Density is a physical property that reveals how much matter is packed into a given space, offering insight into a planet’s composition and internal structure. Calculating a planet’s total mass relative to its volume determines its average density. This measurement allows scientists to differentiate between the solar system’s two main classes of worlds: the small, high-density terrestrial planets and the large, low-density gas giants. This comparison highlights the physical reasons for the least dense world’s unique lightness.
Defining Planetary Density
Planetary density is a measure of a celestial body’s compactness, calculated by dividing its total mass by its total volume. This measurement is typically expressed in grams per cubic centimeter (g/cm³). The resulting average density figure then acts as a signature for the materials that constitute the planet’s interior.
The nature of the constituent materials dictates a planet’s density. High-density worlds are dominated by metallic and silicate rock compounds, such as the iron and nickel found in Earth’s core. Conversely, low-density planets are composed primarily of lighter elements, such as hydrogen, helium, and various ices.
Saturn: The Solar System’s Lightest Planet
The planet with the lowest density in our solar system is Saturn, the sixth planet from the Sun. Saturn’s mean density is approximately 0.687 g/cm³, a figure that is remarkably low for an object of its tremendous size. This low density is famously illustrated by the hypothetical scenario that Saturn would float if placed in a colossal body of water.
The density of pure water is about 1.0 g/cm³, meaning Saturn is roughly 30% less dense than water. No other planet in the solar system shares this specific characteristic. This low measurement is the result of a massive volume holding a comparatively smaller total mass.
Compositional Factors Driving Low Density
The primary reason Saturn exhibits such a low average density is its bulk composition, dominated by the two lightest elements in the universe. Saturn is composed of approximately 96% hydrogen and 3% helium by volume in its upper atmosphere. These gases are extremely light, and even when compressed under immense pressure, they do not contribute significantly to the overall density when averaged across the planet’s vast volume.
Deep within the planet, the pressure converts the hydrogen into a liquid metallic state, which is much denser than the gaseous and liquid hydrogen layers above it. Despite this compression, the sheer size of the gaseous and liquid envelope keeps the mean density low. Models suggest that Saturn’s dense, rocky core is considerably smaller relative to the planet’s total size than the core of Jupiter.
While Saturn is incredibly massive, about 95 times the mass of Earth, its volume is disproportionately large. The planet’s rapid rotation also causes a notable equatorial bulge, increasing its volume. This combination of a predominantly hydrogen-helium composition and a massive, relatively uncompressed volume determines Saturn’s status as the solar system’s least dense world.
Density Comparison Across the Solar System
The density of the planets in the solar system broadly falls into two groups, with Saturn representing the bottom of the scale. The terrestrial planets—Mercury, Venus, Earth, and Mars—are all highly dense due to their composition of metal and rock. Earth is the densest planet overall at 5.514 g/cm³, followed closely by Mercury at 5.427 g/cm³.
The outer planets, known as the Jovian worlds, are significantly less dense but still mostly denser than water. After Saturn, the ranking of increasing density continues with Uranus at 1.27 g/cm³, Jupiter at 1.326 g/cm³, and then Neptune at 1.638 g/cm³. The higher densities of Uranus and Neptune are attributed to their greater proportion of heavier elements and ices, such as water, methane, and ammonia, compared to the predominantly hydrogen and helium makeup of Saturn and Jupiter.