Neptune 

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PHYSICAL DATA Sidereal period:   16.8 years Rotation period:   18 hours, 12 min. Mean orbital velocity: 5.43 km/s Diameter: 49,500 km Mass (Earth is 1): 17.2 Volume (Earth is 1): 57 Mean Surface Temp: -220°C Distance from Sun: 4,456 to 4,537 million km

    Neptune is a rather typical gas planet, with many things in common with the other gas planets (Uranus, Jupiter, and Saturn). It is large (60 Earths would fit inside it), radiates heat, takes a long time to orbit the Sun (165 years), and its atmosphere is mostly hydrogen and helium. If you look in the right spot, you can see Neptune (as a tiny disk) with just binoculars. For more detailed information, a large telescope is necessary.

General
First things first: Neptune has a diameter (at the equator) of 49,500 km, making it smaller across than Uranus but more massive. It has 16 hour days, its core is about the size of the whole Earth, and the fastest winds in the solar system (2000 km/hr). Neptune is huge compared to the Earth (see image at right) and tilts a bit more off its axis (left). In general Neptune is next to the last planet in the solar system. But Plato travels a strange course and is sometimes actually closer to the Sun than Neptune.

Although Neptune is a gas planet, its composition is more like Uranus than Jupiter. First of all, Neptune is also made up of various ices and rock, mixed with hydrogen (15%) and a little bit of helium. Also like Uranus, it is blue in color (because of the presence of methane that absorbs red light) and it has less layering than Saturn and Jupiter.

Also, both Uranus and Neptune have strange magnetic fields, although Neptune's may be the result of internal conditions. Like Jupiter, Neptune has a distinctive spot. In Neptune's case the GDS (Great Dark Spot), left, is a storm system about the size of Earth. Oddly, the GDS does a kind of disappearing trick. In 1994, HST recorded the GDS was gone (picture right) and then found it again a few months later. It could be that the atmosphere occasionally blocks our view of the Great Dark Spot or that it is a storm that comes and goes. Perhaps Neptune's winds simply blow the GDS around the planet in odd patterns. Neptune's atmosphere might change rapidly because of temperature shifts or changes in internal pressure.

History
In Roman mythology, Neptune is the god of the Sea. For the Greeks, it was Poseidon. Both give a sense of being lost in a vast sea and reflect the mystery of Neptune's discovery. Basically, Neptune was discovered because Uranus didn't act as it should. Uranus didn't seem to be following the same rules as the rest of the planets so scientists began looking for an explanation, something that might affect Uranus. In 1846, they found their answer: another planet, previously unknown, was hiding behind Uranus.

All in all, it was a lucky shot. John Adams (1819-1892), a 24 year old English astronomer, predicted the location of an unseen planet. Several months later and independent of Adams, a French astronomer Urbain LeVerrier (1811-1877) made the same prediction. On September 23, 1846, the German astronomer Johann Galle (1812-1910) actually spotted the new planet. There was a major dispute between England and France over whose planet it was. In fact, both Adams and LeVerrier were wrong. They had done the equivalent of adding 2 plus 2 and getting 5. Neptune isn't usually where Adams and LeVerrier said it would be; their prediction of it path was wrong. But during the brief time Neptune was in that region, Galle spotted it. It's like getting the answer right on a test because the test key had been erroneously marked.

Voyager 2 visited Nepture143 after its discovery. Almost everything we know about Neptune comes from that 1989 visit.

Rings
Like the other gas planets, Neptune also has rings. The 4 sets of rings completely encircle the planet but look like faint arcs from Earth. One of the rings in still unnamed; the other three are called Adams, LeVerrier and Gale (after the astronomers who discovered Neptune).

Surprisingly, Neptune's rings may have a twisted structure with bright lumps in the individual rings.

* Distance is from the center of Neptune to the ring's inner edge.

Satellites
There are eight known moons of Neptune. Only two of them, Triton and Neroid, were discovered before Voyager 2. Triton, the largest of Neptune's moons, is to date the coldest object in the solar system (-235° C; -391° F). It has a thin atmosphere of frozen nitrogen. Its icy characteristics can be seen in the composite picture (left) of Triton in the foreground and Neptune in the distance.

Triton's second call to fame is that it is the only large moon to circle backwards (retrograde) around its planet. Triton.was discovered in 1846 by an English brewer turned astronomer, William Lassell (1799-1880).

Unknowns

1. What causes Neptune's magnetic field to be off center?
2. Why is there so much less hydrogen and helium in Neptune and Uranus compared to Jupiter and Saturn?
3. What caused Neptune's winds?
4. What happened to the Great Dark Spot?
5. What are the bright clumps in Neptune's rings?

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