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Terrestrial planets form through hierarchical accretion of planetesimals and planetary embryos in the inner solar system where temperatures prevent ice formation. These planets—Mercury, Venus, Earth, Mars—are small, rocky, and dense. Their varied internal structures (Mercury's enormous iron core, Venus's thick atmosphere, Earth's layers, Mars's smaller size) reflect differences in formation conditions and subsequent evolution.
Compare terrestrial planets' masses, densities, and compositions. Discuss how planet size affects internal differentiation. Examine how proximity to the Sun influenced composition and atmospheric retention.
From your study of solar system zones and architecture, you know that the snow line (or frost line) divides the solar nebula into an inner region where only metals and silicates could condense from the hot gas, and an outer region where water ice and other volatiles also solidified. The terrestrial planets — Mercury, Venus, Earth, and Mars — formed inside this line, which is why they are made primarily of rock and metal rather than the hydrogen, helium, and ice that dominate the giant planets.
The formation process began with dust grains in the solar nebula sticking together through collisions, growing from micrometer-sized particles to kilometer-sized planetesimals over perhaps a million years. Once planetesimals reached sufficient mass, gravity took over from random sticking: larger bodies swept up smaller ones in a process called runaway accretion, where the biggest objects grew fastest because their gravitational reach expanded with each capture. This produced a few dozen Moon-to-Mars-sized planetary embryos within the inner solar system. The final stage was the most violent: over tens of millions of years, these embryos' orbits crossed and they collided in giant impacts, gradually assembling into the four terrestrial planets we see today. Earth's Moon is thought to have formed from debris ejected in one such giant impact.
The differences among the four terrestrial planets reflect their formation conditions and subsequent evolution. Mercury, closest to the Sun, has an outsized iron core comprising about 60% of its mass — possibly because a giant impact stripped away much of its rocky mantle, or because intense solar radiation prevented lighter silicates from condensing nearby. Venus and Earth are similar in size and bulk composition, but Venus's thick CO₂ atmosphere and runaway greenhouse effect created surface conditions radically different from Earth's. Mars, farther from the Sun and smaller, lost most of its atmosphere early because its weaker gravity could not retain it against solar wind stripping, and its small size meant its interior cooled quickly, shutting down the magnetic dynamo that might have protected its atmosphere.
A planet's size is the single most important factor in its long-term evolution. Larger planets retain internal heat longer, sustaining geological activity (volcanism, plate tectonics, magnetic dynamos) that recycles atmospheres and surfaces. Earth's size places it in a sweet spot: large enough to maintain a protective magnetic field and active geology, but not so large as to retain a massive hydrogen envelope. Understanding terrestrial planet formation illuminates not only our own solar system but also the thousands of rocky exoplanets now being discovered around other stars.