Planets primarily composed of atmosphere are defined by the fact that the majority of their mass and volume is not concentrated in a solid, rocky, or metallic body. Instead, their structure is dominated by deep layers of light elements, primarily hydrogen and helium. These elements gradually transition from gas to exotic forms of liquid under immense pressure. This composition makes them fundamentally different from Earth and the other inner, rocky planets.
The Massive Gaseous Worlds
The four largest planets in our solar system—Jupiter, Saturn, Uranus, and Neptune—are mostly composed of atmosphere. All reside in the outer solar system. Jupiter is the most massive, with a diameter nearly 11 times that of Earth, while Saturn is about nine times as wide. Uranus and Neptune are smaller, with diameters approximately four times that of Earth.
These giant worlds formed from the light gases abundant in the outer regions of the early solar nebula. Their bulk composition is overwhelmingly dominated by hydrogen and helium, the two lightest and most abundant elements. This prevalence of light elements gives them a low average density compared to the inner, rock-dominated planets. Their massive scale allows them to retain these light gases, which would have easily escaped the gravitational pull of smaller bodies.
Internal Structure and Composition
The term “mostly made of atmosphere” is a simplification, because the extreme pressure inside these worlds converts the gas into states far removed from what we experience on Earth. As one descends into a giant planet, the atmosphere does not end abruptly at a surface, but instead becomes progressively denser and hotter. The gaseous hydrogen gradually compresses into a super-dense liquid molecular hydrogen layer.
In the case of Jupiter and Saturn, the pressure becomes so enormous that it strips the electrons from the hydrogen atoms. This creates a vast, deep layer of liquid metallic hydrogen, which acts as a conductor of electricity, much like a metal. This layer makes up the bulk of these enormous planets and is the source of their powerful magnetic fields.
The lack of a clear, solid surface is a defining characteristic. The visible atmosphere simply fades into the deeper, high-pressure liquid layers. Scientists believe that deep within these layers, the planets contain a core of heavier elements, such as rock and ice. This core is small relative to the planet’s total mass and accounts for only a small percentage of the total planetary mass.
Gas Giants Versus Ice Giants
Within the family of gas-dominated planets, there is an important distinction based on composition that separates the Gas Giants from the Ice Giants. Jupiter and Saturn are classified as Gas Giants, with their mass consisting of 90% or more hydrogen and helium. This composition closely mirrors the primordial material of the early solar system. Their massive size and high internal pressure allow them to maintain the deep, metallic hydrogen layers.
Uranus and Neptune are classified as Ice Giants because their bulk composition contains a much higher proportion of heavier elements. These volatile compounds—including water, methane, and ammonia—are what astronomers collectively refer to as “ices.” These ices form a dense, fluid mantle that is proportionally much larger than the metallic hydrogen layer found in the Gas Giants. In the Ice Giants, hydrogen and helium account for less than 20% of the total mass, making the “ice” material the dominant component beneath the atmosphere.
This compositional difference suggests that the Ice Giants formed under different conditions compared to the Gas Giants. Their smaller overall mass means they do not generate the extreme internal pressures necessary to create a substantial layer of liquid metallic hydrogen. The Ice Giants are composed of a rock and ice core surrounded by a vast, hot, dense ocean of water, ammonia, and methane, capped by a hydrogen-rich atmosphere.
Why Terrestrial Planets Differ
Planets like Earth, Venus, and Mars are categorized as terrestrial planets. While these planets possess atmospheres, the gaseous envelope makes up a minuscule fraction of their overall structure. For example, Earth’s atmosphere accounts for only about one part per million (ppm) of the planet’s total mass.
Even Venus, which has an extremely dense atmosphere resulting in crushing surface pressure, is still structurally dominated by its rocky interior. The atmosphere of Venus, though massive compared to Earth’s, only accounts for an estimated 98 ppm of its total mass. This demonstrates that the vast majority of a terrestrial planet’s mass is concentrated in its solid mantle and core, which is the key structural difference separating them from the gas-dominated giants.