Measurements by Voyager 2 in extreme-ultraviolet and radio frequencies revealed that Neptune has faint and weak but complex and unique aurorae; however, these observations were limited in time and did not contain infrared. Subsequent astronomers using the Hubble Space Telescope have not glimpsed the aurorae, in contrast to the more well-defined aurorae of Uranus. [96] [97] The average distance between Neptune and the Sun is 4.5 billion km (about 30.1 astronomical units (AU)), and it completes an orbit on average every 164.79years, subject to a variability of around ±0.1years. The perihelion distance is 29.81AU; the aphelion distance is 30.33AU. [g] Neptune's atmosphere is subdivided into two main regions: the lower troposphere, where temperature decreases with altitude, and the stratosphere, where temperature increases with altitude. The boundary between the two, the tropopause, lies at a pressure of 0.1 bars (10kPa). [21] The stratosphere then gives way to the thermosphere at a pressure lower than 10 −5 to 10 −4 bars (1 to 10Pa). [21] The thermosphere gradually transitions to the exosphere. Shortly after its discovery, Neptune was referred to simply as "the planet exterior to Uranus" or as "LeVerrier's planet". The first suggestion for a name came from Galle, who proposed the name Janus. In England, Challis put forward the name Oceanus. [45]

Neptune has a planetary ring system, though one much less substantial than that of Saturn. [149] The rings may consist of ice particles coated with silicates or carbon-based material, which most likely gives them a reddish hue. [150] The three main rings are the narrow Adams Ring, 63,000km from the centre of Neptune, the Le Verrier Ring, at 53,000km, and the broader, fainter Galle Ring, at 42,000km. A faint outward extension to the Le Verrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57,000km. [151] Neptune resembles Uranus in its magnetosphere, with a magnetic field strongly tilted relative to its rotational axis at 47° and offset at least 0.55radius, or about 13,500km from the planet's physical centre. Before Voyager 2 's arrival at Neptune, it was hypothesised that Uranus's tilted magnetosphere was the result of its sideways rotation. In comparing the magnetic fields of the two planets, scientists now think the extreme orientation may be characteristic of flows in the planets' interiors. This field may be generated by convective fluid motions in a thin spherical shell of electrically conducting liquids (probably a combination of ammonia, methane and water) [85] resulting in a dynamo action. [92] Neptune's mass of 1.0243 ×10 26kg [7] is intermediate between Earth and the larger gas giants: it is 17 times that of Earth but just 1/19th that of Jupiter. [f] Its gravity at 1 bar is 11.15m/s 2, 1.14 times the surface gravity of Earth, [68] and surpassed only by Jupiter. [69] Neptune's equatorial radius of 24,764km [10] is nearly four times that of Earth. Neptune, like Uranus, is an ice giant, a subclass of giant planet, because they are smaller and have higher concentrations of volatiles than Jupiter and Saturn. [70] In the search for exoplanets, Neptune has been used as a metonym: discovered bodies of similar mass are often referred to as "Neptunes", [71] just as scientists refer to various extrasolar bodies as "Jupiters".The spacecraft verified the existence of a magnetic field surrounding the planet and discovered that the field was offset from the centre and tilted in a manner similar to the field around Uranus. Neptune's rotation period was determined using measurements of radio emissions and Voyager 2 also showed that Neptune had a surprisingly active weather system. Six new moons were discovered, and the planet was shown to have more than one ring. [144] [169]

The axial tilt of Neptune is 28.32°, [123] which is similar to the tilts of Earth (23°) and Mars (25°). As a result, Neptune experiences similar seasonal changes to Earth. The long orbital period of Neptune means that the seasons last for forty Earth years. [104] Its sidereal rotation period (day) is roughly 16.11hours. [11] Because its axial tilt is comparable to Earth's, the variation in the length of its day over the course of its long year is not any more extreme.

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On 11July 2011, Neptune completed its first full barycentric orbit since its discovery in 1846, [120] although it did not appear at its exact discovery position in the sky, because Earth was in a different location in its 365.26day orbit. Because of the motion of the Sun in relation to the barycentre of the Solar System, on 11 July Neptune was also not at its exact discovery position in relation to the Sun; if the more common heliocentric coordinate system is used, the discovery longitude was reached on 12 July 2011. [11] [121] [122] A second symbol, an ‘LV’ monogram for 'LeVerrier', analogous to the ‘H’ monogram for Uranus. It was never much used outside of France and is now archaic.