MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Image comparing the size of Earth (left) with the planet Neptune. Neptune is the farthest planet from the Sun, with an average distance from it of 29.8 times the Earth-Sun distance. Being a fluid world of mostly hydrogen and helium, rich in ices of methane, water and ammonia, astronomers label it (along with Uranus) an ice giant. With a diameter of 3.9 times that of the Earth, Neptune is the Solar System's fourth largest planet.
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