Description
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 constitutes most of the planet, whereas the hydrogen/helium only makes up an outer envelope on Uranus and Neptune which are sometimes called ice giants, as they are mostly composed of water, ammonia, and methane molten ices.
Among extrasolar planets, Hot Jupiters are gas giants that orbit very close to their stars and thus have a very high surface temperature. Hot Jupiters are currently the most common form of extrasolar planet known, perhaps due to the relative ease of detecting them.
Gas giants are commonly said to lack solid surfaces, but it is closer to the truth to say that they lack surfaces altogether since the gases that make them up 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.
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