A rocky planet, or terrestrial planet, is a celestial body primarily composed of silicate rocks and metals. This composition distinguishes them from the gas giants and ice giants, which are dominated by lighter elements like hydrogen, helium, and various ices. These dense worlds feature a solid surface, unlike the outer planets that lack a well-defined solid boundary. The term “terrestrial” signifies their structural similarity to Earth. Within our Solar System, this group includes the four innermost planets.
The Four Terrestrial Planets
The Solar System contains four planets that fit the definition of a rocky world: Mercury, Venus, Earth, and Mars. Each of these worlds possesses a compact, solid surface, but they exhibit vast differences in their individual environments and features.
Mercury, the closest planet to the Sun, is the smallest of the group, with a diameter only slightly larger than Earth’s Moon. Its extremely thin atmosphere offers little protection, leading to dramatic temperature swings and a heavily cratered surface. Venus follows, similar in size to Earth but featuring a crushing, super-hot atmosphere of carbon dioxide. This creates a runaway greenhouse effect, making Venus the hottest planet.
Earth is unique among the four, being the only one currently known to harbor a vast liquid hydrosphere and active plate tectonics. It also generates a powerful, global magnetic field that shields the planet from solar radiation. Mars, the final terrestrial planet, is significantly smaller than Earth and Venus, possessing an atmosphere about 100 times thinner than Earth’s, composed mainly of carbon dioxide. Scientists actively study Mars for evidence of past liquid water on its surface, noting that it currently lacks a global magnetic field.
Shared Internal Structure and Composition
Despite their surface differences, all terrestrial planets share a fundamental, layered internal architecture that resulted from a process called differentiation. This process caused denser materials to sink to the center while lighter components rose toward the surface early in the planets’ histories.
The innermost layer is a dense, metallic core, typically composed of iron and nickel. Surrounding this core is the silicate mantle, a thick layer of rock that is less dense than the core. Earth’s core is known to have a liquid outer layer and a solid inner layer, a structure responsible for generating its protective magnetic field.
The outermost layer is the crust, a relatively thin shell of the least dense silicate rocks. The bulk density of these rocky planets is consistently high, all exceeding 3 grams per cubic centimeter, which contrasts sharply with the much lower densities of the giant gas and ice planets. Mercury is unusual because its massive core extends to about 80% of its radius, contributing to its exceptionally high overall density.
How Location Influenced Formation
The formation of these rocky worlds in the inner Solar System is a direct result of the temperature gradient within the primordial solar nebula, a vast disk of gas and dust. This protoplanetary disk had a critical boundary known as the frost line, which separated the inner, warmer region from the colder outer region.
Inside the frost line (roughly 2.7 astronomical units from the Sun), temperatures were too high for volatile compounds like water, methane, and ammonia to condense into ice. Only materials with high melting points, such as silicates and metals, could solidify and accrete to form planetesimals. This limitation meant the resulting inner planets were relatively small and composed almost entirely of rock and metal.
Beyond the frost line, cold temperatures allowed water ice and other volatile ices to condense, significantly increasing the total amount of solid mass available. This abundance enabled the outer planets to grow much larger, quickly accumulating enough mass to gravitationally capture vast envelopes of hydrogen and helium gas, leading to the formation of the gas and ice giants. The frost line acted as a cosmic sorting mechanism, dictating the composition and size of the planets based on their distance from the Sun.