Planets that are mostly comprised of atmosphere are fundamentally different from Earth, which is a solid, rocky body. This description refers to worlds whose mass and volume are overwhelmingly dominated by fluid envelopes rather than solid or liquid surfaces. For these worlds, the term “atmosphere” transitions from a thin gaseous layer to a deep, dense fluid layer under immense pressure, making the entire planet essentially a giant ball of compressed fluid. The sheer scale and composition of these planets place them in a distinct category from the small, dense worlds of the inner solar system. They represent the largest planets in our solar system, with interiors that lack a distinct boundary between gas and liquid.
Defining Planetary Composition Types
Planetary scientists categorize the worlds of our solar system into two main groups: Terrestrial and Jovian. Terrestrial planets are the rocky worlds, like Earth and Mars, characterized by a distinct solid surface and a composition dominated by silicates and metals. In contrast, Jovian planets are the giant worlds of the outer solar system whose composition is gas- or fluid-dominated. These Jovian planets are so massive that the immense pressure generated by their own gravity compresses their gaseous components into exotic liquid or supercritical states. This process means that the atmosphere constitutes the vast majority of the planet’s total volume and mass, and the lack of a distinct, accessible solid surface is a defining feature.
The Gas Giants: Jupiter and Saturn
Jupiter and Saturn are the two primary examples of planets overwhelmingly made of atmosphere, officially classified as gas giants. Their composition closely mirrors the Sun’s, consisting of approximately 75% hydrogen and 25% helium by mass. Beneath the visible cloud tops, the gaseous hydrogen transitions seamlessly into a liquid state without a phase boundary, becoming an extremely dense fluid. Deeper within Jupiter and Saturn, the pressure exceeds three million times Earth’s atmospheric pressure, forcing the hydrogen atoms to shed their electrons. This intense state transforms the fluid hydrogen into a bizarre substance known as liquid metallic hydrogen, which conducts electricity and makes up the bulk of the planet’s interior, surrounding a comparatively small, dense core of rock and ice.
The Ice Giants: Uranus and Neptune
Uranus and Neptune represent a distinct category of fluid-dominated worlds, known as ice giants. While they also lack a solid surface, their internal composition differs significantly from the gas giants. These planets contain a lower proportion of hydrogen and helium, making up only about 20% of their mass, compared to the 90% found in Jupiter and Saturn. Instead, their bulk composition is rich in heavier volatile compounds referred to as “ices” in planetary science, such as water, methane, and ammonia. The extreme internal pressures and temperatures mean that these “ices” exist as a dense, hot, supercritical fluid, giving them a much higher concentration of heavy elements than the gas giants.
Compositional Differences in Terrestrial Planets
The inner solar system worlds, including Earth, Mars, Venus, and Mercury, do not fit the criteria of being “made of atmosphere.” These terrestrial planets are defined by a composition dominated by solid rock and metal, structured into a distinct crust, mantle, and core. For Earth, the atmosphere is a thin, gaseous film, contributing only a tiny fraction of the planet’s total mass. Even on Venus, which has the densest atmosphere of the rocky worlds, the gaseous envelope is minuscule in comparison to the planet’s massive solid body. The existence of a distinct, solid surface and internal layers of silicates and iron completely differentiate them from the fluid-dominated Jovian worlds.