Query: Neptune's orbit is 164 years so observations (1950 to present day) span less than a third of a Neptunian year.

Relevant passages:
Neptune <=SEP=> Neptune is the eighth and farthest known planet from the Sun in the Solar System . In the Solar System , it is the fourth-largest planet by diameter , the third-most-massive planet , and the densest giant planet . Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus , which is 15 times the mass of Earth and slightly larger than Neptune.Neptune is denser and physically smaller than Uranus because Neptune 's greater mass gravitationally compresses the atmosphere more . Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 AU . It is named after the Roman god of the sea and has the astronomical symbol ♆ , a stylised version of the god Neptune 's trident .   Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation . Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet . Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier . Its largest moon , Triton , was discovered shortly thereafter , though none of the planet 's remaining known 14 moons were located telescopically until the 20th century . The planet 's distance from Earth gives it a very small apparent size , making it challenging to study with Earth-based telescopes . Neptune was visited by Voyager 2 , when it flew by the planet on 25 August 1989 . The advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar .   Neptune 's composition can be compared and contrasted with the Solar System 's other giant planets . Like Jupiter and Saturn , Neptune 's atmosphere is composed primarily of hydrogen and helium , along with traces of hydrocarbons and possibly nitrogen , but it contains a higher proportion of `` ices '' such as water , ammonia , and methane . However , its interior , like that of Uranus , is primarily composed of ices and rock , which is why Uranus and Neptune are normally considered `` ice giants '' to emphasise this distinction . Traces of methane in the outermost regions in part account for the planet 's blue appearance .   In contrast to the hazy , relatively featureless atmosphere of Uranus , Neptune 's atmosphere has active and visible weather patterns . For example , at the time of the Voyager 2 flyby in 1989 , the planet 's southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter . These weather patterns are driven by the strongest sustained winds of any planet in the Solar System , with recorded wind speeds as high as 2100 km/h . Because of its great distance from the Sun , Neptune 's outer atmosphere is one of the coldest places in the Solar System , with temperatures at its cloud tops approaching 55 K. Temperatures at the planet 's centre are approximately 5400 K. Neptune has a faint and fragmented ring system ( labelled `` arcs '' ) , which was first detected during the 1960s and confirmed by Voyager 2 .

Pluto <=SEP=> Pluto ( minor-planet designation : 134340 Pluto ) is a dwarf planet in the Kuiper belt , a ring of bodies beyond Neptune . It was the first Kuiper belt object to be discovered .   Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun . After 1992 , its planethood was questioned following the discovery of several objects of similar size in the Kuiper belt . In 2005 , Eris , a dwarf planet in the scattered disc which is 27 % more massive than Pluto , was discovered . This led the International Astronomical Union ( IAU ) to define the term `` planet '' formally in 2006 , during their 26th General Assembly . That definition excluded Pluto and reclassified it as a dwarf planet .   Pluto is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun . It is the largest known trans-Neptunian object by volume but is less massive than Eris . Like other Kuiper belt objects , Pluto is primarily made of ice and rock and is relatively small -- about one-sixth the mass of the Moon and one-third its volume . It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU ( 4.4 -- 7.4 billion km ) from the Sun . This means that Pluto periodically comes closer to the Sun than Neptune , but a stable orbital resonance with Neptune prevents them from colliding . Light from the Sun takes about 5.5 hours to reach Pluto at its average distance ( 39.5 AU ) .   Pluto has five known moons : Charon ( the largest , with a diameter just over half that of Pluto ) , Styx , Nix , Kerberos , and Hydra . Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body .   On July 14 , 2015 , the New Horizons spacecraft became the first spacecraft to fly by Pluto . During its brief flyby , New Horizons made detailed measurements and observations of Pluto and its moons . In September 2016 , astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins , organic macromolecules that may be ingredients for the emergence of life , and produced from methane , nitrogen and other gases released from the atmosphere of Pluto and transferred about 19,000 km to the orbiting moon .