The planets of our solar system are divided into two groups based on their distance from the Sun. The four inner planets—Mercury, Venus, Earth, and Mars—are relatively small, dense worlds orbiting within the asteroid belt. Beyond this boundary lie the four outer planets—Jupiter, Saturn, Uranus, and Neptune—which are vast, low-density giants. This fundamental division creates differences in their physical makeup, size, and orbital characteristics.
Fundamental Differences in Composition
The primary distinction between the two planetary groups is their material composition, classifying the inner planets as terrestrial and the outer planets as Jovian. Terrestrial planets, including Earth, are predominantly composed of silicates and metals such as iron and nickel. This composition results in a solid surface and a layered internal structure where heavier materials form a dense, metallic core through differentiation.
The outer planets, often called gas giants and ice giants, are made of lighter elements and compounds. Jupiter and Saturn are classified as gas giants, consisting almost entirely of hydrogen and helium. Their interiors lack a defined solid surface, instead transitioning from gaseous outer layers to liquid or metallic states under immense pressure.
Uranus and Neptune are categorized as ice giants because their structure contains a higher proportion of heavier volatile compounds, often referred to as “ices.” These ices include water, methane, and ammonia, which surround a smaller, rocky core. This means the ice giants have a greater concentration of elements heavier than hydrogen and helium compared to their gas giant counterparts.
Scale, Mass, and Density
The differing compositions lead directly to variations in physical scale and density between the two planetary groups. The inner planets are relatively tiny, with Earth being the largest terrestrial world at a diameter of about 12,742 kilometers. Their collective mass is dwarfed by the outer planets, which are orders of magnitude larger.
Outer planets are enormous; Jupiter alone has a diameter of nearly 143,000 kilometers and holds more mass than all the other planets in the solar system combined. Despite their immense size and mass, the outer planets have a very low average density due to their light gaseous and icy makeup. Saturn, in particular, has an average density so low that it would hypothetically float if placed in a large enough body of water.
Conversely, the inner planets exhibit a high average density, exceeding 3 grams per cubic centimeter, because of their metallic and rocky makeup. Earth’s average density is approximately 5.5 grams per cubic centimeter, a direct consequence of its substantial iron-nickel core.
Orbital Distance and Planetary Features
Orbital distance from the Sun dictates planetary temperature, orbital mechanics, and the characteristics of the surrounding environment. The inner planets orbit close to the Sun, completing their paths in relatively short periods; Mercury, the closest, finishes an orbit in just 88 Earth days. This proximity results in higher surface temperatures, such as the scorching heat found on Venus and Mercury.
The outer planets are located millions of miles away from the Sun, causing them to have extremely long orbital periods; Neptune takes over 164 Earth years to complete a single revolution. Due to their great distance, these worlds exist in frigid environments. The orbital speeds of the outer planets are significantly slower than the inner planets, as they are less affected by the Sun’s gravitational pull.
A clear distinction is found in the presence of moons and ring systems. The inner planets generally have few or no natural satellites; Mercury and Venus have none, Mars has two small moons, and Earth has only one. In sharp contrast, the outer planets are characterized by extensive systems of numerous moons and prominent rings. Every outer planet possesses a ring system, with Saturn’s being the most visible and complex.