NEPTUNE

Orbiting the Sun in 164.79 years, at a mean distance of 30.06 AU (4.50×109 km), Neptune is the eighth and the farthest planet in the Solar System. It has been named after the Roman God of Sea and is the fourth largest plant in terms of diameter and third largest by mass.  Neptune has a mass of about 17 times that of the Earth. Its orbit is nearly circular and inclined to the Earth’s orbit plane by only 1.77°. Neptune’s outer atmosphere, like those of the other giant planets, is composed predominantly of molecular hydrogen and helium with traces of methane (CH4). Yet Neptune, together with Uranus, forms a distinct subclass of ice-rich Jovian planets. Most of the planet’s mass is in the form of ‘ices’, primarily water, with possibly a ‘rock’ component. 

Despite its remoteness, Neptune’s atmosphere is one of the most dynamically active in the outer solar system. Small bright clouds evolve with a timescale shorter than a day while large dark spots, slowly moving and oscillating in shape, persist for months. In the upper atmosphere,  photolysis  of  methane  initiates  a  suite  of photochemical  reactions  producing  heavier  hydrocarbons and condensation hazes. Neptune possesses a large and complex magnetic field, much like that on Uranus, with a magnetic dipole offset from the planet’s center and tilted by 47° to the spin axis.

Neptune was first observed and identified as a planet in September 1846. This brilliant discovery, based purely on calculations of orbital perturbations, marked the triumph of celestial mechanics. With a visual magnitude of 7.8, Neptune is too faint to be seen by the naked eye.  Through a telescope, it appears as a bluish disk with an angular diameter of 2.3 arcsec at mean opposition. In the 1970s, spectrophotometer observations of the visible and near-infrared spectrum of Neptune yielded a first determination of the CH4 abundance and of the pressure level of the upper haze layer.

Most of what we know about Neptune and its environment comes from the Voyager 2 flyby in August 1989. Thousands of images were recorded and observations were conducted in the visible, infrared, ultraviolet, and at centimeter wavelengths.

Current  theoretical models suggest  that  the atmosphere of Neptune extends down to about 3500 km below the 1 bar  level, up  to a pressure of ~100 kbar, and overlies a denser fluid  comprising  mainly  water,  methane  and ammonia. The mass of this outer layer, between 0.5 and 1  Earth  mass,  is  a  small  fraction  of  the  total  mass  of Neptune, in contrast to the gas-rich Jupiter and Saturn . Neptune has a total of 13 moons, out of which Triton, the largest one has been widely studied.