Neptune, fourth-largest of the planets and eighth major planet in order of increasing distance from the Sun. The mean distance of Neptune from the Sun is 4.5 billion km (2.796 billion mi), and its mean linear diameter is approximately 49,400 km (30,700 mi), or about 3.8 times that of the Earth. Its volume is about 72 times, its mass 17 times, and its mean density 0.31 that of the Earth, or 1.7 times that of water. The albedo of the planet is high; 84 per cent of the light falling on it is reflected. The period of rotation is about 16 hours, and the period of revolution about the Sun is 164.79 years. The average stellar magnitude of the planet is 7.8, and it is therefore never visible to the naked eye, but it can be observed in a small telescope as a small, round, greenish-blue disc without definite surface markings.

      The temperature of the surface of Neptune is about -218° C (-360° F), much like Uranus, which is more than 1.5 billion km (1 billion mi) closer to the Sun. Scientists assume, therefore, that Neptune must have some internal heat source. The atmosphere consists mostly of hydrogen and helium, but the presence of up to three per cent methane gives the planet its striking blue colour.