Short answers, pulled from the story.
Absorption spectroscopy measures the absorption of radiation by a sample to reveal its atomic and molecular composition. Photons matching the energy gap of molecules are absorbed while other photons transmit unaffected through the material. This variation in intensity creates an absorption spectrum that identifies specific substances.
The first direct detection and chemical analysis of the atmosphere of an exoplanet occurred in 2001. Sodium in the atmosphere filters the starlight of HD 209458 as the giant planet passes in front of the star. Astronomical spectra contain both absorption and emission spectral information used for this discovery.
Rotational lines appear in the microwave spectral region when radiation matches the energy difference between two quantum mechanical states. Vibrational lines show up in the infrared region while electronic lines fall within the visible and ultraviolet regions. X-ray absorptions involve the excitation of inner shell electrons in atoms.
An absorption spectrum relates quantitatively to the amount of material present using the Beer, Lambert law. Determining absolute concentration requires knowledge of the compound's absorption coefficient found in reference sources. Calibration standards with known concentrations allow scientists to determine coefficients for unknown substances.
Measurements can be made without bringing the instrument and sample into contact because radiation traveling between them contains the necessary spectral information. Toxic or hazardous environments become safe zones when operators stay far away from the danger. Sample material does not have to touch the instrument preventing possible cross contamination.