Interior of Sol System's Gas Giants
A gas giant is a massive planet with a thick atmosphere of hydrogen and helium. They may have a dense molten core of rocky elements, or the core may have completely dissolved and dispersed throughout the planet if the planet is hot enough. The hydrogen and helium in "traditional" gas giants like Jupiter and Saturn constitute most of the planet, whereas they only make up an outer envelope on Uranus and Neptune, which are sometimes called ice giants, because they are mostly composed of water, ammonia, and methane.
In other systems, Hot Jupiters are gas giants that orbit very close to their stars and thus have a very high surface temperature. Hot Jupiters were, until the advent of space-borne telescopes, the most common form of extrasolar planet known, perhaps due to the relative ease of detecting them from ground-based instruments.
Gas giants are commonly said to lack solid surfaces, but it is more accurate to say that they lack surfaces altogether since the gases that constitute them simply become thinner and thinner with increasing distance from the planets' centers, eventually becoming indistinguishable from the interplanetary medium. Therefore landing on a gas giant may or may not be possible, depending on the size and composition of its core.
Ever since the discovery of the Bellebee, as the first sentient Jovian planet species, many Gas Giants have been found to harbor life and some of it sentient.
Since then Union Survey ships pay close attention to gas giants, not only as fuel source but as a planet of potential life.