Jovian planets are the four immense worlds orbiting in the outer solar system: Jupiter, Saturn, Uranus, and Neptune. These planets are fundamentally different from the rocky, dense terrestrial planets like Earth and Mars. Their composition is dominated by light elements and volatile compounds that were abundant in the cooler, outer regions of the solar nebula during their formation. This material makeup resulted in worlds of enormous size and low average density, lacking a solid surface in the conventional sense. Understanding their bulk composition provides insight into the conditions of the early solar system.
Dominant Elements: Hydrogen and Helium
The two largest Jovian planets, Jupiter and Saturn, are classified as Gas Giants because their bulk composition closely mirrors that of the Sun and the primordial solar nebula. The vast majority of their mass consists of the lightest elements, hydrogen and helium. Their immense gravity allowed them to capture large amounts of the surrounding nebula’s gas before it was dissipated by the young Sun’s activity.
By the number of atoms, hydrogen makes up approximately 90% of the material in the atmospheres of Jupiter and Saturn, with helium accounting for the remaining 10%. This ratio is very close to the composition of the original cloud of gas and dust from which the solar system formed. The low density of these elements explains why the Gas Giants are so massive, yet have low average densities compared to the rocky inner planets.
These elements dominate because the outer solar system was cool enough for them to be retained by the forming planets’ gravitational pull. In contrast, the inner planets, which formed closer to the Sun’s heat, could only hold onto heavier, high-melting-point materials like rock and metal.
Internal Structure and Layers
The extreme mass of the Jovian planets means their internal structure is organized by pressure and temperature, causing the dominant hydrogen to exist in exotic states. The uppermost layer is a deep atmosphere of molecular hydrogen and helium gas, which transitions seamlessly into a liquid state with no distinct surface boundary. As depth and pressure increase, the molecular hydrogen becomes a dense, fluid layer under immense compression.
Deeper within Jupiter and Saturn, the pressure exceeds a threshold that forces the hydrogen atoms to behave differently. At this point, the hydrogen atoms are squeezed so tightly that their electrons are freed, allowing them to move freely and conduct electricity. This phase change creates a vast ocean of liquid metallic hydrogen, a substance not naturally found on Earth.
This electrically conductive layer is the source of the Gas Giants’ powerful magnetic fields, generated by the convective flow of the metallic fluid. At the center of both planets, astronomers expect a dense core composed of rock and ice. Data from the Juno mission suggests Jupiter’s core is not a compact body but rather a “fuzzy” or partially dissolved mixture of heavy elements that extends far into the metallic hydrogen envelope.
The Role of Ices and Heavier Compounds
While hydrogen and helium dominate the Gas Giants, the composition of Uranus and Neptune is distinct enough that they are classified separately as Ice Giants. These two outer worlds contain a much higher proportion of elements heavier than hydrogen and helium, which planetary scientists refer to as “ices.” These “ices” are volatile compounds such as water (\(\text{H}_2\text{O}\)), methane (\(\text{CH}_4\)), and ammonia (\(\text{NH}_3\)).
Uranus and Neptune have a smaller hydrogen and helium envelope compared to their larger siblings, with these light gases accounting for only about 20% of their total mass. The bulk of the Ice Giants’ material is this dense, hot, and highly compressed icy mixture. This composition suggests they were less efficient at attracting the massive amounts of nebular gas that Jupiter and Saturn captured during formation.
The internal structure of Uranus and Neptune consists of a small, dense rocky core surrounded by a thick mantle of these heavy compounds. The extreme pressure and temperature in this mantle mean the “ice” is not frozen solid but exists as a hot, dense fluid or a supercritical state, sometimes referred to as an “icy slush.” The presence of methane in this layer gives the Ice Giants their distinct blue-green color in their upper atmospheres.