The planets in our solar system fall into two distinct categories: the Terrestrial planets and the Jovian planets. The Terrestrial planets—Mercury, Venus, Earth, and Mars—are found in the inner solar system. The Jovian planets—Jupiter, Saturn, Uranus, and Neptune—reside in the outer reaches. These two groups represent fundamentally different outcomes of planetary formation, separated by location and material makeup.
Defining Differences in Composition and Density
The most profound distinction between the two groups lies in their primary constituent materials. Terrestrial planets are often called rocky planets because they are primarily composed of silicates and metals like iron and nickel. The Earth, for instance, has a mean density of about 5.5 grams per cubic centimeter, a direct result of this dense, heavy element composition. This density is a shared trait among the inner planets, with values ranging from about 3.9 to 5.5 g/cm³.
Jovian planets, in contrast, are formed from much lighter elements, leading to significantly lower overall densities. They consist mainly of the gases hydrogen and helium, which were abundant in the outer solar system during their formation. Saturn, the least dense of all the planets, has an average density of only 0.69 g/cm³, so low that it would float in water. Even the densest Jovian world, Neptune, is only about 1.64 g/cm³, a density far lower than any of the inner, rocky worlds.
This compositional split is a direct consequence of the temperature gradient in the early solar nebula. The inner region was too warm for volatile compounds, such as hydrogen and helium, to condense into solid or liquid forms, leaving only the heavier rocky and metallic materials to build the Terrestrial planets. The outer solar system, however, was cold enough for these lighter elements and various ices to accumulate, allowing the Jovian worlds to grow massive enough to retain the abundant gases of the nebula.
Contrasting Physical Structure and Size
The difference in materials results in radically different internal structures and overall dimensions. Terrestrial planets have a clearly defined physical surface, consisting of a solid crust that transitions into a mantle and a dense metallic core. This layered structure is characteristic of worlds built from molten rock that separated into different density layers as they cooled over time.
Jovian planets lack a definable solid surface. Their structure involves a gradual transition where the atmosphere becomes denser with depth, eventually turning into a liquid or supercritical fluid under immense pressure. In the massive worlds of Jupiter and Saturn, hydrogen gas compresses into a layer of liquid metallic hydrogen, a state that conducts electricity and is responsible for generating their powerful magnetic fields. Uranus and Neptune, sometimes called ice giants, have lower pressures and lack the metallic hydrogen layer, instead possessing deep layers of super-critical fluids composed of water, methane, and ammonia surrounding a small, dense core of rock and ice.
In terms of scale, the Jovian planets are vastly larger and more massive than their inner counterparts. Jupiter’s diameter is approximately 11 times that of Earth, and its mass is greater than all the other planets in the solar system combined. Even the smallest of the giants, Neptune, is about four times the diameter of Earth. The Terrestrial planets are comparatively small and modest in mass.
Orbital Distance and Satellite Systems
The physical separation of the two planet types is a defining characteristic of our solar system’s architecture. The Terrestrial planets orbit relatively close to the Sun, all lying within 1.5 astronomical units (AU) of the star. This inner region is separated from the outer solar system by the main asteroid belt, which orbits between Mars and Jupiter.
The Jovian planets reside in the outer solar system, with orbital distances ranging from Jupiter’s 5 AU to Neptune’s approximately 30 AU. Another significant difference is the number of associated orbiting bodies. Terrestrial planets are isolated worlds, with Mercury and Venus possessing no moons, Earth having one, and Mars having two small satellites. Conversely, all Jovian planets host extensive satellite systems, with dozens of moons each, many of which are worlds of significant size. All four Jovian planets also possess ring systems, composed of countless small pieces of rock and ice, a feature entirely absent from the Terrestrial planets.