How were the inner planets formed?

• A. Accretion of gas from the outer nebula
• B. Gravitational capture from passing stars
• C. Impact, flooding, plate tectonics, cooling
• D. Collision with comets

Answer: (C)

Inner planets form from solid material in the early solar system by collisions and sticking together—accretion of rocky planetesimals. As these bodies grow, the energy from impacts heats them, causing partial melting and differentiation. Later, volcanic activity can flood surfaces with lava, and over time the planets cool and their interiors solidify, leading to tectonic processes that shape the crust. Among the options, this sequence—growth through impacts and subsequent surface and interior evolution—best captures both how rocky planets build up and how their surfaces and interiors develop. The other choices describe gas capture, capture by passing stars, or isolated comet impacts, which don’t explain the primary formation of the rocky inner planets.