A planet’s composition separates it into two major categories: those made primarily of rock and metal, and those made mostly of gas and ice. Planets in our solar system that fall into the former group are known as terrestrial planets, a name derived from the Latin word Terra, meaning Earth. These rocky worlds share many structural similarities with Earth, possessing a compact, solid surface. This solid surface distinguishes them from their much larger, gaseous neighbors. This discussion focuses on the eight major planets recognized in our solar system.
Identifying the Rocky Worlds
The solar system contains four planets classified as terrestrial, or rocky, worlds: Mercury, Venus, Earth, and Mars. These four planets occupy the inner region of the solar system, orbiting relatively close to the Sun. Their proximity to the Sun during formation dictated their composition, as intense solar radiation drove away lighter, volatile materials like hydrogen and helium.
These inner planets are characterized by relatively small sizes and high average densities, all greater than 3 grams per cubic centimeter. This high density results from their composition, which is dominated by heavy elements that condensed in the hot inner solar nebula. Although they share the same classification, their individual surface conditions vary drastically, from the scorching atmosphere of Venus to the cold, thin atmosphere of Mars. The presence of a solid surface is a defining feature, meaning a spacecraft can land on their terra firma.
Defining Planetary Composition
A planet is considered “rocky” because its bulk composition is dominated by silicate minerals and metallic elements, primarily iron and nickel. Silicates are compounds containing silicon and oxygen, which form the common rock-forming minerals found in the crusts and mantles. The presence of dense metals like iron and nickel, which make up the core, gives these planets their high overall density.
All four terrestrial planets share a differentiated internal structure, meaning their material has separated into layers by density. At the center of each is a dense metallic core, consisting mostly of iron and nickel, which sank inward during formation. Surrounding this core is the mantle, a thick layer composed of silicate rock.
The outermost layer is the crust, a thin, solid shell of rock that forms the planet’s surface. This layered structure, from a heavy metallic core to a silicate mantle and crust, is the physical signature of a rocky planet. This segregation of materials occurred through a process called differentiation, where heat caused the planet’s elements to melt, allowing the heavier components to sink inward.
The Contrast: Non-Rocky Planets
The four non-rocky planets in our solar system—Jupiter, Saturn, Uranus, and Neptune—are fundamentally different in composition and structure from the terrestrial worlds. These planets are located in the outer solar system, far from the Sun’s heat, which allowed them to retain massive amounts of lighter elements. They are collectively known as the giant planets, which are separated into two subcategories.
Jupiter and Saturn are the Gas Giants, composed predominantly of the lightest elements, hydrogen and helium. These two planets are so massive that they lack a distinct, accessible solid surface. Instead, they transition from a thick atmosphere to a deep layer of liquid metallic hydrogen under immense pressure. Their average densities are significantly lower than the rocky worlds, with Saturn’s density being less than that of water.
Uranus and Neptune are classified as Ice Giants, composed primarily of heavier volatile compounds, often referred to as “ices.” These ices include water, methane, and ammonia, alongside a hydrogen and helium atmosphere. The interior of these planets is thought to be a dense, slushy mixture of these ices and rock, separating them from the gas-dominated giants.