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Jupiter's enormous gravity would turn its atmosphere first into a liquid from a certain depth, and then into a solid further towards its centre.

So Jupiter has a solid core, above which is a liquid layer and an atmosphere above that.

How is Jupiter fundamentally different than the (solid) inner planets? Why is Jupiter called a Gas Giant?

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    $\begingroup$ Keep in mind that most of those models on the inner workings of the larger planets have not been thoroughly sampled. They are projections based on what is known concerning the obvious: composition, mass, volume, hydrodynamics visible on the surfaces, magnetic fields, et al. Very few probes have been dropped into the interiors of those planets, and so there is quite a lot of room for uncertainty. $\endgroup$ – can-ned_food Jun 16 '17 at 16:46
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    $\begingroup$ Shouldn't this be asked on astronomy SE? $\endgroup$ – Walter Jun 17 '17 at 10:55
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    $\begingroup$ Clearly off topic here. No reference to observations of a probe. Flagged but flag declined. $\endgroup$ – James K Jun 17 '17 at 19:25
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    $\begingroup$ @JamesK: Probably because study of the sun and planets (but not necessarily exoplanets) is on-topic here in its own right, without regard to how this is accomplished. $\endgroup$ – Nathan Tuggy Jun 18 '17 at 2:50
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Jupiter being a gas giant is not about its appearance, as another answer stated. It's only about the mass distribution of a planet.

Jupiter's mass is 320 Earth masses, while we know from the Juno mission that the rock/ice in the core account for 5–25 of these Earth masses. So the rest of about 300 Earth masses is gas.

Thus Jupiter is a gas giant. It is really that simple. The more exotic states of matter that the solids in its interior probably exist in, don't affect the definition of a gas giant.

To put that into perspective:

  • The same thing goes for Saturn, 95 Earth masses total, with estimated 20 Earth masses in the core. So 75 Earth masses of gas. Gas giant.
  • Uranus and Neptune are different. Their masses are 14 and 17 Earth masses, respectively, with at least half of that in rock/ice. Thus they're not gas giants, but loosely named 'ice giants'.

Another way of inferring what Jupiter is made of, is taking a look at its average density. This is how it was done historically, before we had sophisticated computer models and high-pressure lab experiments.

Knowing Newton's laws, one can measure Jupiter's mass $M$ from its orbiting moons. Its size $r$ is known from the distance. Thus one can calculate its average density $\bar{\rho} = \frac{M}{4/3 \pi r^3}$ and finds a value of $\bar{\rho}_♃ = 1.326~\mathrm{g/cm^3}$. This is significantly lower than what Earth has ($\bar{\rho}_{\oplus} = 5.55~\mathrm{g/cm^3}$) and the other rocky planets.

Through simple weighted averaging with some rock mass of a few percent, it was then realized early on that one needs a lot of gas to reach such a low average density for such a planet as Jupiter.

An interesting detail here is, that one cannot build Jupiter of gas only, the resulting average density would be lower than the real one. This is how it was realized that some very dense material needs to exist somewhere in Jupiter, possibly a core of solid material.

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    $\begingroup$ "Jupiter's mass is 320 Earth masses, while we know from the Juno mission that 5-25 of those is in rock/ice in the core. So the rest of about 300 Earth masses is gas." No liquid? $\endgroup$ – David Richerby Jun 16 '17 at 16:40
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    $\begingroup$ "It's only about the mass distribution of a planet." "The more exotic states of matter that the solids in its interior probably exist in, don't affect the definition of a gas giant." It would be nice if the answer actually stated the definition of a gas giant. $\endgroup$ – JiK Jun 16 '17 at 18:08
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    $\begingroup$ @JiK: There is no hard definition. In the case of the solar system the boundaries are clear, because our planets are nicely ordered in a hierarchy of masses. But when you look into the exoplanet database, then all we have are average densities, and those are all over the place. If your interested I can edit a plot for that in. There are definitions for gas giants from planet formation theory, but there the physics is not 100% clear and classifications are still disputed. $\endgroup$ – AtmosphericPrisonEscape Jun 16 '17 at 19:06
  • $\begingroup$ @DavidRicherby: Sure, there's gonna be varius liquids floating around there at high pressures. But I was presenting the fundamental argumentation why one needs another very dense component besides hydrogen to explain Jupiters mass at given average density. $\endgroup$ – AtmosphericPrisonEscape Jun 16 '17 at 19:09
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    $\begingroup$ @DavidRicherby The majority of the "gas" is in a supercritical fluid phase, which may be considered a hybrid between liquid and gas - the distinction doesn't exist at those pressures/temperatures. "Supercritical fluid giant" doesn't quite have the same ring to it though. $\endgroup$ – pericynthion Jun 16 '17 at 19:14
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One reason they are called gas giants is because they are mostly composed of elements that are gaseous at Earth like temperatures and pressures.

Jupiter is primarily composed of hydrogen with a quarter of its mass being helium, though helium comprises only about a tenth of the number of molecules.

Jupiter's upper atmosphere is about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules. A helium atom has about four times as much mass as a hydrogen atom, so the composition changes when described as the proportion of mass contributed by different atoms. Thus, Jupiter's atmosphere is approximately 75% hydrogen and 24% helium by mass, with the remaining one percent of the mass consisting of other elements. The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia. The interior contains denser materials - by mass it is roughly 71% hydrogen, 24% helium, and 5% other elements.[21][22] Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found.

https://en.wikipedia.org/wiki/Jupiter

So Jupiter and Saturn are almost totally composed of hydrogen and helium, elements that are gaseous at Earth like temperatures and pressures. Of course the temperatures and pressures deeper inside Jupiter and Saturn are not exactly Earth like!

But the elements that they are composed of are commonly called gases even though they might be in exotic conditions such as liquid metallic hydrogen under the immense pressure and temperatures inside the planets. Most of us think of hydrogen and helium as gaseous elements, not liquids, or solids, or highly exotic forms of matter.

Thus Jupiter and Saturn are "gas giants".

A second reason they are called "gas giants" is historical. Famed science fiction writer James Blish wrote a science fiction story called "Solar Plexus", published in Astonishing stories, September, 1941. "Solar Plexus" was rewritten and republished in an anthology Beyond Human Ken, edited by Judith Merrill, 1954. The 1954 rewritten version contained the line:

A quick glance over the boards revealed that there was a magnetic field of some strength near by, one that didn't belong to the invisible gas giant revolving half a million miles away.

Science fiction readers who knew anything about the structure of giant planets thought that "gas giant" was a very fitting phrase to describe them. And some of them were professional astronomers. Thus the phrase began to be used by astronomers to describe the giant planets in the solar system.

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    $\begingroup$ @DiegoSánchez - Why do you say that "those numbers don't add up"? Are you maybe assuming that all molecules/atoms are equally massive? That wouldn't be correct. For instance, a helium atom is about four times as massive as a hydrogen atom is. $\endgroup$ – Mico Jun 16 '17 at 8:20
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    $\begingroup$ @AtmosphericPrisonEscape. Helium molecules are monoatomic, hence He is both an atom and a molecule in itself. Also, some ways of addressing people are politer than others, just saying. $\endgroup$ – Diego Sánchez Jun 16 '17 at 9:49
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    $\begingroup$ @DiegoSanchez: Helium, under exotic conditions can in fact bond into an actual molecule, such that it has a chemical bond (covalent or otherwise). A single Helium atom, in absence of any bondings, it not a molecule. $\endgroup$ – AtmosphericPrisonEscape Jun 16 '17 at 9:51
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    $\begingroup$ @AtmosphericPrisonEscape. Did you readme first comment? The Wikipedia article mentions those figures and i wanted to know how that can be possible. Things spiralled out of control from there, but I honestly understood you knew more about it. I know its not very relevant, but somehow sheds some doubt about the main reference used for this answer. $\endgroup$ – Diego Sánchez Jun 16 '17 at 11:06
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    $\begingroup$ Perhaps the answer could make its statement clearer, but regarding @AtmosphericPrisonEscape concern that the gaseous state at STP conditions has nothing to do with whether a planet is classified as a “gas giant” or not: the term is historical, back when the physical properties of the constituent elements in a “gas giant” were not well studied or theorized. People like Blish probably thought “So, it is mostly hydrogen and helium, and has no visible terrestrial surface? And, it is a giant planet? ‘Gas giant’!” $\endgroup$ – can-ned_food Jun 16 '17 at 16:42
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Jupiter is composed primarily of gaseous and liquid matter. It is the largest of the gas giants, and like them, is divided between a gaseous outer atmosphere and an interior that is made up of denser materials. It's upper atmosphere is composed of about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, and approx. 75% hydrogen and 24% helium by mass, with the remaining one percent consisting of other elements.

The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds as well as trace amounts of benzene and other hydrocarbons. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. Crystals of frozen ammonia have also been observed in the outermost layer of the atmosphere.

The interior contains denser materials, such that the distribution is roughly 71% hydrogen, 24% helium and 5% other elements by mass. It is believed that Jupiter's core is a dense mix of elements – a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. The core has also been described as rocky, but this remains unknown as well.

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