Terrestrial, Gas Giant & Ice Giant Worlds: Structures & Vocabulary

The major planets fall into physically different classes rather than forming one smooth sequence. Terrestrial planets are comparatively small, dense, and dominated by rock and metal; Jupiter and Saturn are gas giants dominated by hydrogen and helium; Uranus and Neptune are ice giants with larger fractions of water-, ammonia-, and methane-rich material beneath their atmospheres.

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 have solid surfaces and high mean densities. Jupiter and Saturn possess deep hydrogen-helium envelopes with no simple solid surface, while Uranus and Neptune have smaller hydrogen-helium outer layers above interiors enriched in water-, ammonia-, and methane-related constituents. Giant planets also have extensive satellite systems, rings, and magnetospheres.