Terrestrial, Gas Giant & Ice Giant Worlds: Foundations

Terrestrial planets have solid surfaces and differentiated interiors containing metallic cores and silicate mantles. Gas giants contain deep hydrogen-helium envelopes surrounding denser interiors, while ice giants have smaller hydrogen-helium envelopes relative to interiors enriched in heavier volatile compounds. Mass, density, radius, composition, and distance from the Sun distinguish these groups.

Planet type reflects both the composition of the protoplanetary disk and how quickly a growing core accumulated mass before disk gas dispersed. In the warmer inner disk, volatile ices were scarce and rocky planets remained relatively small. Beyond colder condensation regions, more solid material was available, allowing large cores to form and capture substantial gas.

Planet masses and radii determine bulk density, while spectroscopy reveals atmospheric composition. Spacecraft gravity measurements, magnetic fields, ring dynamics, and atmospheric probes constrain internal structure. The sharp compositional contrast between inner rocky worlds and outer giant planets supports formation in a disk with a strong temperature gradient.

Terrestrial planets, gas giants, and ice giants differ because of formation location, available solids and volatiles, mass, and later evolution. The categories describe broad composition and structure: rocky worlds are dominated by silicate and metal, gas giants by hydrogen and helium, and ice giants contain larger fractions of heavier volatile-forming materials.