Formation

The formation of Neptune has long been a mystery. If the planet formed in the manner that Jupiter and Saturn did, it would need to be older than the solar system. An alternative scenario considers the fact that bodies as far from the Sun as Neptune likely contained large amounts of ice. Collisions between bodies would have been powerful enough to vaporize the ices. While most would simply condense due to the extremely cold temperatures, some of the vapors would mix with the gaseous nebula, increasing its density. As the density of gasses increases, the ease with which a smaller planet core could trap them increases. As a result, cores as small as Mars could have acquired very dense gaseous envelopes at the distance of Neptune. This result allows for Neptune to have formed within the solar system.

Composition

Neptune is believed to have the same internal composition as Uranus. Neptune’s average density is 1640 kg/m³ which is greater than that of Jupiter, Saturn and Uranus.

For more information about the composition of Neptune, see Uranus:Composition.

Atmosphere

Like Uranus, Neptune’s atmosphere is very hydrogen rich with helium some methane and traces of water and ammonia. Neptune, like Uranus, gets its blue green color from the methane present in its atmosphere.

Neptune also has a very high proportion of heavy elements compared when compared with Jupiter and Saturn. This may possibly be a direct result of the late heavy bombardment and the elements that polluted Neptune once impactors entered the atmosphere.

Neptune has a variable haze which is probably composed of aerosol particles or methane ice crystals.

Like Uranus, the temperature at Neptune’s cloud tops is 57 K and its atmosphere is cold to great depths.

The rise in Neptune’s brightness from 1999-2002 is due to the dramatic increase in restricted latitude bands. This idea is based on the increase of bright cloud mid latitude activity taken by the Hubble telescope. Recently there has been an increase in cloud activity in the southern hemisphere. This is because Neptune has dominant stages where there are no particular storms dominating the atmosphere, however the activity is ever changing. As of right now it is predicted that Neptune’s current features will persist for another few years.

As the patterns on Neptune are changing, the long term visible brightness rises steadily and smoothly. From the past it can be predicted that there may be another bright feature forming soon.

Neptune has the fastest winds in the solar system. Winds whip clouds of frozen methane across the planet at speeds of more than 556 m/s.

Neptune has three distinct storm features. The Great Dark Spot which is accompanied by bright, white clouds that undergo rapid changes in appearance, “Scooter,” and “Dark Spot 2” which has a very bright core. Each storm moves eastward and they are all moving at different velocities so they only occasionally appear close to one another.

For more information about the atmosphere of Neptune, see Uranus:Atmosphere.

Magnetic Field

The magnetic field of Neptune is tilted 47° from its axis of rotation and is also offset from the planets center similar to Uranus. While this misalignment is believed to be caused by the same sources as that or Uranus, there is no evidence which suggests a collision is responsible for the offset magnetic axis. The presence of its magnetic field is also believed to be caused by the electric currents present in the liquid water and ammonia mantle.

For more information about Neptune's magnetic field, see Uranus:Magnetic Field.

Unique Features

One of the most unique facts of Neptune is the way in which it was discovered. The existence of this Planet was first determined through a calculation rather than an actual discovery. These calculations were the gravitational imbalance in the orbit of Uranus.

Another interesting fact is the length of Neptune's year. Because it is so far from the Sun, Neptune takes approximately 164 Earth years to complete one orbit. As a result, it has yet to complete such an orbit since its discovery.

The Numbers

Discovered By	Johann Galle
Discovery Date	1846
Average Distance from the Sun	4,498,252,900 km (30.069 AU)
Perihelion	4,459,630,000 km (29.811 AU)
Aphelion	4,536,870,000 km (30.327 AU)
Equitorial Radius	24,764 km
Equitorial Circumference	155,597 km
Volume	6.2526 * 1013 km³
Mass	1.0244 * 1026 kg
Density	1.76 g/cm³
Surface Area	7.6408 * 109 km²
Equitorial Surface Gravity	10.71 m/s²
Escape Velocity	85,356 km/h
Sidereal Day	0.67125 Earth days
Sidereal Year	164.79 Earth years
Mean Orbital Velocity	19,720 km/h
Orbital Eccentricity	.00859
Orbital Inclination to Ecliptic	1.769 degrees
Equitorial Inclination to Orbit	29.58 degrees
Orbital Circumference	2.8142 * 1010 km
Effective Temperature	-214 °C
Namesake	Roman god of the sea

Source: http://sse.jpl.nasa.gov/planets