Orbit and Rotation of Neptune. Neptune has an equatorial diameter of about 30,700 mi (49,400 km), nearly four times that of the earth, and a mass about 17 times the earth's mass. The average distance between the Sun and Neptune is 4.55 billion km, and Neptune completes its orbit every 164.79 years; a year on Neptune is 164.79 Earth years. Neptune’s diameter is about four times greater than Earth’s. A year on Neptune lasts 165 Earth years. Neptune is what's called an ice giant, basically a gigantic ball of hydrogen and helium gas with loads of methane, ammonia, and other molecules in it (that for historical reasons, planetary scientists refer to as "ices" even if they're gaseous). It is not visible to the naked eye and was first observed in 1846. Neptune was not known to the ancients. Diameter and Mass: Neptune is the fourth largest planet in terms of diameter and third largest in terms of mass. The core of Neptune is an extremely hot and pressurized rocky material near the mass of the Earth.The surface of Neptune is an ocean of hydrogen, helium, and water. The enormous weather pattern was about the size of Earth. Neptune is the most distant planet from the Sun. No solid surface: because Neptune is a gas giant, there is no solid surface on Neptune for a spacecraft to land on the planet. Neptune has 6 faint rings. It is much like Uranus, the solar system's other ice giant, with a thick atmosphere of hydrogen, helium, methane, and ammonia, a relatively low density, and a rapid period of rotation. Neptune is about 2.8 billion miles (4.5 billion kilometers) from the sun. Neptune is spinning, and that makes the equator bulge a little bit, so the diameter at the equator is larger than its diameter at the poles. Neptune is very far away. Although Neptune is large, it cannot be seen from Earth unless a telescope is used. Its volume is 57.7 times the volume of Earth which means that 57 Earths could fit inside of Neptune with a little room left over. Despite this fact, it never appears in the same position in our sky because the Earth would have rotated in a different location during its 365.25 day orbit. Neptune is the smallest gas giant. Neptune is the fourth largest planet in our Solar System and it is the smallest of the giant gas planets. 14. 15. Watch the video to know its diameter. Neptune is named after the Roman god of the sea. Neptune Observational Parameters Discoverer: Johann Gottfried Galle (based on predictions by John Couch Adams and Urbain Leverrier) Discovery Date: 23 September 1846 Distance from Earth Minimum (10 6 km) 4305.9 Maximum (10 6 km) 4687.3 Apparent diameter from Earth Maximum (seconds of arc) 2.4 Minimum (seconds of arc) 2.2 Mean values at opposition from Earth Distance from Earth (10 6 km) … In our solar system, planet Neptune is located next to planet Uranus. And so it appears small and dim. Because the pressure is so high at the center of Neptune, helium and hydrogen are compressed to a liquid, and the 5000 degree water cannot evaporate. However, while Jupiter's famous feature has lasted for hundreds of years, Neptune's first … Neptune has a diameter of 34,503 miles (55,528) kilometers.