Terrestrial, Gas Giant & Ice Giant Worlds: Observation, Measurement & Evidence

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 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.

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.

Mass, radius, gravity harmonics, spectra, magnetic fields, satellite motions, and atmospheric probes constrain interiors. Mean density alone is insufficient because pressure strongly compresses giant planets. Comparative data from spacecraft show that Jupiter and Saturn are not simply enlarged Earths and that Uranus and Neptune are not merely smaller Jupiters.