Composition of astronomical objects

The chemical and isotopic makeup of celestial bodies, inferred from emitted or absorbed light, collected samples, and physical properties.

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Astronomical spectroscopy: The study of astronomical objects through the spectra of their emitted, absorbed, or scattered light. Element-specific spectral lines provide the main remote evidence for composition.
Chemical element: A substance whose atoms all have the same number of protons. Elemental abundances are the basic inventory used to describe a body's chemistry.
Planetary differentiation: The separation of a planetary body's materials into layers with distinct compositions and densities. Bulk chemistry helps reconstruct how a body separated into core, mantle, and crust.
Remote sensing: The acquisition of information about an object without direct physical contact. It reveals distant bodies through radiation, unlike laboratory analysis of collected material.
Spectral line: A narrow feature in a spectrum produced when matter emits or absorbs radiation at a particular wavelength. Each element and ion produces characteristic lines used to identify atmospheric and stellar constituents.
Isotope: One of multiple forms of an element whose atoms differ in neutron number. Isotope identities and proportions add detail beyond a body's elemental inventory.
Exoplanet atmosphere: The layer of gases surrounding a planet beyond the Solar System. Atmospheric molecules can be detected in starlight filtered through or emitted by an exoplanet.
Sample-return mission: A space mission that collects extraterrestrial material and brings it back to Earth. Returned samples allow detailed laboratory tests but are limited to a few accessible locations.
Absorption spectroscopy: A method that identifies substances by measuring which wavelengths they absorb from transmitted light. Starlight passing through a planet's atmosphere can acquire absorption features from its gases.
Meteorite: A fragment of extraterrestrial material that survives passage through a planetary atmosphere and reaches the ground. Meteorites provide laboratory samples of asteroids and early Solar System materials.