Why are the first four planets solid and not gaseous?

Answer 1

We used to think we had this figured out... planets close to the Sun had their thick atmospheres stripped away by radiation pressure and the solar wind, so they could never accumulate enough mass to become gas giants.

Then we discovered "hot Jupiters" orbiting other stars.

Now we don't know. It might just be because that's the way it happened to turn out in our solar system. We know for a fact that it hasn't worked out that way in other systems.

Answer 2

The first four planets in our solar system - Mercury, Venus, Earth, and Mars - are solid because they formed closer to the Sun where temperatures were higher, preventing the gases in the disk of dust and gas from condensing into giant gas planets like Jupiter and Saturn. Instead, these solid planets accumulated rock and metal in their formation.