Let's think of Saturn, because of its density. Can there be a place near the center of the planet where the gravity upwards is more than towards the center, and with an atmosphere that is more suitable for life. Yes, I am calling the hollow Earth extreme, but is it that for a giant gas planet too. Can it be possible?

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    $\begingroup$ Even in a puffy planet like Saturn, the pressure well below the surface is crushing and the temperature enormous. There are no hollow planets, and certainly no hollow gas giants. (A rocky world might have an extensive cave system but only near it's surface. Hollow in the center isn't possible). $\endgroup$ – userLTK Jun 13 '16 at 14:35
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    $\begingroup$ The gravity might be close to zero near the center but the pressure will still be extreme. $\endgroup$ – Floris Jun 13 '16 at 19:24
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    $\begingroup$ There can be absolutely no hollow planets, that's that. Note that even if the Earth was hollow, just in theory. Say you were inside the middle bit. Perhaps surprisingly, there is no gravity: you do not get pulled towards the "shell" part, even if you a right next to it: it kind of evens out. All hollow spheres have NO gravity inside. it's just one of those weird physics results! You can find thousands of posts explaining this online. $\endgroup$ – Fattie Jun 13 '16 at 22:38
  • $\begingroup$ Lol. This is a crazy idea. $\endgroup$ – wogsland Feb 12 '17 at 4:58

Newton's shell theorem proves that inside a gas giant, any layers that are further than you from the centre have a zero gravitational effect on you. So if you are inside a gas giant (and by some magic not dead) the only gravity comes from the layers that are closer to the centre. So the gravitational pull is always down.

If fact, the pressure increases as you go down, as the weight of the layers above you causes pressure (just as the the Earth's air is more dense the lower you go). Since the gas is under extreme pressure, it would fill up any gap that formed, (just like the air fills a vacuum on Earth) In a gas giant the pressures are extreme, there is certainly no way that a hole or cavity could ever form.

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