Giant planet
A planet substantially more massive than Earth and composed largely of volatile materials such as hydrogen, helium, water, ammonia, or methane.
Planet: A celestial body that orbits a star, is massive enough for gravity to make it nearly round, and has cleared its orbital neighborhood. Giant planets are one broad class of planets, defined by their mass and volatile-rich composition.
Core accretion: A planet-formation model in which solid material builds a core that later accretes surrounding gas. It explains how some giant planets could gather massive hydrogen-helium envelopes around growing cores.
Jupiter: The largest planet in the Solar System, a hydrogen-helium giant with powerful atmospheric jets and a strong magnetic field. Jupiter is the archetypal giant planet and the most massive member of the class in the Solar System.
Terrestrial planet: A rocky planet composed mainly of silicate rock and metal, with a solid surface or mantle. Terrestrial planets contrast with giant planets in bulk composition, size, and atmospheric mass.
Exoplanet: A planet that orbits a star other than the Sun. Exoplanet surveys have revealed giant planets with masses, orbits, and temperatures unlike Solar System examples.
Hydrogen: The lightest chemical element, abundant in stars and in the atmospheres of many giant planets. Hydrogen is the dominant constituent of Jupiter and Saturn and a major atmospheric gas elsewhere.
Disk instability: A planet-formation model in which a region of a young circumstellar disk collapses directly under its own gravity. It offers an alternative route to giant planets, especially where rapid formation is difficult for core accretion.
Saturn: The second-largest planet in the Solar System, a hydrogen-helium giant distinguished by its extensive ring system. Saturn shows how a giant planet can have a lower mean density than water while retaining a massive atmosphere.
Ice giant: A giant planet whose interior contains substantial water, ammonia, and methane relative to hydrogen and helium. The term distinguishes Uranus- and Neptune-like composition from Jupiter- and Saturn-like gas giants.
Hot Jupiter: A gas giant on a close orbit around its star, with an orbital period typically measured in days. Hot Jupiters expose giant planets to intense irradiation and tidal effects absent from the Solar System's giants.