The question of the smallest gas planet depends entirely on whether the search is limited to our Solar System or expanded to the thousands of exoplanets discovered orbiting distant stars. The local answer differs significantly from the true cosmic minimum. To find the smallest example, it is necessary to first understand how a planet earns the “gas” classification, then look beyond the four familiar giants to the distant, smaller worlds.
Neptune: The Smallest Giant Planet in Our Solar System
Within the eight planets of our Solar System, Neptune holds the title for the smallest of the four “giant planets.” It has a diameter approximately four times that of Earth, making it the most compact of the large, non-rocky worlds. The four outer planets are often distinguished between the Gas Giants (Jupiter and Saturn) and the Ice Giants (Uranus and Neptune).
Jupiter and Saturn are predominantly composed of hydrogen and helium. Uranus and Neptune contain a much higher concentration of heavier volatile compounds, such as water, methane, and ammonia, referred to as “ices.” These dense volatile layers mean that Uranus and Neptune are categorized separately as Ice Giants.
Defining the Boundary: What Classifies a Planet as “Gas”?
A planet is classified as a gas world primarily by its low overall density and the lack of a defined, solid surface. Instead, atmospheric pressure and temperature gradually increase with depth until the gaseous material is compressed into a supercritical fluid state. In this state, the distinction between a gas and a liquid disappears entirely.
The composition is also a determining factor, as a gas planet must have accumulated a massive envelope of light elements, most commonly hydrogen and helium. This envelope must be thick enough to dominate the planet’s volume and mass, significantly lowering its mean density compared to a rocky world. For a true gas giant, this hydrogen layer is so extensive that the pressure can convert the gas into liquid metallic hydrogen deep within the interior.
The Smallest Known Gas Planets: Mini-Neptunes and Exoplanets
The smallest examples of gas planets are found outside our Solar System among the exoplanets, where they are commonly referred to as “Mini-Neptunes” or “gas dwarfs.” These worlds are generally larger than Earth but smaller than Neptune. Mini-Neptunes typically have radii between 1.6 and 3.9 times that of Earth, and they are the most frequently discovered type of planet in the galaxy.
These planets represent the smallest size where a world can still hold onto a thick, hydrogen-rich atmosphere. Below a radius of about 1.6 Earth radii, planets are overwhelmingly rocky Super-Earths that have failed to retain a substantial gaseous envelope. Worlds in the Mini-Neptune class possess a small, dense core of rock and ice surrounded by a massive, deep atmosphere of hydrogen, helium, or water vapor.
One of the smallest exoplanets suspected to possess a significant gas envelope is Kepler-138d, which has a radius about 60 percent larger than Earth. Despite its small size, its low measured density suggests it cannot be purely rocky and must have a thick outer layer of volatiles. Determining the exact minimum size remains challenging, as the line between a large rocky world with a steam atmosphere and a true gas dwarf remains fuzzy, often depending on the planet’s mass, temperature, and distance from its star.