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Why are Mercury, Venus, Earth, and Mars rocky planets?

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Illustration showing the eight planets of the solar system in order from the Sun-Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune-depicted to scale by size, with labeled diameters in miles and kilometers. The Sun is partially visible on the left for comparison. — Solar system The planets (in comparative size) in order of distance from the Sun.

Mercury, Venus, Earth, and Mars are rocky planets because of their proximity to the Sun during the formation of the solar system.12

According to the nebular hypothesis, the planets formed from a solar nebula—a rotating disk of gas and dust. In the inner regions of this disk, temperatures were too high for volatile substances like water, carbon dioxide, and ammonia to condense into ice. Consequently, the solid material available to form planetesimals in these inner orbits consisted primarily of silicates and other high-melting-point materials.342

These rocky building blocks aggregated through accretion to form the four dense, terrestrial planets. In contrast, the outer planets formed beyond the "snow line," where colder temperatures allowed ices to condense, providing enough mass to attract and retain vast amounts of hydrogen and helium from the nebula.542