Uranus and Neptune are the two outermost planets in the solar system, often grouped together as the “Ice Giants.” They share similar atmospheric compositions, consisting primarily of hydrogen, helium, and methane, and contain a large proportion of heavier elements referred to as “ices” and rock in their deep interiors. Despite these fundamental commonalities in their bulk chemical makeup, the two planets exhibit subtle but significant differences in their physical dimensions that often cause confusion about their relative sizes.
The Direct Answer: Volume and Radius
Neptune is indeed physically smaller than Uranus when measuring their spatial dimensions. Uranus holds the title of the third-largest planet by diameter in the solar system, while Neptune is the fourth largest. The difference in their equatorial radii, while slight, confirms Uranus’s greater size. Uranus has an equatorial radius of approximately 25,559 kilometers, making it the more voluminous of the two Ice Giants. Neptune’s equatorial radius is slightly smaller, measuring about 24,764 kilometers. To put this in perspective, Uranus is roughly four times the diameter of Earth, while Neptune is just under four times. The volume of Uranus is therefore greater than Neptune’s, even though they appear nearly identical in visual representations.
Mass and Density Contrast
While Uranus is the larger planet by volume, Neptune is the more massive planet, which is the key counter-intuitive fact about the pair. Neptune possesses a mass of about 17.1 Earth masses, making it the third-most-massive planet in the solar system. Uranus, by comparison, is less massive, weighing in at approximately 14.5 Earth masses. This disparity in mass combined with the difference in size is directly reflected in their overall density. Because Neptune packs more mass into a smaller volume, it is significantly denser than Uranus. Neptune’s mean density is estimated to be around 1.64 grams per cubic centimeter, making it the densest of all the giant planets. Uranus, with its larger size and lower mass, has a mean density of only about 1.27 grams per cubic centimeter.
Distinct Internal Structures
The reason Neptune is both smaller and more massive than Uranus lies in the distinct thermal and structural conditions within their interiors. Both planets are theorized to have a rocky core surrounded by an icy mantle composed of water, ammonia, and methane under immense pressure. However, the energy balance of the two planets is drastically different. Neptune radiates a substantial amount of energy, emitting about 2.6 times more energy than it absorbs from the sun, which indicates a robust internal heat source. This excess heat drives vigorous convection and greater compression within the planet’s layers. The resulting high internal pressure squeezes Neptune’s material into a smaller, denser configuration. Uranus, in contrast, has a very low thermal flux, radiating barely more heat than it receives from the Sun. This lack of significant internal heat means the material inside Uranus experiences less compression compared to Neptune. Consequently, Uranus’s internal material is less compacted, allowing the planet to puff out to a slightly larger size despite having less overall mass.