Planetary Structure and Composition

Core Idea

Planets exhibit distinct internal structures shaped by their mass, composition, and thermal history. Terrestrial planets have rocky silicate mantles and iron cores; gas giants have rocky cores surrounded by thick hydrogen/helium envelopes; ice giants are primarily water/methane/ammonia ices. Mean density, atmospheric composition, and magnetic fields provide constraints on internal structure. Composition varies systematically with distance from the host star, reflecting nebular temperature gradients.

Explainer

We cannot drill into Jupiter or slice Saturn in half, so understanding planetary interiors requires indirect reasoning. The starting point is mean density, which you can calculate once you know a planet's mass (from its gravitational influence on moons or spacecraft) and its radius (from transit observations or direct imaging). Earth's mean density of 5.5 g/cm³ far exceeds the density of surface rocks (~2.7 g/cm³), immediately telling us the interior must contain denser material — an iron-nickel core. Jupiter's mean density of only 1.3 g/cm³ tells us it is overwhelmingly composed of the lightest elements, hydrogen and helium, consistent with its massive gaseous envelope.

The solar system's planets fall into three structural categories that reflect where they formed in the protoplanetary disk. Close to the young Sun, temperatures were too high for volatile ices to condense, so only metals and silicates survived — producing the terrestrial planets (Mercury, Venus, Earth, Mars) with iron cores, silicate mantles, and thin or absent atmospheres. Beyond the frost line (roughly 3–5 AU), water ice and other volatiles could condense, providing far more solid material for planet building. Cores that grew massive enough — around 10 Earth masses — gravitationally captured enormous hydrogen and helium envelopes, becoming gas giants like Jupiter and Saturn. Uranus and Neptune, the ice giants, accumulated substantial ice-rich mantles but captured less gas, giving them a fundamentally different internal structure dominated by water, ammonia, and methane in exotic high-pressure phases.

Within each category, internal structure is layered by density through a process called differentiation: denser materials sink toward the center while lighter materials float upward. In a terrestrial planet, this produces a dense metallic core overlain by a silicate mantle and a thin crust. Whether the core is liquid or solid matters enormously — Earth's liquid outer core generates its magnetic field through convection-driven dynamo action, while Mars's mostly solidified core explains its lack of a global magnetic field today. For gas giants, the interior transitions from molecular hydrogen gas to liquid metallic hydrogen at pressures exceeding a million atmospheres, a state where hydrogen conducts electricity and drives the planet's powerful magnetic field.

Additional clues to internal structure come from a planet's moment of inertia (how mass is distributed radially, measured from its rotational flattening and precession), seismic waves (which reveal density and phase boundaries inside Earth), magnetic field geometry (which constrains core size and dynamics), and gravitational harmonics (measured by orbiting spacecraft). Together, these observations let us build models of planetary interiors that, while never directly observed, are tightly constrained by physics. The systematic relationship between a planet's position in the solar system and its internal makeup is one of the strongest pieces of evidence for how planetary systems form from collapsing clouds of gas and dust.