Terrestrial, Gas Giant & Ice Giant Worlds: Physical Mechanisms

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.

Disk temperature influenced which materials condensed. Beyond frost lines, abundant ices increased the solid mass available for rapid core growth, helping some planets capture nebular gas before the disk dispersed. Later compression, atmospheric escape, impacts, migration, and internal heat changed the initial structures.