The terrestrial planets, also known as the rocky planets, are a distinct class of worlds in our solar system that share fundamental traits. This group includes the four innermost planets: Mercury, Venus, Earth, and Mars. They are fundamentally different from the gas giants (Jupiter, Saturn, Uranus, and Neptune) due to their composition and structure. Exploring the similarities among these four planets reveals a common history and a shared blueprint for planetary formation close to a star.
Shared Composition and High Density
The primary commonality among the terrestrial planets is their bulk composition, consisting predominantly of non-gaseous, high-melting-point materials. They are built from two main classes of substances: silicate rock and metals. Silicate rock, rich in silicon and oxygen, makes up the outer layers, while metals, specifically iron and nickel, form the central mass.
This combination results in a high average density compared to the gas giants. Iron and nickel are much denser than the hydrogen and helium dominating the outer planets’ composition. The concentration of these heavy elements defines the terrestrial planets’ density profile.
While the planets vary in their exact proportions—Mercury, for example, has an exceptionally high metal-to-silicate ratio—the fundamental building blocks remain the same. Earth, Venus, and Mars have roughly similar bulk compositions, with approximately one-third of their mass being iron-nickel combinations and two-thirds being silicates. This shared material makeup reflects the conditions under which they formed.
Layered Internal Structure
All four terrestrial planets exhibit a structurally differentiated interior, a similarity arising from their formation process. Differentiation occurs when denser materials sink to the center while lighter materials rise, resulting in distinct layers. This process, which happened when the planets were hot and molten, created a consistent three-part structure.
The innermost layer is a dense, metallic core, composed primarily of iron and nickel. Surrounding this core is the mantle, a thick layer composed of denser silicate rocks.
The outermost layer is a relatively thin, solid crust, consisting of the lowest-density silicate materials. The existence of these distinct layers—core, mantle, and crust—is a defining structural commonality. The size, state (liquid or solid), and thickness of these layers vary, but the core can be a source of internal heat and, as on Earth, generates a magnetic field.
Common Origins in the Inner Solar System
The similarities in composition and structure are traceable to the shared birth environment of these planets. All terrestrial planets formed relatively close to the Sun, inside the asteroid belt. This location within the inner solar nebula was characterized by high temperatures due to the proximity of the young Sun.
The intense heat had a crucial effect on the materials available for planet formation. Volatile compounds, such as water ice and light gases like hydrogen and helium, could not condense into solid form. They were instead vaporized or pushed outward by the solar wind. This left behind only refractory materials, which are compounds with high melting points, such as metals and silicates.
The planets grew through accretion, where small dust grains collided and stuck together to form increasingly larger bodies. These bodies, called planetesimals, were made almost exclusively of dense rock and metal. Over tens to a hundred million years, these planetesimals merged through collisions to form the planets we see today. This process ensured that Mercury, Venus, Earth, and Mars inherited the same fundamental, dense, rocky, and metallic makeup, explaining their classification as terrestrial worlds.