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Neptune, the eighth and farthest known planet from the sun, is the fourth-largest planet
in the solar system. Neptune is the densest of all the gas giants, and so even though
it is slightly smaller than Uranus, Neptune is more massive. Like its near twin Uranus,
Neptune is made mostly of hydrogen and helium, with traces of other materials and 'ices'
like water, ammonia, and methane. Because of this, Neptune, like Uranus, is sometimes
referred to as an "ice giant."
While Uranus has a hazy, mostly unremarkable atmosphere, Neptune has active and easily
visible weather patterns, which gives it a more distinctive appearance. Like Uranus,
Neptune has methane in its atmosphere which gives it a blue color. However, Neptune appears
a darker blue than Uranus, and scientists are not sure why. When the Voyager 2 spacecraft
flew by Neptune in 1989, the planet had a "Great Dark Spot" much like the "Great Red
Spot" on Jupiter, but by 1994 the spot had disappeared completely. Neptune has the strongest
winds of any planet in the solar system, with speeds recorded as high as 1300 miles or 2100
kilometers per hour. Neptune is also one of the coldest places in the Solar System, with
atmospheric temperatures of about -360 F or -218 degrees Celsius.
Neptune was the first planet to be discovered through mathematical predictions instead of
being seen, as it is too faint to be observed with the unaided eye. As early as 1821, astronomers
noticed that Uranus's orbit was behaving in a way that could best be explained by the
gravity of another planet tugging on it. In 1843, John Couch Adams calculated where the
orbit of this mysterious eighth planet would be, but nothing came of it. Three years later,
in 1846, French mathematician Urbain le Verrier made his own calculations and convinced German
astronomer Johann Gottfried Galle to search for the undiscovered planet, which he spotted
on September 23, 1846.
The newly discovered planet was called 'Neptune', after the Roman god of the sea, probably because
of its deep blue color. A name from Roman mythology was chosen to match the names of
the other planets.
Neptune is nearly 2.8 billion miles or 4.5 billion kilometers from the Sun. Because it
is so far, it takes an incredibly long time to complete one orbit, about 165 years. From
the time it was discovered in 1846, Neptune did not complete a full orbit until 2011,
and it will not complete its next orbit until 2176, the 400th birthday of the United States!
Another interesting fact about Neptune's orbit is the way it intersects with Pluto's orbit.
Because Pluto's orbit is so irregular, it crosses inside of Neptune's path for 20 years
of every 248. Between 1979 and 1999, Pluto was actually closer to the Sun than Neptune,
something that won't happen again for more than 200 years.
Neptune has a system of six known rings, although they are so faint that they were not discovered
until 1968. Neptune's rings are unusual in that they are not smooth and even like the
rings of other planets, but are instead distributed in uneven clumps of dust and ice.
There are 14 known moons orbiting Neptune. Only the largest moon, Triton, is large enough
to be round - the rest are irregular in shape.
Only one spacecraft has visited Neptune: Voyager 2, which passed the planet in 1989. Neptune
was the final objective for the Voyager probe, and it passed extremely close to the planet.
Voyager 2 discovered 5 of Neptune's moons, 4 of its rings, and took photos of the Great
Dark Spot. It also made some detailed observations of the largest moon, Triton, before continuing
on its path out of the Solar System.
Distant and mysterious, Neptune may still hold many discoveries for the scientists of
the future. I hope you enjoyed learning about Neptune, the last planet. Goodbye till next
time!