A giant planet (or Jovian planet) is a planet such as Jupiter, Saturn, Uranus or Neptune: much larger than Earth. A minimum of 10 Earth masses is a common rule of thumb. Jupiter and Saturn are known as gas giants, being mostly hydrogen and helium. In contrast, Uranus and Neptune are now classified as ice giants, having water, ammonia, and methane. The term Jovian planets is often used to indicate a planet more specifically like Jupiter or Saturn. There is a correlation between the presence of such a planet and high host-star metallicity.
Like Earth and the Sun, giant planets can have magnetic fields, but they may be multipole rather than like the Earth's dipole magnetic field. Some giant planets are thought to have primordial atmospheres, i.e., representative of atmospheres in their system at earlier times, as are the giant planets of the solar system.
Known extra-solar planets include hot Jupiters, gas giants very close to the host star, and eccentric Jupiters, gas giants with considerable eccentricity. The planet formation of giant planets is of interest, requiring a means by which they can grow to observed sizes within the time that gas is available to them. The gravitational instability model aims to provide a sufficiently rapid means to do this, while the core accretion model requires some theory as to how sufficient gas comes close enough to the growing planet. Some theorize the emptied region near the planet will draw in sufficient gas from the protoplanetary disk to accomplish this. One factor is that the cooling of the atmosphere shrinks it, creating more relatively-vacant space within the Hill radius, allowing continued accretion of disk gas. Given this latter model, a cooling timescale of the gas determines the amount of gas accretion, and thus the ultimate size of the planet. Some of the classic stellar structure equations can assist in modeling this process. The time does depend upon how much of the gas envelope is radiative, i.e., the larger the transparent portion, the faster the cooling, thus the larger the ultimate size of the planet, making that depend upon the boundary between convective and radiative layers.