Neptune, like any planet, has its own unique characteristics and attributes. Compared to the only planet we know very well, it is very strange by comparison to Earth.
Named after the Roman god of the sea (Poseidon in the Ancient Greek pantheon), Neptune is around 2.8 billion miles (4.47 billion kilometers) from the Sun and has only been visited by a single human-made object.
Relative to some of the other planets, we know next to nothing about this distant planet. However, from what we have been able to glean since its relatively late discovery, is truly a fascinating place.
So, what do we know about this most remote of our planetary neighbors?
Is Neptune all water?
But, before we get into that, let's clear up one common question that appears to be frequently asked about the planet Neptune. As the planet is a very deep blue in color, it appears many believe that this might be because the planet could be a rich source of potential water (ice or otherwise).
The fact is that this is sadly not true. While there is believed to be water on the planet, the blue color is actually a consequence of the methane in the atmosphere of Neptune. Although methane makes up a relatively small amount of Neptune's atmosphere (about 1%), clouds of methane gas absorb red wavelengths of light and reflect blue light outward.
Most of the planet's mass is made up of a hot dense fluid of "icy" water, methane, and ammonia above a small, rocky core.
But, more on that later.
What is the most interesting fact about Neptune?
There are so many interesting facts about Neptune that it is almost impossible to pick the "most" interesting fact.
So, we'll let you choose from the following handpicked selection. Bon appetite.
1. Neptune is, once again, the furthest planet from the Sun
Prior to the discovery of Pluto in the 1930s, Neptune was, for almost a century, the furthest known planet from the Sun. However, in a strange twist of fate, once the definition of what constituted a planet was revised in 2006, the title once again belonged to Neptune.
Neptune itself was first discovered in 1846, but not by direct observation. That far out, Neptune's only neighboring planet is Uranus.
2. Neptune would not have been known to the ancients
Neptune, as we know it today, is about 30 times as far from the Sun as Earth is. This is incredibly far away, and though it is technically observable in our night's sky, it is so dim that it is lost in the "noise" of other stars and brighter celestial bodies.
Since Neptune cannot readily be seen with the naked eye at night, its existence, for most of human history, was not known. In fact, up until about the 17th century, the only known celestial bodies were the Sun, Earth and the Moon (obviously), Mercury, and Venus, Mars, Jupiter, and Saturn.
Since all of these can be easily seen without the need for highly specialized equipment or analytical techniques, most cultures around the world are well aware of their presence.
Neptune, on the other hand, was only really discovered in 1846 after its position and existence were surmised using a series of mathematical calculations. The discovery was something of a joint effort with French astronomer Urbain Le Verrier and British astronomer John Couch Adams both working on the problem at the same time. Le Verrier made calculations and predictions of Neptune's likely position at a given date and time.
His work was independently verified by Adams who reached the same, or a similar conclusion as Le Verrier. Based on these calculations, the planet was first observed using a telescope by Johann Gottfried Galle on the night of the 23rd to the 24th of September 1846.
That was amazing enough, but only 17 days later, another astronomer, William Lassel, actually managed to identify a large moon orbiting the new planet too. This moon is now known as Triton, but it wouldn't receive that name until more moons were discovered well into the 20th century.
While the works of Le Verrier, Adams, and Galle are officially recognized as the first to identify Neptune as a new planet, they may not technically have been the first to actually "find" it. From a study of records kept by Galileo, some historians believe that he may have actually found the planet centuries early.
However, he appears to have thought of it as a new star and didn't appear to recognize the possibility of it actually being a planet.
3. Here's why Neptune is so blue
Another interesting fact about Neptune is the makeup of its atmosphere. Like other gas giants (though also classified as an "ice giant"), of which Neptune is a member, the planet is shrouded in a thick blanket of gases.
As best we can ascertain from data collated from probes like Voyager 2, Neptune's atmosphere is made up of around 74% hydrogen, 25% helium, and about 1% methane and other minor constituents.
Neptune also has clouds, of a sort. These are made up, primarily, of particles of icy methane (and other minor gases) in the extremities of the atmosphere. It is these icy particles of methane, in fact, that give Neptune its characteristic deep blue color.
It is not only the striking blue but also the white features of Neptune that help to distinguish it from Uranus.
Like other large gas planets, Neptune's atmosphere is also divided into a series of layers. Roughly speaking, these are the lower troposphere and stratosphere with the troposphere being the boundary between the two.
Temperatures in the lower troposphere of Neptune decrease with altitude (as you'd expect), but in the higher stratosphere, temperatures actually appear to increase with altitude. This is very weird.
Hydrocarbons form hazes of 'smog' that appear in the entire upper atmosphere of Neptune and hydrocarbon snowflakes that form in Neptune's atmosphere melt before they reach its surface due to the high pressure.
So, as we said, a very strange atmosphere indeed.
4. Neptune and its vanishing (then reappearing) "Great Dark Spot"
We've already seen how strange Neptune's atmosphere is, but it actually gets a little weirder. When NASA's Voyager 2 probe passed by the planet in 1989, it picked up something very unexpected in the southern hemisphere - an enormous dark spot.
Further study of the data recorded about it from the probe quickly revealed that it was some form of an enormous rotating storm system. And it was also huge, about the size of the entire Earth!
Much like Jupiter's Great Spot, Neptune's spot consisted of incredibly fast-moving winds of up to around 1,500 mph (2,414 kph).
These winds are so fast, in fact, that they are the fastest of their kind we've yet recorded on any planet. Data also indicated that the so-called "Great Dark Spot" appeared to vary considerably in size throughout the brief contact Voyager 2 had with the planet.
The source of the energy of these storms is yet to be discovered, but given Neptune's distance from the Sun, it is unlikely to be from sunlight. To this day, scientists are still completely at a loss as to what actually drove such ferocious winds.
But, the story of the spot gets even stranger.
When Neptune was observed directly with the Hubble Space Telescope, it was discovered that the "Great Dark Spot" had apparently vanished completely. However, another new dark spot could be observed in Neptune's northern hemisphere.
Since then, Hubble has been keeping a very close eye on Neptune and has been able to discover these spots come and go on a roughly biannual basis.
5. There is something very strange about Neptune's temperature
We've already covered the strange storm systems on the planet and its atmospheric conditions, but it seems to get even stranger. Recently, NASA scientists noticed that the temperature of Neptune appears to be changing fairly rapidly over time.
In 2019, a team of scientists from the University of Leicester, UK, observed Neptune over a number of years using the ground-based European Southern Observatory's Very Large Telescope (ESO's VLT).
The team used a set of thermal cameras to track the average temperature of the planet to improve our already limited knowledge of it. Since beginning to make detailed recordings in 2003, they noticed that the planet appears to be getting cooler over time.
"This change was unexpected," says Michael Roman, a postdoctoral research associate at the University of Leicester and the lead author of the study. "Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder."
They found that the southern hemisphere dropped in temperature by 46 degrees Fahrenheit (7.8 degrees Celsius) between 2003 and 2018. However, this appears to have now reversed, with the temperature having rapidly risen by 51 degrees Fahrenheit (10.6 degrees Celsius) between 2018 and 2020.
The cause for this is not really known, but we do know that Neptune has its own seasons too, just like all planets. The difference with Neptune as compared to Earth, say, is that its seasons last roughly 40 years.
The data recovered by the team covers roughly half of one season in what should be Neptune's "summer".
6. At the heart of Neptune is a solid core about the size of Earth
Underneath Neptune's thick atmosphere is believed to be a more solid core about the size of the planet Earth. Little is actually known about the core (as we can't see it), but it is likely composed of heavier elements like iron, nickel, and probably silicates.
Modeling of the planet has revealed that the pressure at the center is likely in the order of 7 Mbar (700 GPa). That is about twice as high as that at the center of Earth. It is also believed that the temperature of the core is probably something like 101,060 degrees Fahrenheit (or 5,400 Kelvin).
Like on Earth, surrounding the core is another layer called the mantle. Unlike the hot and plastic rock-type mantle found on Earth, Neptune's consists of water, ammonia, and methane.
The total mass of this portion of the planet is believed to be about the equivalent of 10 to 15 Earths. Often termed "icy", simply out of planetary science convention, this layer is actually incredibly hot, dense fluid.
Interestingly, it is believed that at a depth of around 4,350 (7,000 km), conditions might be such that methane could conceivably decompose and form diamonds that actually rain down like hailstones.
Very-high-pressure experiments at the Lawrence Livermore National Laboratory suggest that the top of the mantle may be an ocean of liquid carbon with floating solid 'diamonds. Imagine that!
Others believe that there might be an ocean of super hot water under Neptune's cold clouds. It does not boil away because incredibly high pressure keeps it locked inside.
7. Neptune actually has some rings, but they are really hard to see
Believe it or not, Neptune actually has a set of rings too. While nowhere near as impressive as those of the planet Saturn, they are still very beautiful in their own way.
As they are very faint, the rings were only discovered by astronomers in the mid-1980s. They were discovered by Patrice Bouchet who was working for the European Southern Observatory at La Silla in Chile.
Closer, more detailed images were collected by the NASA Voyager 2 probe later in the decade.
Even at their densest, Neptune's rings are roughly comparable to the less dense portions of Saturn's main rings like, for example, Saturn's C ring and the Cassini Division. However, a lot of Neptune's ring system is pretty weak, faint, and dusty, and more closely resembles the rings of Jupiter.
The rings have been named in honor of the astronomers who contributed important work on the planet. So, moving outward from the planet the rings are called Galle, Le Verrier, Lassell, Arago, and Adams.
There is also another, as yet unnamed ring that roughly coincides with the orbit of one of Neptune's moons Galatea. Astronomers think these rings are a relatively new addition to the planet, and may also be very short-lived.
It should be noted, however, that Neptune's ring system also has peculiar clumps of dust called arcs.
The four most prominent arcs are called Liberté (Liberty), Egalité (Equality), Fraternité (Fraternity), and Courage which are located in the outermost ring, Adams. The arcs are strange because the laws of motion would predict that they would spread out evenly rather than stay clumped together. Scientists now think the gravitational effects of Galatea probably stabilize these arcs.
What we know of the rings to date is that they appear to be made of some form of extremely dark material. Scientists are not entirely sure what they are made of, but it has been proposed that they may actually consist of large concentrations of organic compounds that have formed from exposure to sunlight over long periods of time.
8. Neptune may have migrated to its current position billions of years ago
Another interesting fact about Neptune is that its current location may not have always been the case. Like the rest of the Solar System, Neptune formed about 4.5 to 4.6 billion years ago when gravity pulled swirling gas and dust together to form all the planets and the Sun.
The opinion is divided on the exact mechanism of this formation, with two possible methods being the most likely. The first is the core accretion hypothesis which works well for rocky inner planets but is less logical for the larger gas and ice giants. The second is called the disk instability model which explains more satisfactorily how larger planets, like Neptune, can form relatively quickly.
Most of the planets formed, more or less, in their current orbital positions around the Sun, but that might not be true for Neptune or, for that matter, Uranus.
Astronomers currently believe that both of these planets may have actually formed closer to the Sun but gradually migrated out to their current locations.
9. Neptune is probably out of bounds for lifeforms, probably
To be completely honest, most scientists are not entirely sure if life exists, or could ever exist, on Neptune. The conditions there are very different to planets like Earth and the lack of liquid water likely completely rules out life as we know it.
The atmosphere and surface of the planet are also subject to extreme temperatures, pressures, and substances that would typically make it a very hostile place for early or complex life to develop and/or survive.
However, since we have never actually observed or studied any form of actual alien life that doesn't mean it is not possible.
In fact, various potential modes of life have been proposed. But, there may be pockets of relatively hospitable areas where life might be able to eke out a living.
If we were to find life on Neptune, the planet would need some form of energy source that lifeforms could exploit for their own ends. The planet would also need some form of liquid water or other as-yet unknown substance with similar properties to water.
If life were to exist on Neptune's surface, it would need to tolerate temperatures as low as 55 Kelvin. At this temperature, liquid water is out of the question.
However, as you travel towards Neptune's interior, temperatures do increase to more comfortable levels, although at the cost of increased pressures. To this end, there could very well be a point inside the planet where water remains as a liquid, and life could exist inside it.
Given what we know about Neptune, this would likely be hundreds of kilometers below the surface and would be impossible for us to study. Until we develop probes that could ever penetrate and observe these parts of Neptune, we'll have to simply speculate about whether life exists there or not.
Right now, scientists don't know if there's any life on Neptune, and the conditions on the planet seem very hostile for life. It's unlikely we'll ever find any there.
And that, Neptune-nerds. is your lot for today.
The most recently discovered and most distant planet in our Solar System, we have more questions than we have answers about Neptune. Since it is so far away, we will probably have to wait a very long time before many of the holes in our knowledge of it could ever be addressed.
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