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Key Facts & Information

  • Neptune is the fourth largest and the furthest planet of the Solar System, with the most powerful wind speeds out of all the planets. It is the smallest of the gas giants and is the first planet to be discovered by mathematical predictions in 1846. It is the only planet that is invisible to the naked eye, thus it eluded ancient astronomers until it was officially discovered in 1846 by Le Verrier and Johann Galle.

Quick Figures

  • Surface Temperature: -201°C
  • Orbit Period: 60,190.03 Earth days
  • Known Rings: 5
  • Known Moons: 14
  • Equatorial Circumference: 155,600 km
  • Equatorial Diameter: 49,528 km
  • Neptune was given the name of the Roman god of the sea due to its bluish, ocean-like color. Its satellites also received names of water deities.
  • Neptune’s color is believed to be influenced by the presence of methane in its atmosphere and also an unknown factor.
  • Neptune is the smallest ice giant with a mass of 1.024 × 10^26 kg.


  • Neptune was observed in 1613 by Galileo Galilei, however he thought that it was just a star. When he wanted to further research it, the motion of the planet began to be far too slight for it to be detected again.
  • Johann Galle found the planet one degree away from the predicted point thus making Neptune the first planet to be discovered by mathematical calculations and predictions.
  • Urbain La Verrier proposed the name Neptune. Soon after, it became widely accepted. Neptune was the Roman god of the sea, identified with the Greek Poseidon as the planet appeared water-like in color.

Distance, Size & Mass

  • From its discovery until 1930 when Pluto was discovered, Neptune was considered the furthest planet. After the discovery of Pluto, Neptune was thought to be the second furthest planet. When Pluto’s eccentric orbit was understood and its status dropped from that of a planet to a dwarf planet in 2006, Neptune regained the title of the furthest planet in the Solar System.
  • Neptune is an average distance of 4.5 billion kilometers or 30.1 AU away from the Sun, and is currently 29.4 AU away from Earth with its light taking up to 4 hours before we can see it.
  • The mass of Neptune is about 17 times that of Earth or 1.0243×1026 kg, but when compared to the largest gas giant Jupiter, it has only 1/19th of Jupiter’s mass. It has an equatorial radius of 15,387 miles or 24,764 kilometers, about four times wider than Earth, and a diameter of 49,244 km or 30,598 mi, making it the fourth largest planet of the Solar System.

Orbit & Rotation

  • Since it is the furthest planet from the Sun, it has the longest orbital duration, completing a trip around the Sun in about 165 years. However, one sidereal rotation or day on Neptune is completed in 16.11 hours.
  • The elliptical orbit of Neptune is inclined 1.77° compared to that of Earth.
  • The axial tilt of Neptune is 28.32°. It is similar to the tilts of Earth (23°) and Mars (25°). As a result, Neptune experiences similar seasonal changes to Earth but due to its long orbital period, the seasons last for 40 Earth years.
  • Neptune’s orbit has a profound impact on the region directly beyond it, known as the Kuiper belt. The Kuiper belt is a ring of small icy worlds, similar to the asteroid belt but far larger, extending from Neptune’s orbit at 30 AU out to about 55 AU from the Sun. Many dwarf planets are located here.


  • Like the rest of the gas giant planets in the Solar System, Neptune can be broken up into various layers. The outermost layer of Neptune is the atmosphere, forming about 5-10% of the planet’s mass, and extending up to 20% of the way down to its core.
  • Beneath the atmosphere is the planet’s large mantle. This is a superheated liquid region where temperatures can reach as high as 1727–4727 °C. The mantle is equivalent to 10–15 Earth masses and is rich in water, ammonia, and methane. This mixture is referred to as icy even though it is a hot, dense fluid, and is sometimes called a “water-ammonia ocean”.
  • The core is estimated to be about 1.5 times the mass of Earth. The pressure at the center is 7 Mbar or 700 Gpa, twice as high as that at the center of Earth with temperatures of around 5,400 K.

Atmosphere & Climate

  • The atmosphere is made up mostly of hydrogen, helium, and methane. Very similar to Uranus, its vivid blue color is influenced by the presence of methane, and some unknown factor causes the more intense color.
  • Neptune may not be as cold as Uranus but it has the most powerful winds out of all the planets in the Solar System despite being the furthest planet from the Sun and receiving the lowest energy input from it. Having the wildest and strangest weather in the entire Solar System, it is not understood how it gets this much energy in order to produce such weather.
  • Neptune has an average temperature of -214 degrees Celsius; -353 degrees Fahrenheit. Wind speeds blowing westward on the equator reach up to 2,160 kilometers or 1,324 miles per hour, nearly a supersonic flow. Bands and colossal storms also form on the planet.
  • Neptune’s magnetic field is offset 47° relative to its rotational axis.

Planetary Rings

  • A common feature of the gas giants are their ring systems, which Neptune also has. It is very faint due to its low denseness and extremely dark color, a reddish hue. Neptune has 5 ring systems named after the people involved in the discovery and research of Neptune.
  • The innermost ring is the Galle Ring, which is faint and wide at 2,000 km or 1,242 miles.
  • The second is the first bright ring, named Le Verrier. It is only 113 km wide or 70.2 mi.
  • The third is the Lassell Ring, a very faint band 4,000 km or 2,485 mi across.
  • At the edge of the Lassell Ring is the Arago Ring, slightly brighter and less than 100 km or 62 mi wide.
  • The last known and outermost ring is named the Adams Ring. This ring is about 35 km or 21.7 mi wide but it is one of the brightest rings. It has a special feature.


  • A total of 14 known moons surround Neptune. The first moon discovered was Triton, the largest of the 14 moons. The 14 moons of Neptune can be divided into regular, irregular, and unusual irregular moons.
    • Out of the 14 moons, only 7 are inner regular moons, meaning they orbit along Neptune’s ecliptic with very circular orbits or orbits with very low eccentricity.
    • In order of their distance from Neptune: Naiad, Thalassa, Despina, Galatea Larissa, Hippocamp, and Proteus.
    • Irregular moons follow an inclined, eccentric, and often retrograde orbit.
    • In order of their distance: Triton, Nereid, Halimede, Sao, Laomedeia, Psamanthe, and Neso.
    • The smallest of the irregular moons is Psamanthe, while the biggest is Triton.

Did You Know?

  • Uranus is Neptune’s near twin in size and composition.
  • In 2011, Neptune completed its first 165-year orbit since its discovery in 1846, meaning we have witnessed just 1 Neptunian year.
  • Neptune is 30 times further than Earth’s orbit from the Sun.
  • A total of 58 Earths can fit inside Neptune.
  • The Voyager 2 is the only spacecraft to have reached Neptune to this day.
  • Neptune has a very strong magnetic field. It is around 27 times stronger than the one on Earth.
  • Because of the dwarf planet Pluto’s elliptical orbit, Pluto is sometimes closer to the Sun and Earth than Neptune is.