Terrestrial planet

A terrestrial planet is a world composed primarily of silicate, rocks, or metals. These worlds are also known as tellurian or telluric planets. They are fundamentally different from gas giants. Gas giants are composed mostly of hydrogen, helium, and water in various physical states. In contrast, terrestrial planets possess a solid planetary surface. This characteristic allows geologists, astronomers, and geophysicists to study them as distinct solid bodies.

Most terrestrial planets in our Solar System share a specific internal structure. They typically feature a central metallic core, which is mostly made of iron. This core is surrounded by a silicate mantle, which is a layer of rocky material. The four inner planets—Mercury, Venus, Earth, and Mars—all follow this basic pattern. Even some large bodies follow this rule. The large rocky asteroid 4 Vesta has a similar structure. The moon Io also has a structure similar to a terrestrial planet.

Not every rocky body is organized the same way. The asteroid 2 Pallas is similar in size to Vesta. However, Pallas is significantly less dense. It appears to have never undergone differentiation. Differentiation is the process where a body separates into a core and a mantle. The Earth's Moon also has a similar structure, but its iron core is much smaller. Other bodies, like the Jovian moon Europa, have a significant ice layer on their surface. Because of this ice, Europa is sometimes classified as an icy planet instead.

Surface features on these planets depend on specific environmental processes. Terrestrial planets can have canyons, craters, mountains, and volcanoes. These features form due to tectonic activity or the presence of an erosive liquid. These planets also possess secondary atmospheres. These atmospheres are generated by volcanic out-gassing or by debris from comet impacts. This is a major difference from the outer giant planets. The giant planets have primary atmospheres. These were captured directly from the original solar nebula during formation.

Scientists use density to understand what these planets are made of. The uncompressed density is the average density of materials at zero pressure. A higher uncompressed density usually indicates a higher metal content. In our Solar System, uncompressed densities tend to decrease as distance from the Sun increases. This follows the temperature gradient of the primordial solar nebula. For example, Mercury is 0.39 AU from the Sun and has a density of 5.4 g/cm³. Earth is 1.0 AU from the Sun and has a density of 5.5 g/cm³. Mars is further out at 1.52 AU with a density of 3.9 g/cm³.

Discovery of these worlds has expanded significantly over time. In the early 1990s, astronomers found planets orbiting a pulsar using pulsar timing. In 2005, they found planets orbiting a main-sequence star. One example is Gliese 876 d, which is 15 light-years away. It has a mass seven to nine times that of Earth. In 2011, the Kepler space telescope found the first confirmed terrestrial exoplanet, Kepler-10b. Many other candidates are called super-Earths. These are planets with masses between Earth and Neptune.

Understanding these planets helps us connect different parts of space science. The study of terrestrial planets overlaps with the study of icy planets. Icy planets, like those found beyond the snow line, have solid surfaces but are made of ice and rock. This includes dwarf planets like Ceres, Pluto, and Eris. Some icy worlds, like Europa or Enceladus, may have subsurface hydrospheres. A hydrosphere is a system of liquid water. Even Titan has surface liquids, though they are liquid methane rather than water.

Recent research continues to refine our definitions. In 2016, statistical modeling suggested that large rocky worlds might be rare. This model looked at the relationship between a planet's mass and its radius. As of 2024, the expected transition point between rocky and intermediate-mass planets is roughly 4.4 Earth masses. In 2020, astronomers even detected a rogue planet named OGLE-2016-BLG-1928L. This is an Earth-mass planet that is not bound by any star. It floats freely through the Milky Way galaxy.