Terrestrial planet Terrestrial planet A terrestrial planet, telluric planet, or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun, i.e. Mercury, Venus, Earth, and Mars. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth ("Terra" and "Tellus"), as these planets are, in terms of structure, "Earth-like". Terrestrial planets have a solid planetary surface, making them substantially different from the larger giant planets, which are composed mostly of some combination of hydrogen, helium, and water existing in various
Tradeoffs for locomotion in air and water Tradeoffs for locomotion in air and water Certain species of fish and birds are able to locomote in both air and water, two fluid media with very different properties. A fluid is a particular phase of matter that deforms under shear stresses and includes any type of liquid or gas. Because fluids are easily deformable and move in response to applied forces, efficiently locomoting in a fluid medium presents unique challenges. Specific morphological characteristics are therefore required in animal species that primarily depend on fluidic locomotion. Because the properties of air and water are so different, swimming and flying have