In the Solar System Neptune is the eight and farthest planet from the Sun. The average distance between Neptune and the Sun is 4.50 billion km (2.14 billion miles). It completes an orbit on average every 165 years. The atmosphere of Neptune at high altitudes is 80% hydrogen and 19% helium. It extends between 10 and 20% of the whole radius to reach the core with pressure of about 10 GPa, that is 100,000 times greater than that at see level on Earth. Increasing concentrations of methane, ammonia and water are found in the lower regions of the atmosphere. The mantle is a hot, dense fluid with high electrical conductivity, and is called a water–ammonia ocean. The core of Neptune is composed of iron, nickel and silicates. The pressure at the centre is 700 GPa, about twice as high as that at the centre of Earth, and the temperature may reach 5,130 degree C (9266 degree F).

Neptune has a planetary ring system, which rings may consist of ice particles coated with silicates or carbon-based material, which most likely gives them a reddish hue. The axial tilt of Neptune is 28.32 degrees, which is similar to that of Earth. As a result, this planet experiences similar seasons lasting for fourty Earth years.

Neptune has 14 known moons. Triton is the largest natural satellite of the planet Neptune. It comprises more than 99.5% of all the mass known to orbit Neptune, including the planet's rings and thirteen other known moons. Triton is one of the few moons in the Solar System known to be geologically active. It orbits in a direction opposite to that of the planet. Its mean radius is 1353 km (840 miles). Moons in retrograde orbits cannot form out of the same region of the solar nebula as the planets they orbit, so Triton must have been captured from elsewhere, it might originated in the Kuiper belt. The surface of Triton is covered with frozen nitrogen, water ice comprising and frozen carbon dioxide. Triton has a tenuous nitrogen atmosphere, with trace amounts of carbon monoxide and small amounts of methane near the surface.

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