The Solar System
What are the physical properties of Neptune?
Neptune is the eighth major planet in our solar system, seventeen times more massive than Earth and about four times its diameter. The most remote of the four gas giant planets in our solar system, Neptune takes 165 Earth years to orbit the Sun once. A “day” on Neptune, however, is only sixteen Earth hours. Similar to Uranus, Neptune’s cloud-top temperature is a frosty –350 degrees Fahrenheit (–210 degrees Celsius).
Neptune is bluish-green in color, which might seem fitting for a planet named after the Roman god of the sea. However, the color does not come from water; it is caused by the gases in Neptune’s atmosphere reflecting sunlight back into space. Neptune’s atmosphere consists mostly of hydrogen, helium, and methane. Below the atmosphere, scientists think there is a thick layer of ionized water, ammonia, and methane ice, and deeper yet is a rocky core many times the mass of Earth.
Neptune is so distant that very little was known about it until 1989, when the Voyager 2 spacecraft flew by Neptune and obtained spectacular data about this mysterious gas giant. Today, we know of at least five rings—one of which is composed of numerous partial arcs—and thirteen moons that orbit Neptune.