Basic Data

• Mean Distance from Sun: 4503.4 million km.
• Equatorial Radius: 24,764 km. (3.88 Earth)
• Mass: 1.02 x 10²⁶ kg. (17.1 Earth)
• Surface Gravity: 11.2 m/s² (1.14 Earth)
• Length of Year: 164.79 years
• Length of Day: 16h 6m

Wikipedia - Neptune

About Neptune

Since the demotion of Pluto, Neptune is the furthest of the planets in our solar system from the Sun. It is also the only planet that was discovered as a result of mathematical calculation. In the first half of the nineteenth century, observations of Uranus showed that it was deviating slightly from its predicted path. After several attempts, in 1846 it was finally discovered—within a degree of its predicted position—by an astronomer who took the calculations seriously enough to look.

Neptune is slightly smaller, yet slightly more massive than Uranus, and has a correspondingly higher gravity. In fact, Neptune is the only other planet aside from Jupiter to have a higher surface gravity than Earth. Of course, being a gas giant, Neptune does not actually have a solid surface. Interestingly, it does have the highest sustained wind speeds of any planet in the solar system, reaching over two thousand kilometers per hour. Its internal structure is similar to that of Uranus, with a small, rocky core (slightly bigger than Uranus'), a hot, fluid mantle composed of water, ammonia and trace components, and its atmosphere.

Neptune's atmosphere is similar to that of Uranus in composition, although differences in temperature and other factors give it a blue color, rather than green. And although its upper atmosphere is one of the coldest places in the solar system, temperatures at the core are estimated to be around 7000° C. Neptune is also known for its "dark spots"; ovals in the atmosphere similar in shape to Jupiter's Great Red Spot and similar storms, but dark in color—indeed, they appear essentially black. The current explanation is that they are storms that occur at lower altitudes in the atmosphere, resulting in "holes" in the upper cloud deck.

Like the other gas giants, Neptune has rings. But like those of Jupiter and Uranus, they are thin and faint. Neptune's rings are also clumpy and broken into arcs, and are probably "controlled" by small shepherd moons.

Neptune's Moons

Neptune has one large satellite—Triton—as well as several smaller ones. Triton is interesting in that it has a very thin atmosphere (about a thousandth that of Mars), and orbits backward. The retrograde orbit suggests that it was not originally part of the Neptunian system, but was captured, possibly from the Kuiper Belt (the belt of small, icy objects out beyond Neptune, of which Pluto turned out to have merely been the charter member; see Trans-Neptunian Objects). Triton is also the coldest place yet visited by space probes

Observation from Earth

Neptune is the second planet discovered telescopically. Unlike Uranus, it is not at all visible to the naked eye, nor was its discovery an accident. Like Uranus, it has been extensively studied through telescopes since its discovery. Its large moon Triton was spotted almost immediately after the planet itself was discovered, but the rest of its satellites had to wait until the more sophisticated instruments of the 20th century.

Interestingly, examinations of Galileo's drawings of his observations of Jupiter show that he actually saw Neptune! But because of its slow orbital speed he did not detect any motion and assumed that it was just another star.

Exploration of Neptune

Only one spacecraft has visited Neptune. Voyager 2 flew past in 1989, befor heading out into the Great Beyond. At this time, no further probes are planned, although some scientists have suggested Triton as an interesting world to visit again.