Named for an ancient Roman sea god, Neptune is the eighth planet from the sun at a distance of about 2.8 billion miles or 30.07 AU. It was also the first planet to be discovered by mathematics and not observations.
Galileo was the first to record an observation of an object in the area of space we now know to be Neptune in December 1612. Galileo was studying Jupiter and it’s moons with his telescope when he first saw the planet. However, he believe it to be a star. During the course of his observations, the location of the star changed. Galileo noticed the shift in position, but continued with his task of studying Jupiter. Galileo’s moving star was, in fact, the planet Neptune.
During the 19th century, Alexis Bouvard calculated the path that Uranus was expected to take around the sun. When it did not match calculations it was apparent that something was pulling it off course. In 1845, John Couch Adams, an English astronomer, calculated the location of Neptune. Urbain Leverrier, a French mathematician, independently did similar calculations. In 1846, John G. Galle and Heinrich d’Arrest of the Urania Observatory in Berlin, using Leverrier’s calculations sighted Neptune which became the eight planet of the solar system.
Neptune takes 164.8 earth years to orbit the sun. It’s axis is tilted to 29.5 degrees from the perpendicular, which is similar to Earth and thus experiences seasons. The average surface temperature is minus 2200 Celsius which is similar to that of Uranus which is closer to the Sun. This is because Neptune has an internal heat source which compensates for the difference.
It’s atmosphere, like that of all the gas giants, is comprised mainly of hydrogen and helium. With noticeable amounts of methane and other hydrocarbons in its atmosphere. The planet’s blueish color derives from it’s methane clouds which absorb red light, reflecting green and blue. Although initially named a Jovian planet, the planet has more recently been called a Uranian planet, to indicate Neptune’s greater similarity to Uranus, rather than Jupiter, which only has trace amounts of methane. Neptune has rapid winds confined to bands of latitude and large storms with wind speeds reaching 1,200 miles an hour making them the fastest in the solar system. The winds were measured by the Voyager 2 spacecraft in 1989.
Neptune’s most remarkable feature is a Great Dark Spot in the southern hemisphere. The spot seems to be similar to Jupiter’s Great Red Spot as it may be a large storm. Another theory is that it may be a hole in the atmosphere similar to the hole in the Earth’s ozone. In 1994, the Hubble telescope looked for the Great Dark Spot in the southern hemisphere, it wasn’t there. It was replaced with another spot in the northern hemisphere. A testament to the dynamic nature of Neptune’s atmosphere.
Neptune probably has a small, rocky core about the size of the Earth. Its mantle may be enveloped in a layer of liquid hydrogen as the gaseous layers in its atmosphere descend to the surface.
Neptune has fourteen known moons, by far the largest of which is Triton. Triton orbits Neptune in a retrograde direction, suggesting the two bodies were not formed together. Before it was captured by Neprune’s gravitational field , Triton could have been much like Pluto, a trans-Neptunian object from the Kuiper belt.
Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Much of we know about Neptune comes from this single encounter and more recently ground-based observations.
- It took the space probe Voyager 2, 12 years to reach Neptune
- Neptune only receives 1/900 of the solar energy that reaches Earth
- Neptune is 30 times farther from the sun as is the Earth
- Neptune is the smallest of the gas giants
- Although smaller in diameter than Uranus, Neptune has a greater mass
- Neptune is the coldest planet in the Solar System
- There are currently no plans to visit Neptune again with a spacecraft