A terrestrial body is a planet, moon, or other object in space made primarily of rock and metal, with a solid surface you could theoretically stand on. In our solar system, the four terrestrial planets are Mercury, Venus, Earth, and Mars. The term “terrestrial” comes from the Latin word for Earth, and these bodies share Earth’s basic blueprint: a layered interior with a metallic core, a rocky mantle, and a thin outer crust.
What Makes a Body “Terrestrial”
The defining feature of a terrestrial body is its composition. These worlds are built from heavy elements like iron, silicon, magnesium, and oxygen, which combine to form silicate rocks and metallic alloys. This gives them solid surfaces with recognizable landforms: cliffs, valleys, volcanoes, and impact craters.
Terrestrial bodies are also notably dense. Earth has a density of about 5.5 grams per cubic centimeter, comparable to iron-rich minerals. Mercury is similarly dense. Compare that to Jupiter, which despite being over 300 times Earth’s mass, has a density of just 1.3 g/cm³. Saturn is even less dense than water. That contrast reflects a fundamental difference in what these objects are made of: rock and metal versus hydrogen and helium gas.
Beyond density and composition, terrestrial bodies tend to be small. Mars has only about 11% of Earth’s mass, and Mercury just 5.5%. Even Venus, the closest in size to Earth, comes in at 81.5% of Earth’s mass. The giant planets dwarf them entirely, with Jupiter packing 318 times more mass than Earth into a body 11 times wider.
The Four Terrestrial Planets
Mercury, Venus, Earth, and Mars occupy the four closest orbits to the Sun. Their position isn’t a coincidence. It’s a direct result of how the solar system formed.
Mercury is the smallest and densest, with virtually no atmosphere and a surface covered in impact craters that resemble our Moon. Venus is nearly Earth’s twin in size but radically different in conditions, with an atmosphere that is 96% carbon dioxide and surface temperatures hot enough to melt lead. Earth is the only terrestrial planet with liquid water on its surface and an atmosphere dominated by nitrogen (78%) and oxygen (21%). Mars, the outermost terrestrial planet, has a thin atmosphere that is 95% carbon dioxide and a surface marked by the largest volcano and deepest canyon in the solar system.
All four have few or no moons. Earth has one, Mars has two tiny ones, and Mercury and Venus have none. This contrasts sharply with the giant planets, which can have dozens.
Layered Interior Structure
Every terrestrial body shares a common internal architecture: three distinct layers that formed as heavier materials sank inward during the body’s early molten stage.
The outermost layer is the crust, a relatively thin shell of lighter rock. Beneath it lies the mantle, a thick layer of dense, hot, semi-solid rock. Earth’s mantle alone is roughly 2,900 kilometers thick and contains higher concentrations of iron, magnesium, and calcium than the crust. At the center sits a metallic core made primarily of an iron-nickel alloy. Earth’s core has two parts: a liquid outer core about 2,200 km thick and a solid inner core about 1,250 km thick. This layered structure, with variations in thickness and temperature, exists in all four terrestrial planets and even in Earth’s Moon.
How Terrestrial Bodies Form
Terrestrial bodies are born in the inner regions of a protoplanetary disk, the rotating cloud of gas and dust that surrounds a young star. Close to the star, temperatures are too high for lightweight gases like hydrogen and helium to condense, so only heavier materials like metals and silicates survive. The solar wind blows lighter elements outward, leaving behind the raw ingredients for rocky worlds.
The process starts with micrometer-sized dust grains, which make up roughly 1% of a typical disk’s mass. These grains stick together, forming millimeter-to-centimeter-sized clumps. Some of these clumps experience brief heating episodes that cause partial melting, helping them fuse. Growth continues until kilometer-sized bodies called planetesimals take shape.
From there, gravity takes over. Gravitational interactions between planetesimals trigger rapid growth, producing Moon-to-Mars-sized protoplanets within 100,000 to a million years. These protoplanets then collide with each other in giant impacts over tens of millions of years, eventually assembling into Earth-mass planets. One such collision is thought to have created Earth’s Moon.
Terrestrial Bodies Beyond the Planets
The term “terrestrial body” isn’t limited to the four inner planets. Earth’s Moon qualifies as a terrestrial body because it shares the same basic structure: a silicate crust, a silicate mantle, and an iron-rich core. Much of what scientists know about the Moon’s interior comes from seismic instruments and heat flow measurements placed during the Apollo missions, which revealed an internal structure strikingly similar to Earth’s in arrangement, if not in scale. The Moon is small and airless, more similar to Mercury than to the planet it orbits.
Some of the large moons orbiting the outer planets also have rocky compositions, though many are mixed with significant amounts of ice, placing them in a gray area between purely terrestrial and icy bodies.
Terrestrial Worlds Around Other Stars
Astronomers have identified rocky exoplanets orbiting distant stars, expanding the concept of terrestrial bodies far beyond our solar system. Classifying a planet as terrestrial from light-years away requires measuring both its size (from how much starlight it blocks) and its mass (from how much it tugs on its star). The relationship between these two values reveals density, which tells scientists whether the planet is rocky or gaseous.
One commonly used boundary places the upper size limit for rocky exoplanets at about 1.23 times Earth’s radius. Above that threshold, planets tend to have thick gaseous envelopes and lower densities, making them more like miniature versions of Neptune than like Earth. Researchers use this cutoff to sort the thousands of known exoplanets and identify which ones have the best chance of being true terrestrial worlds, sometimes grouping them by similarity to Mercury, Venus, Earth, or Mars.