And now we come to the last planet in our solar system…

  • You found me… eventually

Up until the discovery of Neptune all planets in our Solar System were found with the naked eye or with the aid of a telescope, Neptune however was the first mathematically predicted planetary discovery. After Uranus was found in 1781 astronomers noticed some odd behaviour – Uranus was being pulled slightly out of its orbit. Things just didn’t add up and it was predicted that another planet was affecting Uranus with its gravity. And they were right! French astronomer Jean Joseph Le Verrier and British astronomer Couch Adams have both been credited with detecting Neptune in September 1846 despite international disputes. From this astronomer, Johann Gottfried Galle was able to observe Neptune for the first time not long after. 

Image of Neptune taken by Voyager 2 Credit: NASA
  • Cool Moon King 

Now would you believe it was only seventeen days after the discovery of Neptune that its largest moon Triton was found by astronomer William Lassell? For over 100 years Triton was the only moon of Neptune and was known simply as ‘the satellite of Neptune’. Since then 13 more moons have been discovered but none as impressive or as interesting as Triton, all of which have been named after sea gods and nymphs in Greek mythology. Triton is unique as it is the only large moon in the Solar System to rotate in a retrograde orbit and for this reason it is believed that Triton is a Kuiper belt object captured by Neptune’s gravity long ago. Triton surface is mostly made up of frozen nitrogen with a water ice crust and a rocky core. Triton was also the last solid body to be studied by the Voyager 2 spacecraft, but more on that later. 

Image of Triton and Neptune
Triton seen with its parent planet in a photomontage. (Image credit: NASA)
  • It’ll blow over 

Neptune is the windiest world in our Solar System; super winds beat clouds of frozen methane across the planet. Wind speeds can reach up to an impressive 1200mph, whereas the top reaching wind speed recorded on Earth was 254 mph, taken in Australia in 1996 when a hurricane hit. Just like Jupiter, Neptune also has many storm systems most notably the Great Dark Spot which was first discovered in 1989 when the Voyager 2 spacecraft flew past, but by 1994 when the Hubble Space Telescope photographed the same spot it had completely vanished! 

Image: Neptune the windiest planet as captured by Voyager 2 in 1989. Credit: Voyager 2, NASA.
  • Flying Visit 

The infamous Voyager 2 is the only spacecraft to fly by Neptune. It approached 5000 km above the planets cloud tops, 12 years after leaving Earth in 1977. From this fly by there were many discoveries, including four faint rings made up of dust and other debris and 6 previously unknown satellites (Larissa, Naiad , Despina, Thalassa, Gealatea and Proteus). This fly by also revealed that tritons surface temperate is a chilling -235 degrees Celsius and discovered the ‘Great Dark Spot’ as previously mentioned. Voyager 2 was the first and last spacecraft to visit Neptune and as of yet no spacecraft has orbited Neptune. 

The voyager 2 space craft drifting into Interstellar space. Credit: NASA/JPL-Caltech.
  • I’m cold and I’ll take my time, thank you 

Neptune is the eighth and most distant planet from the Sun at 4.5 million km away, this is 30 times more distant from the Sun than the Earth. Therefore, it is no surprise that on average Neptune is the coldest Planet in our Solar System with temperature plummeting as low as -214 degrees Celsius. However things can get a little confusing as Uranus actually has the lowest recorded temperature of any planet with -224 degrees Celsius, but on average Neptune is colder. Interestingly, once upon a time the answer would have simply been Pluto before its demotion to a Dwarf Planet back in 2006. But did you know that due to Pluto’s elliptical orbit it is sometime closer to the Sun and Earth than Neptune is? Not only is Neptune cold but also has the slowest orbit of any Planet in the Solar System, taking 165 years to complete one full orbit of the sun. In 2011 Neptune completed its first full orbit since it discovery in 1846. Although a year on Neptune is very long, a day is only 16 hours, called a Neptunian day!

The sun as seen from Triton – Neptune visible. Illustration by Ron Miller

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