The four outer gas and ice giants—Jupiter, Saturn, Uranus, and Neptune—are far less dense than the four inner, rocky terrestrial worlds: Mercury, Venus, Earth, and Mars. The average density of terrestrial planets is approximately 5 grams per cubic centimeter, significantly higher than the average density of Jovian planets, which is only about 1.3 grams per cubic centimeter. This difference in density is the fundamental feature separating the two planetary classes, revealing a contrast in their composition and formation history.
Understanding Planetary Density as a Metric
Density is a fundamental physical property used by astronomers to infer a planet’s internal structure and composition. Mathematically, it is defined as mass divided by volume, revealing how tightly material is packed within a given space. For example, a bowling ball has a much higher density than a beach ball of the same size because it contains far more mass.
Calculating a planet’s average density allows scientists to understand what materials dominate its interior. A high average density suggests the presence of heavy elements, such as metals and silicate rocks. A low average density points to an abundance of lighter elements, such as hydrogen and helium gas. This metric is a starting point for modeling a planet’s layers, from its core to its atmosphere.
The Structure of Terrestrial Planets
Terrestrial planets achieve their high density because they are composed almost entirely of heavy elements that condensed close to the young Sun. These worlds are differentiated, meaning their interiors separated into distinct layers based on density when they were molten early in their history. The densest materials sank to the center, while lighter materials rose toward the surface.
The innermost layer of these planets is a metallic core, dominated by iron and nickel. This core is surrounded by a mantle and crust made primarily of silicate rock. Earth has the highest average density in the solar system, measuring about 5.5 grams per cubic centimeter, partly because its immense gravity compresses its core materials. Mercury also exhibits a high density of approximately 5.3 grams per cubic centimeter, attributed to its disproportionately large metallic core.
The Gaseous Composition of Jovian Planets
Jovian planets have low densities because their vast volumes are overwhelmingly filled with the lightest elements. These giant planets formed in the colder outer solar system, where volatile compounds like hydrogen and helium were plentiful and captured in massive amounts. Jupiter and Saturn are classified as gas giants, composed of about 75% hydrogen and 25% helium by mass.
Even under intense pressure deep within these planets, hydrogen and helium do not achieve the density of solid rock or metal. Jupiter’s interior pressures are so immense that hydrogen transforms into liquid metallic hydrogen, which conducts electricity. Despite this compression, the overall average density of Jupiter is only about 1.3 grams per cubic centimeter. The ice giants, Uranus and Neptune, are slightly denser than Saturn because they contain a higher proportion of heavier compounds, often referred to as “ices,” such as water, methane, and ammonia, surrounding their core.
Direct Density Comparison Across the Solar System
Density data confirms that terrestrial planets are consistently more compact than Jovian planets. The inner worlds—Mercury, Venus, Earth, and Mars—have bulk densities that fall within the range of roughly 3.9 to 5.5 grams per cubic centimeter. This range is defined by the density of rock and metal.
All four outer giants have average densities well below 2 grams per cubic centimeter. Saturn has the lowest density of any planet in the solar system, averaging only 0.69 grams per cubic centimeter. This remarkably low value is less than the density of liquid water (1.0 gram per cubic centimeter), leading to the classic thought experiment that Saturn would float in a large enough body of water. The fundamental difference in bulk density—heavy elements versus light elements—establishes the terrestrial planets as the dense worlds and the Jovian planets as the low-density giants.