So – you’re still interested in astronomy? Neptune6 min read

Hello again. If you remember, last time we took a look at 10 things you wanted to know about Jupiter. This time, I thought it would be a great idea to consider 10 facts you might find interesting regarding one of the strangest influences in our Solar System – Neptune.

  1. 1. So, what do we know about Neptune?

At an average of 4.5 billion kilometers – (2.8 billion miles) – Neptune is the eight and farthest planet from the Sun and takes 165 Earth years to complete one orbit. Neptune is named after the Roman god of the sea. She is the fourth largest planet by diameter, and the third largest by mass.




Note: Of the Gas giants, (Jupiter, Saturn, Uranus, and Neptune), although she is the smallest, Neptune is by far the densest.
Also, she has a highly elliptical orbit, (Varying by over 101 million kilometers – 62.7 million miles) between her closest-perihelion and farthest-aphelion points. So at certain times in her orbit; Neptune is father away from the sun than Pluto.


  1. 2. How big is Neptune?

This is the brilliant thing about gas giants. Depending which way you measure them, their size can differ quite considerably. Neptune has a diameter of 49,500 km (30,700 miles) around her equator. If you measure her through the Polar Regions, then her diameter is only 48,682 kilometers (30,249 miles) so she possesses a noticeable bulge.


  1. 3. Neptune is known to possess ‘Great Spots’. What are they?

Neptune is noticeable for its active and visible weather patterns. As with Jupiter, massive anti-cyclonic storms ravage the upper and lower atmosphere creating huge spots.




Unlike Jupiter, Neptune’s don’t last as long, but they can still be massive. For example, a great dark spot seen in 1989 by Voyager 2 was 13000 X 6600 km in size. Although that storm died out, a number of others have sprung up in its place.

Note: These storms are created by the strongest sustained winds of any planet in the solar system. Those winds are almost supersonic, exceeding 2,100kph (1,300mph).


  1. 4. What is Neptune actually made of then?

The upper atmosphere consists of 80% hydrogen and 19% helium with the rest being made up of methane and other gases. Important absorption bands of methane happen at wavelengths above 600 nm, in the red and infrared portion of the spectrum. This absorption of red light by the atmospheric methane gives Neptune its very distinctive blue color.

Beneath this, comes the mantle of water, ammonia, and methane ices. At the very center comes the core of rock (silicates and nickel-iron).


  1. 5. Any other cool facts?

There certainly is. Because Neptune orbits so far away from the Sun, it gets very little heat, and the upper regions of the atmosphere usually have a temperature around -218⁰C (55K). However, when you go deeper, the temperature rises slowly. Now, scientists aren’t sure just why this is. Several theories have been put forward, including: radiogenic heating from the planets core, gravity waves above the area of the troposphere, and the continued radiation into space of leftover heat made by infalling (material drawn into a central mass by gravity) during Neptune’s birth. Whatever the reason, the extra heat appears to bleed into the superheated interior of Neptune, making the pressure at the center of the planet millions of times more than it is on the surface of the Earth. That is cool!


  1. 6. How many moons does Neptune have?

She has 17 known moons. Because Neptune a god of the sea, the moons are named after lesser gods or goddesses. The largest one – and only one big enough to form a sphere – is Triton. Unlike the other moons, Triton has a retrograde orbit, which means she was probably captured from within the Kuiper Belt. Although locked in synchronous orbit, Triton is slowly moving in toward Neptune, and will one day be torn apart when it passes the Roche Limit. (The distance within which a celestial body, held together by its own gravity, will disintegrate due to the tidal forces inflicted by another body).

Fact. Triton is the coldest known place in our Solar System, with temperatures of -235⁰C (38K). Its diameter is 2700km (1678 miles) (80% of our own Moon).


  1. 7. How does Neptune have any interaction with the Kuiper Belt?

Neptune’s orbit regularly takes her into part of an area known as the Kuiper Belt (a ring of small icy world – and occasional large bodies – similar to the main Asteroid Belt, but much larger).


  1. 8. Does Neptune have a ring system?

Although much less substantial than Saturn, Neptune has a three ring system.




Scientists think the particles are ice particles covered in silicates or carbon, which gives them their reddish hue. In total, there are three main rings. The Adams Ring, 63,000 km (39,146 miles) from the center of the planet. The Le Verrier Ring, 53,000 km (33,000 miles) up, and finally, the Galle Ring, at 42,000 km (26,100 miles). A faint outward extension to the Le Vernier Ring has been named Lassell, and extends up to 57,000 km away (36,000 miles) until it blends into the Arago Ring.


  1. 9. Is Neptune hard to see?

Unfortunately, yes. Neptune is never visible to the naked eye. With brightness of magnitudes between +7.7 – +8.0, there are many other objects between Neptune and Earth which are much brighter. A telescope or strong binoculars will resolve Neptune as a small blue disc, similar at a distance to Uranus.



  1. 10. So how did Neptune get discovered?

This is quite a story!
She was the first planet ever to be discovered by mathematical prediction rather by observation. In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus. He was puzzled by apparent deviations in the actual orbit to the tables published, leading him to hypothesize an unknown body was perturbing Uranus through gravitational interaction. In 1843, John Couch Adams began work on the orbit of Uranus. Intrigued by what he found, he continued his investigations throughout 1845/46, and produced estimates as to the position of a new planet. For some reason, fellow scientists weren’t all that interested. At the same time Urbain Le Verrier ran his own set of calculations, and likewise, ran into a similar wall of reticence.

In June 1846, the Astronomer Royal of England – Sir George Airy – noticed similarities in the results produced by Adams and Le Verrier, and tasked Cambridge Observatory director James Challis to search for a new planet.

Challis did so through August and September, but unfortunately, due to his casual attitude, failed to confirm the planet’s position.

In the meantime, Le Verrier urged Berlin Observatory astronomer, Johann Gottfried Galle to search for this elusive sprite in a certain area of the night sky suggested by his recent calculations. Fortunately for Galle, an astute pupil, Heinrich d’Arrest proposed they compare a recently drawn chart of the night sky with the region indicated by Le Verrier’s prediction.

On the evening of September 23rd 1846 – on the very evening Galle received the letter – Neptune was discovered within 1⁰ of where Le Verrier had predicted it would be, and about 12⁰ from Adam’s.


But it wasn’t over!

Quite a bit of wrangling went back and forth between advocates of le Verrier and Adams, saying they should both credited with the discovery. At first, that’s what happened. However, a review took place in 1998, that stated Adams should not have equal credit with Le Verrier, and the distinction should rest only with the person who predicted both the new planet’s position AND in convincing independent astronomers to search for it. Therefore, Urbain Le Verrier is now known as the man who discovered the final planet of our Solar System.


So, there you go. I hoped you enjoyed this brief look at Neptune. Next time, we’ll look at 10 interesting things about Neptune’s neighbor, Uranus.


See you the.




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Andrew Weston
Andrew Weston

Andrew P. Weston is Royal Marine and Police veteran from the UK who now lives on the beautiful Greek island of Kos with his wife, Annette, and their growing family of rescue cats. An astronomy and law graduate, he is the creator of the international number one bestseller, The IX, and also has the privilege of being a member of the Science Fiction and Fantasy Writers of America and the British Fantasy Society. When not writing, Andrew devotes some of his spare time to assisting NASA with one of their remote research projects, and writes educational articles for and Amazing Stories. He also enjoys Greek dancing and language lessons, being told what to do by his wife, and drinking Earl Grey Tea. If you would like to find out more, visit his blog or website at:

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