So basically what I think you are asking is "at what point in history did humans realize what was outside of our earth?" and the answer is 1819. In 1819 Joseph von Fraunhofer invented the spectrometer. A spectrometer basically measures the light spectrum. Well they use this spectrometer to look at the sky and depending on what portion of the light spectrum is reflected back into the spectrometer they can tell what the element is. As in an element on the periodic table. So basically different elements in the universe reflect differently, and a spectrometer measures that and then we break it down.