How can scientists measure chemical composition (oxygen, nitrogen etc) and temperature on other planets without being there?
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Scientists can measure the chemical composition of the atmosphere of other planets using spectroscopy. Each molecule in the atmosphere absorbs and re-radiates at specific frequencies of light that are a function of temperature and unique to the specific type of molecule. For more information see https://en.wikipedia.org/wiki/Extraterrestrial_atmosphere and http://www.physics.uu.se/research/astronomy-and-space-physics/research/planets/exoplanet-atmospheres/ .
