How Big Is Neptune?

QUICK ANSWER

Neptune has a diameter of about 30,600 miles (49,200 km), roughly 4 times Earth's diameter. It's the fourth-largest planet in our solar system, slightly smaller than Uranus but more massive due to higher density. About 57 Earths could fit inside Neptune by volume, and Neptune's mass is 17 times Earth's.

Neptune is the smallest of the four giant planets, but it's still vastly bigger than Earth. It's slightly smaller than Uranus in diameter but more massive, making Neptune the denser of the two ice giants. Despite the size, Neptune is hard to see from Earth without a telescope because it's so far away.

How big is Neptune compared to Earth?

About four times wider. According to NASA, Neptune has an equatorial diameter of about 30,600 miles (49,200 km), compared to Earth's 7,918 miles. By volume, you could fit roughly 57 Earths inside Neptune. Neptune's mass is about 17 times Earth's, the largest mass-to-volume ratio of any planet beyond the rocky inner four. Neptune's surface gravity is about 14 percent stronger than Earth's, despite being so much bigger, because of how the mass and radius work out together.

How does Neptune compare to Uranus?

Smaller but heavier. Neptune is the fourth-largest planet by diameter (just slightly smaller than Uranus), but more massive than Uranus despite being the smaller of the two. The reason is density: Neptune has an average density of about 1.64 g/cm³, while Uranus is about 1.27 g/cm³. The difference comes from Neptune having a slightly more compact interior with more rock and less hydrogen-helium envelope. The two planets look similar from the outside but have meaningfully different internal structures.

Where does Neptune rank among the planets?

Fourth by size. The order from largest to smallest is Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury. Neptune is dramatically smaller than Jupiter (which is about 4.5 times Neptune's diameter) and noticeably smaller than Saturn. Neptune and Uranus are sometimes lumped together as the ice giants, distinct from the gas giants Jupiter and Saturn. Among the four giants, Neptune is the smallest but contains the most mass relative to its size.

Why is Neptune denser than Uranus?

Probably because of internal structure differences. Both planets formed from similar materials in similar regions of the solar system, but Neptune's interior appears to be slightly more compact, with proportionally more rock and ice and less hydrogen-helium envelope. Some models suggest Neptune's interior contains more water-ice and less puffy outer layers than Uranus's. The exact reason isn't fully understood, partly because both planets have only been visited once by spacecraft, and Voyager 2's data from 1989 doesn't have the detail needed to fully resolve the question.

Neptune has a diameter about four times Earth's and a mass 17 times greater, making it the fourth-largest planet in our solar system. It's slightly smaller than Uranus but more massive due to higher density. Among the four giant planets, Neptune is the runt by size but punches above its weight in terms of mass and density.

How Big Is Neptune?