by 
sean
ANSWERS: 4
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I'd say from the information sent back by the various probes that have been sent to them.
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Because we said so.
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The answer lies in the one of the many amazing things to do with light. (Come to think of it, light seems to be the answer most of the time in astronomy.) In this case, we use spectroscopy, which is studying the light from an object by the different wavelengths. Every atom and every molecule like to emit or absorb light at certain wavelengths and not at others. This means that by breaking the light from an object into it's various wavelengths (for visible light, different wavelengths is the same as different colors; for other parts of the spectrum – ultraviolet, x-rays, gamma rays, infrared, and radio – we just say "wavelengths"), we can look for the fingerprints of different atoms and molecules. By now you've guessed that this is exactly how we work out what stars and planets are made of, for the most part. (This technique is even used for the Earth to map out vegetation types and other surfaces from orbit. Besides getting useful information in its own right, this also allows us to check our ability to interpret spectra.)
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Well,,, I've heard Jupiter pass gas. Most people say I'm just hearing thunder but I know better... ;~)
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