Terrestrial planets are rocky planets that are primarily composed of solid surfaces. They include Mercury, Venus, Earth, and Mars. These planets are characterized by their dense materials and relatively small sizes compared to gas giants. They have surfaces that can support landforms like mountains, valleys, and craters. Unlike gas giants, terrestrial planets have a well-defined surface and are closer to the Sun. They also have varying atmospheres, with Earth having the most suitable conditions for life. The study of terrestrial planets helps scientists understand the formation and evolution of planetary systems.