What Is Neptune?
Once just a theoretical planet Neptune is what is known as an Ice Giant. It is the eighth and most distant planet from our solar system, the Sun. Neptune is so distant from Earth, the third planet from the Sun, that it is the only planet that is not visible with the naked eye from Earth’s surface.
Its vast distance from the Sun means that from its discovery in 1846 until 2011 it had made just one full rotation of its orbit around our solar system’s only star. It takes roughly 165 years for Neptune to complete its orbit of the Sun.
The fourth largest planet in our solar system it is 17 times the mass of Earth.
What Is an Ice Giant Planet?
Ice giants are massive planets composed mainly of elements heavier than hydrogen and helium. These might include oxygen, carbon, nitrogen and sulfur. Our solar system is home to two ice giants, Uranus and of course Neptune.
Ice giants form when gravity pulls together swirling gas and dust. They may form closer to a star but migrate further away over time becoming colder as they go. They usually have a smaller rocky core with most of their mass being made up of materials such as water, methane and ammonia.
Neptune’s Structure
Over 80% of Neptune’s mass consists of a hot dense fluid which includes water, methane and ammonia. This covers a rocky core which produces a very intense gravitational pull. This is why even though Neptune is not the largest planet in the solar system it is the densest.
Scientists believe there may be a boiling ocean of super hot water below Neptune’s cold clouds. The density and gravitational pull of the planet however means it does not boil away due to incredibly high pressure. Essentially the boiling water layer is under too much pressure to actually evaporate as it would on Earth.
Surface
There is no solid surface to Neptune as the atmosphere extends to great depths in the planet. This consists of mainly hydrogen, helium and methane. As this atmosphere gets closer to the core it merges into water and other melted ices. This core is estimated to have the same mass as the entirety of the planet Earth.
Rings of Neptune
Neptune has at least five main rings as well as 4 prominent ring arcs which have been discovered so far. The main rings are known as:
- Galle
- Leverrier
- Lassell
- Arago
- Adams
These rings are not thought to be especially old and may not be around for long in stellar terms.
The ring arcs of Neptune Liberté (Liberty), Egalité (Equality), Fraternité (Fraternity), and Courage are interesting in that they are clumps of dust. Laws of motion suggest they should spread out evenly however it is thought that the moon Galatea may be stabilizing these arcs with its gravitational effects.
Does Neptune Have Moons?
According to NASA Neptune has 14 known moons the largest of which is Triton which was itself discovered just over 2 weeks after Neptune was. Named for the Sea God Neptune, its moons all bear the names of other lesser known sea deities of antiquity.
Triton is unique in our solar system in that it is the only moon that circles its planet in a direction opposite to Neptune’s own. This is a strong indication that Triton may be a captured body caught in Neptune’s orbit by its intense gravitational pull.
The moons of Neptune are:
- Triton
- Thalassa
- Sao
- Psamathe
- Larissa
- Naiad
- Nereid
- Neso
- Proteus
- Despina
- Galatea
- Halimede
- Hippocamp
- Laomedeia
History of the Observation of Neptune
Neptune was unknown to ancient astronomers as it could not be seen with the naked eye from Earth. It would not be until telescopes were invented and they reached a certain level at development that Neptune was finally officially confirmed.
Galileo Galilei’s drawings produced on 28th December 1612 and 27th January 1613 show plotted points which match up with the positions of Neptune in the sky on those dates. Of course Galileo did not realize what he had seen so he is not considered the discoverer of Neptune.
Over two centuries later in 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus. Uranus being a near neighbor of Neptune and known since antiquity it was noted that substantial deviations from the tables indicated an unknown body may be affecting its orbit. It was theorized that gravitational interactions from a potential unseen planet may be causing the deviations.
This captured the attention of several astronomers in the next few decades. It was predictions as to location by Urbain Le Verrier that led Johann Galle to finally locate Neptune through his telescope in 1846. Several astronomers had seen and noted Neptune over the years but none had realized they were seeing this elusive 8th planet of our solar system.
Space Exploration
To date the only spacecraft to travel to Neptune is Voyager 2. It passed by the planet on 25th August 1989. During the encounter, signals from the spacecraft took 246 minutes to reach Earth. This is why Voyager 2’s mission relied on preloaded commands. On its way there the spacecraft performed a close pass of the moon Nereid. On the same day as Voyager 2 passed Neptune it also passed close to Triton.
There are several current proposed missions to return to Neptune seeking to confirm and discover some aspects of its structure and the nature of its moons. Both China and the US are still considering future missions.
How Far Is Neptune from Earth?
There is no set distance between any planets in our solar system as they are constantly in orbits around the Sun. These orbits are not precisely circular so at various points any two planets can be at vastly varied distances from each other.
Neptune for example is at its closest 2.7 billion miles from Earth. When they are furthest apart from each other they are 2.9 billion miles apart. This may not seem like a huge difference but it does equate to a variance of 200 million miles.
It takes around 12 years to make the journey from earth to Neptune, around a decade longer than it takes for probes and spacecraft to reach Mars. This is why several missions have been abandoned before launch and it is still unclear if we will try to reach Neptune again in the future.
Final Thoughts
Neptune is an intensely dense ice giant planet that was not discovered officially until 1846. It is a long way from Earth and would take around 12 years to travel to. At its furthest point it is almost 3 billion miles from our own planet.