ELI5: How do we know that Jupiter is made of gas?

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1) we can see the outer layers of clouds moving like gas clouds. 2) we can estimate it’s total mass based on the orbital periods of its moons, and we can see how big it is, and thus estimate its average density, which leads us to conclude that it is mostly gas


Some of the light that hits Jupiter reflects back towards the innersolar system and towards us, and using spectrography we can detect what wavelenghts of light the component materials of Jupiter absorbed, and comparing them with known absorbtion spectrums, we know it’s made from hydrogen, helium, and trace amounts of others gases.


We know the mass of the gas giants Jupiter, Saturn, Uranus and Neptune, from their orbital characteristics. We also know their size. That gives us density. Their densities are so low that they can only be, at least mostly, gas.


In a word, physics. As a rocky planet gets beyond a certain size, its gravity becomes so strong that it does more than capture a gaseous atmosphere for itself, it captures a gaseous atmosphere that itself becomes so dense, so gravitationally significant that the atmosphere itself contributes gravity to capture more atmosphere. This is a snowball effect that turns a big rocky core into a gas giant. At this point, Jupiter is so big that the gaseous atmosphere contributes the vast majority of its gravity compared to it’s core. The solar system formed because a nebula of gas swirled, the law of averages took effect, and that gas flattened out into a disk. From that disk, the sun and planets formed. The inner planets have a smaller circumference than the outer planets, which means there was just less stuff for Earth to sweep up in its orbit to form itself. The larger planets are in the outer solar system and have much more area to cover, and much more area which to gather their mass from. If Jupiter were actually a solid, orbital mechanics would be very different. That alone is enough to deduce the density of the planet to the point we could conclude it’s at least gaseous. There is a lot of overlapping evidence that we’ve known for hundreds of years now, not to mention several flyby space probes that have gotten close enough to observe the planet in detail.


Technically it’s not, it’s mostly liquid and liquid metallic hydrogen. The pressure is too high deep down for it to be gas all the way through. Anyway, the two main ways we get information about the composition of Jupiter are spectroscopy and orbital mechanics. Spectroscopy is looking at the color of light reflected from Jupiter. Different substances are different colors, and if you look at light spectra (the exact wavelengths of light being reflected) you can use that to figure out what substances are reflecting the light. Looking at Jupiter we can see that what we see is mostly hydrogen gas. In addition to this we can figure out the mass of Jupiter by looking at how its moons orbit it. Orbital speeds are related to mass, we can measure orbital speeds, so we can figure out mass. Since we can see how big Jupiter is, once we have the mass we can figure out the density. And since different materials have different densities that also tells us something about Jupiter. Jupiter has a density of 1.33 g/cm³ which is far too light to include much rock, but about right for a planet with a lot of metallic hydrogen in the middle surrounded by liquid and then gas at higher levels. We know the density of different forms of hydrogen by doing experiments with it on earth as well as by calculating properties. Space probes like Juno also give us more information by looking at things like magnetic fields.