From what I know, there has been trace amounts (much, much less than 1% of the mass of the planet) of water (H2O) observed in Jupiter. Both carbon and oxygen have also been observed in trace amounts, and if they come together in the form of CO2, it would be a very, very small percentage of the total composition of the planet. The majority of Jupiter consists of helium and hydrogen gas.
Scientists determine the chemical elements in a planet from far away (such as with orbiting unmanned spacecraft) by recording the light that is emitted from the planet. Each element absorbs a small and unique portion of visible light, so if scientists see a dark spot in the spectrum of light they record from the planet, they know what element caused the dark spot, which tells them which elements are on the planet.
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