Giant Planets

Abstract

Giant planets are mostly made of hydrogen and helium. They formed around and at the same time as their parent star, from the same reservoir of gas and solids. Their study thus informs us directly on conditions present in the protoplanetary disks that eventually led to the formation of planetary systems including smaller planets. We review our current knowledge of the interior structure, evolution, and formation of giant planets, both for ‘our’ Jupiter, Saturn, Uranus, and Neptune, and also in the case of extrasolar giant planets. Giant planets are fluid, warm, relatively compressible, and with low internal viscosity (compared to terrestrial planets). Jupiter in particular, but also Saturn, Uranus, and Neptune, has strong magnetic fields. Their interior is generally mostly convective, but with important exceptions in the upper layers of irradiated planets. The structure and composition of giant planets in our solar system is consistent with the presence of a central core made of dense material (rocks and/or ices) of about 10 Earth masses, but with large possible variations around that value (between 0 and 25 Earth masses), and an envelope, mostly made of hydrogen and helium, but significantly enriched in heavy elements. Around other stars, giant planets can be characterized by the transit method, which enables a measurement of their radius, combined to additional mass measurement. There is mounting evidence from these observations and modeling of their evolution that some of them possess large amounts of heavy elements (up to 100 Earth masses), possibly in the form of central cores. The processes that led to the present structure and composition of these planets are not well known. Accretion most probably played a key role in their formation, but the processes that led to the presence of significant amounts of heavy elements in their interiors are not well understood. We present the various physical processes that affect the evolution of these planets in terms of luminosity and sizes, and discuss mass–radius relations to be compared to observations. We discuss how improvements in the knowledge of planet formation and the origin of the solar system will stem from parallel studies of ‘our’ giant planets for which detailed measurements are available, and from observations to characterize a statistically significant number of extrasolar giant planets.