giant planet

A giant planet (or Jovian planet) is a planet such as Jupiter, Saturn, Uranus or Neptune: much larger than Earth and the other solar system planets. Extra-solar planets with sizes between these two groups are now known, and various criteria have been used to classify a planet as giant, one example being "exceeds ten Earth masses". 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 planet is also used more specifically, for a planet much like Jupiter (and 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 some are observed to 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 planetary systems at earlier times, as are the giant planets of the solar system.

Known exoplanets include hot Jupiters, gas giants very close to the host star, and eccentric Jupiters, gas giants with considerable eccentricity, and how they came to be that way is of interest. The planet formation of giant planets is also definitely 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. A theory is that the emptied region near the forming 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 be of use in modeling this process. The length of time of the significant accretion 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.