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14.14: Neptune

Difficulty Level: At Grade Created by: CK-12
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How do you think Neptune got its name?

Because of its blue color, Neptune was named for the Roman god of the sea. This statue of Neptune is at the Trevi Fountain in Rome.


Neptune, shown in Figure below, is the only major planet that can’t be seen from Earth without a telescope. Scientists predicted the existence of Neptune before it was discovered because Uranus did not always appear exactly where it should appear. They knew that the gravitational pull of another planet beyond Uranus must be affecting Uranus’ orbit.

Neptune was discovered in 1846, in the position that had been predicted, and it was named Neptune for the Roman god of the sea because of its bluish color.

Picture of Neptune

This image of Neptune was taken by Voyager 2 in 1989. The Great Dark Spot seen on the left center in the picture has since disappeared, but a similar dark spot has appeared on another part of the planet.

In many respects, Neptune is similar to Uranus (Figure below). Neptune has slightly more mass than Uranus, but it is slightly smaller in size. Neptune is much farther from the Sun, at nearly 4.5 billion km (2.8 billion mi). The planet’s slow orbit means that it takes 165 Earth years to go once around the Sun.

Diagram of Neptune's composition

Neptune’s composition is that of a gas giant: (1) upper atmosphere, (2) atmosphere composed of hydrogen, helium and methane gas, (3) mantle of water, ammonia and methane ice, (4) core of rock and ice.

Extremes of Cold and Wind

Neptune’s blue color is mostly because of frozen methane (CH4). When Voyager 2 visited Neptune in 1986, there was a large dark-blue spot, which scientists named the Great Dark Spot, south of the equator. When the Hubble Space Telescope took pictures of Neptune in 1994, the Great Dark Spot had disappeared, but another dark spot had appeared north of the equator. Astronomers think that both of these spots represent gaps in the methane clouds on Neptune.

The changing appearance of Neptune is caused by its turbulent atmosphere. The winds on Neptune are stronger than on any other planet in the solar system, reaching speeds of 1,100 km/h (700 mi/h), close to the speed of sound. This extreme weather surprised astronomers, since the planet receives little energy from the Sun to power weather systems. Neptune's core is 7000°C (12,632°C) which means that it produces more energy than it receives from the Sun. Neptune is also one of the coldest places in the solar system. Temperatures at the top of the clouds are about -218°C (-360°F).

Neptune’s Rings and Moons

Neptune has faint rings of ice and dust that may change or disappear in fairly short time frames.

Neptune has 13 known moons. Triton, shown in Figure below, is the only one of them that has enough mass to be spherical in shape. Triton orbits in the direction opposite to the orbit of Neptune. Scientists think Triton did not form around Neptune, but instead was captured by Neptune’s gravity as it passed by.

Picture of Triton, Neptune's largest moon

This image of Triton, Neptune’s largest moon, was taken by Voyager 2 in 1989.


  • Neptune is so far from Earth that it can't be seen without a telescope.
  • Neptune has a turbulent atmosphere, which changes the planet's appearance. The blue color is due to frozen methane.
  • Neptune has 13 moons, including Triton, which orbits in the opposite direction from Neptune.


  1. Why did scientists think that there was a planet beyond Uranus before it had been identified as a planet in a telescope?
  2. Describe the moons of Neptune and where they originated.
  3. What causes the dark spots in Neptune's appearance? Why do they come and go?

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Use this resource to answer the questions that follow. 

  1. How was Neptune found?
  2. What is the structure of Neptune?
  3. What is the atmosphere composed of? Why is Neptune blue?
  4. What is the atmosphere of Neptune like?
  5. What causes the intense winds on Neptune? Why aren't the winds caused by the Sun?
  6. How long is a season on Neptune?
  7. What is Triton? What is unusual about it?
  8. What is the probably origin of Neptune's rings?
  9. Why do we know so little about Neptune?

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    Date Created:
    Feb 24, 2012
    Last Modified:
    Aug 10, 2016
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