Gas Giants?

I have an embarrasingly elementary question that I've been unable to answer through Googling and searching this (and other) sites. I hope that someone out there has the patience to educate an enthusiastic amateur who's recently developed an interest in astronomy and cosmology but currently lacks the background knowledge and experience to even get started:

Planets both in our own solar system and many of those observed/hypothesised extrasolar bodies are described as "gas giants" - what does this mean? Is Jupiter literally a massive ball of gas with no "solid" mass? Surely there's a solid "core" around which a gravitational field is formed?

Once again, sorry for the dumb (but honest) question.

A large ball of gas will form a gravitational field just the same as a large ball of rock, it just tends to be less dense.
As I understand Jupiter might have a rocky core somewhere deep within but it's not a given. Regardless, hydrogen itself will turn solid and metallic under great pressure, so the deeper parts of a gas giant will be solid.

It's probably better to think of the term as a comparative rather than an absolute. Jupiter and the other gas giants are gaseous in comparison with rocky planets like the Earth or even Venus (which has a thick atmosphere). That is, the mass and height of atmosphere on the rocky planets is small, sometimes negligibly small, in comparison with the mass and radius of the rocky part of the planet.

Also, with the rocky planets there's a very obvious divide between the rocky bit and the gaseous bit (that divide is, of course, the ground you're standing on), and the two parts are made of very different materials. With gas giants, the solid bit of the planet emerges almost imperceptibly out of the atmosphere, is (I understand) made of pretty much the same stuff as the atmosphere, and is largely a consequence of the large mass of the planet. That is, even if you had a really well-built Jupiter truck, there wouldn't be an obvious surface for it to drive on.

astrolover wrote:

A large ball of gas will form a gravitational field just the same as a large ball of rock, it just tends to be less dense.
As I understand Jupiter might have a rocky core somewhere deep within but it's not a given. Regardless, hydrogen itself will turn solid and metallic under great pressure, so the deeper parts of a gas giant will be solid.

Hydrogen would turn solid and metallic under great pressure and low temperature. But the interior of Jupiter is hot. At high temperatures hydrogen will not be solid even at high temperature - it would be molten metal or plasma.

Solid metal, whether iron or hydrogen, would not support dynamo and would not have magnetic field (except ferromagnetic).

Worth pointing out that the state of hydrogen is a function of temperature and pressure. For high temperatures there is a pressure above which metallic hydrogen will form. Just because it is hot doesn't mean metallic hydrogen is not going to form. Having said that Jupiter's core has a predominance of liquid metallic hydrogen with maybe a bit of solid stuff at the centre according to current models.

Thanks for the learnin' guys. Much appreciated.

I don't think we are absolutely sure what is near the middle of gas giant planets, but metallic hydrogen is likely for Jupiter. A probe did penetrate 100? kilometers below the cloud tops and sent back data, but our understanding of the next 23,000 miles is mostly computer modeling, which involves quite a few assumptions. The pressure is likely not high enough at the center of Saturn, Uranus nor Neptune to produce metallic hydrogen. The cloud tops of all 4 of our gas giant planets are not as cold as calculations suggest, so the centers are likely hotter than the center of Earth. One estimate was 25,000 degrees for Jupiter. If so, highly ionized plasma may fill 1/2 of the volume. Since comets and asteroids fall into the gas giant planets occasionally, most of the elements are likely plus at least 80% hydrogen near the center. A solid surface is unlikely due to the very high temperature.

The core of a gas giant should be comprised of elements heavier than H and He simply because those heavy elements would sink into the centre of the planet.

Proton wrote:

The core of a gas giant should be comprised of elements heavier than H and He simply because those heavy elements would sink into the centre of the planet.

Those elements would not appear in the photosphere of the gas giant, because the photosphere is cooler than Sun, and the elements would condense and sink out of photosphere. But why should they sink all the way to the centre? The core of a gas giant is hot. Is it hot enough to evaporate/dissolve the heavy element in hot dense metal/plasma hydrogen and mix them uniformly over large volume of interior?

Chris_H wrote:

Proton wrote:

The core of a gas giant should be comprised of elements heavier than H and He simply because those heavy elements would sink into the centre of the planet.

Those elements would not appear in the photosphere of the gas giant, because the photosphere is cooler than Sun, and the elements would condense and sink out of photosphere. But why should they sink all the way to the centre? The core of a gas giant is hot. Is it hot enough to evaporate/dissolve the heavy element in hot dense metal/plasma hydrogen and mix them uniformly over large volume of interior?

Good question. I suspect the answer to all this lies in discussions about the equation of state of gas giant planets, and I don't know nearly enough to go further.

With Jupiter vacuuming up asteroids and comets over billions of years, surely it would have a substantial non-gas core? Or is all that debris like a fly on an elephant? I remember reading a science fiction story about aliens who extract gold from the core of planets on the theory that the heaviest metals fall to the middle.

Stuper Nova wrote:

With Jupiter vacuuming up asteroids and comets over billions of years, surely it would have a substantial non-gas core? Or is all that debris like a fly on an elephant? I remember reading a science fiction story about aliens who extract gold from the core of planets on the theory that the heaviest metals fall to the middle.

Heaviness? That´s fiction.

Carbon dioxide is heavier than air. Yet it does not fall to Earth surface. Salt is heavier than water. Yet it does not sink to the bottom of the sea.

astrolover wrote:

Stuper Nova wrote:

With Jupiter vacuuming up asteroids and comets over billions of years, surely it would have a substantial non-gas core? Or is all that debris like a fly on an elephant? I remember reading a science fiction story about aliens who extract gold from the core of planets on the theory that the heaviest metals fall to the middle.

Heaviness? That´s fiction.

Carbon dioxide is heavier than air. Yet it does not fall to Earth surface. Salt is heavier than water. Yet it does not sink to the bottom of the sea.

it does actually. Remember lake Nyos in Cameroon? All those people died there because the CO2 release from the lake hugged the ground and suffocated them in their houses.

Salt's dissolved in the water though. Sediments on the other hand (which certainly are denser than water) do sink to the bottom of the sea.

So some elements and compounds sink below the cloud tops of Jupiter, but convection currents and change of state due to increasing pressure and temperature likely keep most from sinking all the way to the core center. The interactions are quite complex and largely untested.

Jupiter is an environment with the volume of 108 Earths most of which exists in conditions we cannot reproduce in the laboratory for more than a millionth of a second or so. On the other hand, there is no sign of extensive activity that violates known scientific law.

Jupiter has more than a thousand times the volume of Earth.
Re = 6370km
Rj = 71490km
Rj/Re = 11.22
so Vj/Ve = 1400 (roughly).