have a metallic state hydrogen core. Hydrogen is an odd element that can act as a metal in certain situations. It is only described as such as we know there's a lot of hydrogen in the planet and we know there must be some metallic form as Jupiter has the most gigantic and powerful magnetic field of any of the planets. In the model described here, Jupiter's core would probably remain a hot, dense fluid - at least the outer core would. Fusion can happen in this situation, eventually there must be a tipping point where the outer core gets to burning and the inner core becomes molten due to this. Or maybe it is the 'failure' to become a star by low mass objects that allows them to cool down enough to have a solid core, instead of the gas being stopped from contracting due to the heat generated by fusion reactions created by the contraction. Sounds more plausible, but I don't know.

As for Venarth, it has been noted that the ratios of the orbits of the planets seem to follow certain mathematical patterns, with each one representing a stable orbit between the masses of the other bodies. Perhaps gravity has its own version of quantum effects, turning the random collisions of coallescing planets into a fight between the planets hoovering up what bodies are around and the need for them to be in a stable orbit, undisrupted by other bodies. The rocky planets seem to cease being rocky at a certain distance from the Sun (and then become rocky again at pluto distance). This could be due to the new hot body driving lighter materials away from the centre of the disk of material left behind (shockwaves from new stars clear away the surounding nebula in quite an efficient way), leaving less chance of enough material forming a big body close in. But then the new exoplanets have to be accounted for - close in and big.

That's research for you, I suppose.

Hope that's clear and not too overlong. Its been a long day...
joe
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Post by joe »

Hi Cliff,

If you mean comments about cores then it was a comment on an earlier suggestion that Jupiter and perhaps all gas giants could have rocky cores. From a hierarchical point of view they can't because if by adding more mass (hydrogen) to a gas giant makes it a brown dwarf then surely if it had a rocky core in the first place it would not be able to as it would hinder the process. That is to say it would need a hydrogen core.

I look at Jupiter as a planet that does not have a rocky core. It has a hydrogen core but hydrogen that is under enormous pressure, understandably, and the molecules behave as if they are metallic. Of course no one is certain... and that goes for me too.


(I posted 2 mins after KendalAstronomer and my response now seems puny but thanks for a comprehensive explanation! )
200mm Newtonian, OMC140, ETX90, 15x70 Binoculars.
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