Which group of planets includes Mercury, Venus, Earth, and Mars?

The group of planets that includes Mercury, Venus, Earth, and Mars is known as terrestrial planets. These planets are characterized by solid, rocky surfaces and are comparatively smaller in size than the gas giants. Terrestrial planets typically have a well-defined surface with geological features such as mountains, craters, and valleys.

The term "terrestrial" comes from the Latin word "terra," meaning Earth, which emphasizes that these planets have Earth-like qualities. They are primarily composed of silicate rocks or metals and have relatively high densities compared to gas giants, which are mainly composed of hydrogen and helium and lack solid surfaces.

In contrast, gas giants (like Jupiter and Saturn) are large planets composed mostly of gases and do not have a well-defined surface, while ice giants (like Uranus and Neptune) have compositions that include significant amounts of "ices" such as water, ammonia, and methane. Dwarf planets (like Pluto) are smaller celestial bodies that do not meet all the criteria to be considered full-fledged planets, primarily because they have not cleared their orbits of other debris.

Thus, recognizing Mercury, Venus, Earth, and Mars as terrestrial planets highlights their physical characteristics, composition, and position within our solar system.