The phrase “made of atmosphere” describes a class of planets fundamentally different from rocky worlds. Terrestrial planets like Earth have a distinct, solid surface topped by a relatively thin gaseous layer. In contrast, planets considered “mostly atmosphere” are so immense they lack any defined solid surface, with their gaseous exteriors transitioning seamlessly into pressurized fluid interiors. These bodies are categorized into two types based on composition: Gas Giants and Ice Giants.
Identifying the Gas and Ice Giants
The four outer planets in our Solar System are mostly composed of gas and fluid: the Gas Giants (Jupiter and Saturn) and the Ice Giants (Uranus and Neptune). These planets possess extreme mass and depth, exerting immense gravity that compresses the bulk of their material into fluid states. For these worlds, the atmosphere constitutes the majority of the planet’s total volume.
A “giant planet” refers to a body that continued to accrete and compress a massive envelope of surrounding gas after forming a rocky core. This process resulted in worlds where the gaseous envelope is the primary feature. The lack of a clear boundary between the outer gas and the inner fluid means the entire planet is effectively a giant, deep atmosphere.
The Internal Structure of Atmospheric Planets
The defining characteristic of these planets is the lack of a clear phase boundary where the atmosphere ends and a surface begins. As one descends, pressure and temperature continuously increase due to the overwhelming mass above. This gravitational compression causes the atmospheric gas to transition directly into a dense fluid state without forming a conventional liquid surface.
This transition occurs through a state of matter known as a supercritical fluid, where the gas and liquid phases become indistinguishable. The material is so hot and under high pressure that it possesses properties of both a gas (like diffusing) and a liquid (like dissolving other materials). This supercritical state forms the deep, pressurized envelope that makes up most of the volume of all four giant planets.
The pressure deep within Jupiter and Saturn is so extreme that it forces hydrogen atoms into an exotic form called liquid metallic hydrogen. In this state, electrons are stripped from the hydrogen nuclei, allowing the material to conduct electricity like a metal, similar to liquid iron in Earth’s core. This conductive fluid is believed to drive their powerful magnetic fields.
Uranus and Neptune, being less massive, do not achieve the pressure necessary to create a significant layer of liquid metallic hydrogen. Instead, their interiors consist of a thick, pressurized mantle composed of water, methane, and ammonia, all in a hot, dense fluid state. All four giants are thought to contain a dense, rocky or ice-rock core at their center, though these cores represent only a small fraction of the total mass.
Core Materials: Hydrogen, Helium, and Ices
The two classifications of giant planets reflect a fundamental difference in chemical composition. Jupiter and Saturn (Gas Giants) are overwhelmingly dominated by the lightest elements, hydrogen and helium. These two elements make up over 90% of their total mass, similar to the composition of the Sun.
Heavier elements, often referred to by astronomers as “metals,” account for only three to thirteen percent of the mass of Jupiter and Saturn. The dominance of hydrogen and helium is why these planets are referred to as gas giants, as these elements were the most abundant materials in the early Solar System’s nebula.
Uranus and Neptune (Ice Giants), however, contain a much higher proportion of heavier elements. Their composition is only about 20% hydrogen and helium by mass. The bulk of their mass is composed of compounds referred to as “ices,” including water, ammonia, and methane.
These compounds are called “ices” because they were solids in the cold outer regions of the early Solar System where they formed. In the hot and pressurized interiors of Uranus and Neptune, these icy compounds exist as a dense, hot fluid mantle, making their structure fundamentally different from the hydrogen-dominated Gas Giants.