Why are inner planets solid?
In the early solar system, gases coalesced to form the beginnings of our sun (or, the protosun). This caused the temperatures in the early solar system to rise - close to 2,000 K. Meanwhile the temperatures in the outer solar system were as low as 50 K.
That meant that only substances with very very high melting points would stay solid. The remaining planets -- Mercury, Venus, Earth & Mars -- are made up mostly of iron, silicon, magnesium, sulfur, aluminum, calcium and nickel.