Terrestrial planet
A planet composed primarily of rock or metal, with a solid surface. Mercury, Venus, Earth, and Mars are the Solar System’s terrestrial planets.
Planet: A celestial body that orbits a star, is rounded by its own gravity, and has cleared its orbital neighborhood. Terrestrial planets are one class of planet, distinguished by rocky composition and a solid surface.
Mercury: The smallest planet in the Solar System and the closest to the Sun. Its unusually large metal core makes Mercury the most iron-rich terrestrial planet by proportion.
Protoplanetary disk: A rotating disk of gas and dust surrounding a young star, from which planets form. Terrestrial planets assembled from solid material in the inner regions of the Sun’s disk.
Gas giant: A giant planet composed mainly of hydrogen and helium, without a solid surface accessible at its cloud tops. Gas giants contrast with terrestrial planets in size, composition, and lack of a solid outer surface.
Exoplanet: A planet orbiting a star other than the Sun. Thousands of exoplanets extend the study of terrestrial worlds beyond the Solar System.
Rocky planet: A planet made predominantly of silicate rock and metal, with a solid surface. This near-synonym emphasizes the material composition shared by terrestrial planets.
Venus: The second planet from the Sun, with a dense carbon-dioxide atmosphere and an extremely hot surface. Venus shows how a rocky planet can develop a crushing atmosphere and intense greenhouse heating.
Accretion (astrophysics): The growth of a celestial body through collisions and gravitational accumulation of smaller material. Repeated collisions among planetesimals built the precursors of terrestrial planets.
Ice giant: A giant planet whose interior contains substantial water, ammonia, and methane alongside hydrogen and helium. Ice giants are large volatile-rich worlds, unlike the comparatively small rocky terrestrial planets.
Planetary habitability: The capacity of a planet or moon to support conditions suitable for life. Rocky surfaces and atmospheres make terrestrial planets central targets in habitability studies.