How big is Neptune?

Neptune is the eighth and farthest planet from the Sun, and it is 49,244 kilometers (30,599 miles) across on average. That figure is its mean diameter, the width of a perfect sphere with the same volume as Neptune. Neptune is an ice giant: most of its mass is a hot, dense fluid of water, methane and ammonia around a small rocky core.

Size

Neptune has no solid surface, so its size is measured to a level in its atmosphere. Astronomers use the height where the pressure is 1 bar, about the same as the air pressure at sea level on Earth.

Measured that way, Neptune is 49,528 kilometers (30,775 miles) wide at its equator and 48,682 kilometers (30,250 miles) from pole to pole, so it is slightly flattened. Its size is known less precisely than that of any other planet: the mean diameter is uncertain by about 40 kilometers. Only one spacecraft, NASA's Voyager 2, has visited Neptune, flying past on August 25, 1989.

Size comparisons

Neptune is nearly four times as wide as Earth, and its volume is about 58 times Earth's.
Uranus is 50,724 kilometers (31,518 miles) across on average, about 1,480 kilometers (920 miles) wider than Neptune. Yet Neptune has about 18% more mass.
If Earth were shrunk to the size of a basketball 24 centimeters (9.4 inches) across, Neptune would be a ball about 93 centimeters (3 feet) across. At the same scale, the Sun would be 26 meters (86 feet) wide, and Neptune would circle it about 85 kilometers (53 miles) away.
In 1989, Voyager 2 saw a storm on Neptune, the Great Dark Spot, that was large enough to hold all of Earth. That storm has since disappeared.

Found by mathematics

Neptune was the first planet found by calculation rather than by searching the sky. Uranus was not moving exactly as astronomers expected, and the French mathematician Urbain Joseph Le Verrier worked out where an unknown planet pulling on it should be. Johann Gottfried Galle, at the Berlin Observatory, found Neptune on his first night of searching, on September 23, 1846.

Neptune had been seen before without being recognized. Galileo recorded it as a star in 1612 and 1613.

A cold and windy world

Neptune orbits the Sun at an average distance of about 4.5 billion kilometers, 30 times as far out as Earth, and one orbit takes about 165 Earth years. Sunlight on Earth is roughly 900 times as bright as sunlight on Neptune. At the 1-bar level, the temperature is about −201 °C (−330 °F).

Neptune is the only planet that cannot be seen with the unaided eye. It is also the windiest planet: winds carry clouds of frozen methane at more than 2,000 kilometers per hour (1,200 miles per hour).