Description: Neptune is the eighth, and farthest, major planet from the Sun. Like its neighbor Uranus, Neptune is an ice giant. Neptune's radius is just under 25 thousand kilometers (km), a little less than four times Earth's radius. Neptune has a solid rock core surrounded by a layer of high-pressure water, methane, and ammonia. In the early outer Solar System these chemicals were frozen and accreted onto the young Neptune, hence the name "ice giant". Neptune's outer atmosphere is a thick, puffy layer of hydrogen and helium.

Its typical distance from the Sun is about 4.5 billion km, around 30 astronomical units (Earth–Sun distances). Neptune has at least 14 moons and a faint ring system. As the outermost major planet, Neptune's gravity plays a pivotal role in shaping the orbits of smaller bodies in the Kuiper Belt.

Neptune is not visible to the naked eye. It was first identified due to the effect its gravity has on the orbit of Uranus. The mathematicians John Couch Adams and Urbain Le Verrier both predicted Neptune's existence and location in the early 1840s. Based on Le Verrier's calculations, Johann Gottfried Galle first identified Neptune in 1846. Neptune is named after the Roman god of the sea.

Related Terms:

• Astronomical Unit
• Giant Planet
• Ice Giant
• Kuiper Belt
• Outer Planets
• Solar System
• Uranus

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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