Planetary science explains the formation, structure and evolution of planets, their natural satellites and smaller bodies by connecting observations with physical and chemical models. A body's mass and radius constrain its mean density, but several mixtures and interior structures can fit the same pair of measurements.
Spectroscopy probes accessible surface or atmospheric composition, gravity measurements constrain internal mass distribution, and seismic waves probe elastic layers. Comparing these independent observations tests models of differentiation, cooling, planet formation and atmospheric evolution. Nearby Solar-system bodies also provide resolved examples for interpreting the limited signals available from an exoplanet.
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