Photo courtesy of National Air and Space Museum

by Michael McCarthy and Chris MacKay

Neptune is the eighth planet from the Sun; the only planet farther from the Sun is Pluto. Every 248 years, Pluto travels inside Neptune's orbit for about a 20 year period, during which it is nearer the Sun than Neptune. Pluto entered Neptune's orbit in January, 1979, and stayed there until March, 1999. The orbit of Neptune, called the blue planet, is 4,504,000,000 kilometers from the the Sun, about 30 times as far from the Sun as the Earth. Neptune's rotation period as it spins on its axis, in earth hours, is 16.11 hours, compared to our 24 hour rotation periods. Neptune's axis is tilted 30 degrees to the Sun's orbit. Neptune moves around the Sun in an elliptical or oval-shaped orbit; its orbit period in earth years is 163.72 years. One season on Neptune lasts for forty years; its south pole is in darkness for forty years, while the north pole is in sunlight for forty years. Being the smallest gas planet, Neptune's diameter of 49,500 km is smaller than Jupiter's and Uranus', but is larger than the Earth's (almost 4 times the Earth's).

Neptune's surface is almost identical to Uranus'; a combination of ice and glaciers, rocks and craters make up Neptune's surface with about 15% hydrogen and little helium. Neptune also has great geysers of liquid nitrogen. Neptune has a very solid ground, not like Saturn and Jupiter; these two planets both consist of gases. Neptune's core or center is a region of heavily compressed gases that blend into a central rocky interior. Neptune has a mantle of liquid hydrogen while the atmosphere is a mixture of ammonia, helium, and methane. Neptune is blue because methane in the atmosphere absorbs red wavelengths of light and reflects the blue. The methane freezes in the upper atmosphere and forms an ice cloud which casts a shadow on the clouds below. Neptune is covered by a thick layer of clouds with very strong winds and large hurricane-like storms. Neptune's winds are the fastest in the universe; they can reach speeds up to 2000 km an hour.

Neptune's prominent feature is the Great Dark Spot, a large dark oval on its surface which astronomers belive are hurricane storms; it is as big as the Earth. Neptune also has 4 very dark rings, 2 thin and 2 thick, which are made of dark particles that are different in size. Neptune was discovered when astronomers were looking for Uranus; the way they found it was that the gravitational pull of another object, Neptune, was interfering with Uranus' orbit as predicted by astronomers.

Neptune has a very strong magnetic field just like Uranus'; it is probably generated by ground water. Neptune has 8 different moons, or natural satellites, 4 of which orbit within the rings; the largest of these moons is Triton which has a surface temperature of about -235 degrees Celsius. It also has a thin atmosphere of methane and nitrogen.

Bibliography and Additional Sources of Information

Discover the Universe.

Go to the Top | The Solar System and Outer Space | Cyber Themes | SKC Homepage