ElI5: how do they know what other planets are made off, how can the be so sure? Jupiter could be made of a completely unknown gas?
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The answer is [https://en.wikipedia.org/wiki/Spectroscopy](https://en.wikipedia.org/wiki/Spectroscopy). You can by looking at the wavelengths of light atoms/molecule absorbs/emmit because of it jumping between energy states.
This is done all the time on earth as a way of chemical analysis so we can compare what we see from Jupiter to what we see in labs. So we will primary see the line Hydrogen and Helium produce on earth and that means tha same gases are in Jupiter’s atmosphere.
Helium was in fact discover this way. There were line observers in the light of the sun in 1868 that did not match any known element on earth. It was concluded it was an unknown element that was named after the Greek name of the sun Helios. It was seen on earth in gases from a volcanic eruption in 1881 and isolated on in a lab in 1895.
So in the past, we have discovered that a stellar object was made of an unknown gas by observing light from it on earth. It would not be an unknown element like that today because we have discovered the 118 first elements in the periodic table. There are no gaps today because we an element is determined by the number of protons and elements with 1-118 protons are known.
So in the 19th century, there was missing stable light element that was discovered in stars but there are no gaps today.
Theoretically, there is a heaver stable element but we would see unknown lines in the spectra. They will be very rare because of how heavy elements are produced in exploding stars.
There could be some undiscovered chemical compound on Jupiter but natural processes like that tend to produce simple molecules and we have made lots of those on earth. They would also produce unknown spectral lines
So we can determine chemical composition at a distance with light and has not discovered anything that we have not seen in labs on earth.
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