Though there are speculation about life on gas giants, as far as we know a solid crust is necessary to the complex chemical interaction needed by life, and as a long-term storage of chemicals. It's still unclear what factors create a solid (terrestrial) rather than a gaseous (jovian) planet - since terrestrial planets in the Solar System are the closest to the Sun, it was assumed that planets with higher mass were naturally pushed away by their star, but now a number of planets are known to be both gaseous and very close to the star.