Uranus and Neptune, the two outermost planets, are known as Ice Giants because their composition includes compounds like water, methane, and ammonia, often called “ices.” Although these distant worlds are often considered twins, comparing their physical dimensions to their total mass presents a puzzle. Uranus is physically larger in diameter, but Neptune is the more massive of the two, a difference stemming from their internal structure and density.
Size and Density The Direct Comparison
Uranus holds the title for the greater physical size, boasting an equatorial diameter of approximately 51,118 kilometers (31,763 miles). Neptune is only slightly smaller, with a diameter of about 49,528 kilometers (30,775 miles). This difference means Uranus has a greater overall volume.
Despite being smaller in volume, Neptune contains more total matter, making it the denser and heavier planet. Neptune’s mass is roughly 17.1 times that of Earth, whereas Uranus weighs in at about 14.5 Earth masses. This greater mass packed into a smaller space results in Neptune having a mean density of approximately 1.64 grams per cubic centimeter, compared to Uranus’s lower density of about 1.27 grams per cubic centimeter.
The difference in density suggests that Neptune has a higher proportion of heavier elements concentrated within its core and mantle. Scientists theorize that Neptune’s interior is more compressed or contains a larger rocky core than Uranus, allowing it to contain more mass in a slightly smaller volume. This higher density sets Neptune apart from its neighbor, despite their many shared characteristics as Ice Giants.
Uranus The Tilted Ice Giant
The seventh planet from the Sun is known for its peculiar rotation. Uranus rotates on its side, with an extreme axial tilt of approximately 98 degrees. This means its axis of rotation is nearly parallel to the plane of its orbit, causing it to roll around the Sun like a ball.
This extreme tilt results in the most dramatic seasonal variations in the solar system. For nearly a quarter of its 84-year orbit, one pole of Uranus faces the Sun, experiencing a continuous period of sunlight that lasts for about 21 Earth years. Conversely, the opposite pole endures an equally long, continuous stretch of darkness.
Uranus lacks a significant internal heat source, which contributes to its tranquil and featureless appearance. While most giant planets radiate more heat than they receive from the Sun, Uranus emits only about 1.06 times the solar energy it absorbs. This small amount of internal energy results in a much less dynamic atmosphere compared to Neptune, giving it a relatively uniform, pale blue-green color.
Neptune The Windy Ice Giant
Neptune possesses an atmosphere that is far more active and dynamic than Uranus. This activity is powered by a powerful internal heat source, which distinguishes the two Ice Giants. Neptune radiates approximately 2.6 to 2.7 times the energy it absorbs from the Sun, fueling its violent weather.
This abundant internal heat drives the fastest sustained winds measured on any planet in the solar system. These winds can reach speeds of up to 2,100 kilometers per hour (1,300 miles per hour) in some areas, creating massive, temporary storm systems. The most famous of these features was the Great Dark Spot, an enormous, Earth-sized storm observed by the Voyager 2 spacecraft in 1989.
The presence of these storm features, which appear and disappear over time, demonstrates a highly energetic atmosphere. Neptune’s color is a deeper, richer blue than Uranus, which is attributed to a slightly higher concentration of methane gas in its upper atmosphere. This methane strongly absorbs red light, leaving only the blue wavelengths to be reflected back into space.