.. But would this mean a Jupiter-sized Earth (with a radius rougly ten times larger) would have ~10 g for an earthling-human at it's surface?
Yes, but only if the densities were the same. They’re not. Jupiter is ~ 1/4 as dense as Earth, so it has 1/4 the mass it would’ve had if it had our density. Thus the gravity will be about 1/4 as much. (~ 2.5g)
Also, do I understand correctly that if I want a sort of Earth-like planet with earthish gravity, the 'crust', the top layer that people live and build on will always be a fraction (in it's thickness) of the magmafest that is going on underneath? If we would slice this hypothetical Jupiter-sized Earth in half, and look at all the layers - the crust and atmosphere would always be a tiny part of it - is that correct?
I’m no expert, but a thicker crust, if desired, is plausible if the planet formed with more lighter elements, which float upward during its hot formation. Or, if the planet is very old, the crust thickens as it cools. This, however, creates other issues like weaker tectonics and an older star.
I KNOW it is tricky to make this planet habitable, haha, that's why I've come here. I know Jupiter is a gas giant and probably doesn't have anything we could stand on, not even in it's core. I think I kind of understand why planets turn into gasgiants beyond a certain point because of their size, though not fully. Does this mean 'Jupitearth' has to be hollow?
The above shows how important density is. But I don’t see how one can have far less density for larger terrestrial planets. Lighter elements and a smaller, but faster spinning, iron core only help a little. The core is actually small in volume, btw.
Since the 'rocky' part, the crust, would always be a fraction of the total mass, and the inside can't be like Earth's because it would collapse, possibly into a gas giant?0
The star (temp.) determines planet types. Near the star, temperatures are hotter, so gases and many liquids will boil away. But each type of star has a frost line distance where gases are cool enough to not reach escape velocity. This allows rapid growth of large planets. 90% of every element is hydrogen, helium is almost the other 10%. Jupiter and Saturn are cool enough and massive enough to hold these, making them even larger planets.
Migration of planets is not trivial, so with imagination, planets might be able to go where you want them, but temperature changes must be kept in mind.
I've been trying to come up with ways to decrease the density of course. If the majority of the rocks are porous, on a large scale as well as on a local level, and we fill those spaces with water and air, some density is gone - but not nearly enough to approach Earth-like gravity at the planet's surface, is that correct? Does this mean the Jupiter-sized Earth has to be hollow and has to have some sort of miracle alternative for an iron core floating in it's centre, to have a habitable surface? What would happen if we filled the hollow centre with water? A water core? I don't know what would make more sense, ice or steam there - but I'm hoping you might?
. Planet formation temperatures due to impact (KE) make them molten. Hard to dodge this issue. Only smaller bodies might have watery cores (e.g. Europa).
I’ve no idea how a massive planet would become hollow.
The first planets to form in the cosmos should be far less dense since there were far fewer heavy elements then, I think. Water would have been abundant since stars make a lot of C, O.
And if there can't be an iron core, inside this hollow giant - does that means vulcanism and plate tectonics are also out the window?
. You need heat rising up to stir the crust. Water helps lubricate these flows. Earth has two forms of heat transfer from below: radioactivity and convection.
You’ll need very heavy elements for the first, so the early-aged planet idea won’t be that plausible if radioactivity is used. [well, you could toss in some extra SN to fix this, I suppose
] Convection moves heat upward faster. A young planet will have a hotter core and likely more convection through all the fluid layers. So, perhaps a relatively young planet on the outer edge of the hab. zone might work since internal hear transfer would offset reduced star heating. A heavy atmosphere will likely hold temp. as well, so your larger planet would help here.
I know our magnetic field goes way beyond our surface, far into space, but I would think a planet ten times Earth's size would need an equally 'oversized' magnetic field to have the same 'protection'. But if there is no iron core, there won't be a magnetic field either, right? Or is that not true? You connect rotation speed to Earth's magnetism, which makes me hopeful there is a possibility for a magnetic field for the super-Earth even if it doesn't have an iron core. Is that correct?
The star flares, etc., won‘t be worse if the planet radius is larger, so maybe only a little more EM field is needed. You should have an iron core, but if it’s spinning faster it will produce more field strength. Thus allowing a little less density.
If I'm not misunderstanding you, considerable differences in gravity on the giant planet's surface are out of the question? Since that would imply major differences underneath the crust, asymmetry so to speak, that could not exist long-term without solving it's own imbalance into some sort of 'perfectly' spherical state that would then result in equal gravity everywhere on the surface?
Correct, unless it is spinning fast thus someone on the equator would be traveling in a circle at several thousand miles per hour, so gravity effect is a little less. But those at the poles are not, plus there would be bulge in the radius at the equator that also lowers surf. gravity. But the opposite is true at the pole. The equations can give you those differences.
I understand that the seasons would be gone if the super-Earth isn't tilted like Earth. But still, the poles would be cold - which is absolutely unacceptable, because of reasons
Multiple suns is indeed a solution, but I'm pretty sure I don't want that - if I'm understanding you correctly, that is. I've also thought about this possibility, but I think I need the sunlight to come from one place within this fictional solar system. You mentioned the moon slowing down our rotation speed, could a moon, or multiple moons, also affect/control the direction of the rotation on a super-Earth? What I mean is this: could multiple moons, in various different eliptical orbits around the planet, continuously change the direction of the rotation so that all places get equal sunlight? Probably ridiculous?
. Large moons are formed from impacts, apparently, so they aren’t captured. A large moon stabilizes the planets inclination (tilt). Small or no moons allow planets to change inclination. Think of a wobbling top as it spins down. Earth never wobbles more than about 25 deg., but Mars is more extreme due to the lack of a large moon.
But wobbles take eons. Earth takes ~26,000 years.
Finally, some good news: the atmosphere will be huge automatically! That's a relief. I was afraid there was going to be some sort of insurmountable limit I wasn't aware of. The birds thank you sincerely!
. All nice questions that make me think. I‘m not sure all that I’ve said is correct, but I’ve enjoyed trying.