Group A The Rocky Inner Planets

The Inner Planets

The four innermost planets in the Solar System (Mercury, Venus, Earth, and Mars) are sometimes called the "terrestrial" planets because of their proximity to Earth ("Terra" in Latin) and their similarity as compact solid bodies with rocky surfaces. These four planets developed from small grains of dust that collided and stuck together to form pebbles, boulders, kilometer- and mile-sized planetesimals, and larger planetary embryos and protoplanets). They formed in the inner portion of the protoplanetary disk located closer to the developing Sun during the first 100 million years of the System's birth, where it was too warm for the four developing protoplanets to agglomerate water and more volatile ices and bulk up sufficiently in gravitational might to hold onto the abundant but lightest gases of the Solar nebulae (hydrogen and helium) to become "gas giants." Although rocky (and icy protoplanets beyond the Solar System's 2-AU "ice line") formed in the Main Asteroid Belt, the early development of Jupiter prevented protoplanets like icy Ceres from agglomerating into larger planetary bodies, by sweeping many into pulverizing collisions as well as out into the Oort Cloud or beyond Sol's gravitational reach altogether.

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