Look up into a clear night sky with your naked eye, and what planets could you see?
Technically you would be able to see all of them at one time or another, all of them apart
from Neptune.
It is the smallest of the gas giants, and also the furthest away.
And it is a perplexing place.
You would think a planet so far from the Sun wouldn’t have a dynamic atmosphere that
exhibits ginormous storms and super-fast winds.
So why is this planet as interesting as it is?
I’m Alex McColgan, and you’re watching Astrum, and today we’re going to delve into
everything you could want to know about Neptune.
Let’s start right at the beginning.
Neptune is the only planet found through mathematical prediction.
You see, when Uranus was discovered and astronomers were plotting its orbit, they noticed that
Uranus wasn’t following their models.
From the perturbed orbit of Uranus, Urbain Le Verrier in 1846 concluded that there must be another undiscovered planet, and he predicted where it
should be, and remarkably Johann Galle was able to find it only a degree away from the
predicted point.
Triton, Neptune’s biggest moon was discovered a few days later.
Since then, Neptune was little understood as its distance from Earth and very small
apparent size meant it couldn’t be studied from ground based telescopes easily.
It wasn’t until 1989 when Voyager 2 arrived that a huge amount of information about the
planet became available.
Suddenly we could see what the planet looked like, confirmed planetary rings, and discovered
a lot of previously unknown moons.
But let’s get to today.
What do we know about this planet now?
Since Pluto’s demotion to ‘not a planet’ status, Neptune is the 8th and furthest planet
from the Sun.
It orbits at about 30AU from the Sun on average, which means it’s 30 times further than the
Earth’s orbit from the Sun.
30AU in other words is 4.5 billion km, and from that you can see why it would take a
space probe - using current technology - 13 years to reach Neptune.
4.5 billion km is a considerable distance.
Because of this long orbit, it takes a huge 165 years to orbit the Sun once, which means
we’ve only seen one Neptunian year since its discovery.
This distance from the Sun means the average temperature in Neptune’s atmosphere is very
cold, -201c.
Its axial tilt is 28°, meaning it’s similar to Earth and Mars, which have 23° and 25°
respectively.
This means it has seasons similar to Earth and Mars too, the big difference being these
4 seasons last 40 Earth years each.
At this moment in time, the southern hemisphere is experiencing summer.
And during this summer, the southern hemisphere has actually got brighter, thought to be a
result of interactions with the Sun.
Which is strange, as you would have thought because the Sun is 900 times dimmer on Neptune
than on Earth, from that distance it wouldn’t make much of an impact.
But even if it is only a small impact, it makes an impact nonetheless, and the brightening
of the Southern hemisphere is thought to be due to the southern hemisphere warming up
by about 10c compared to the rest of the planet.
This comparably higher temperature has released frozen Methane gas into the stratosphere,
whereas elsewhere on the planet it remains frozen and stays deeper in the troposphere.
Just a quick recap of the spheres of a planet, the troposphere is the lowest atmospheric
layer, followed by the stratosphere.
Above those layers are the Mesosphere, the Thermosphere and then the Exosphere.
That’s a very interesting topic in itself but we’ll save it for another video.
If you look at the weather on Neptune, it actually has the fastest wind speed of any
planet, with wind speeds blowing westward on the equator reaching a staggering 2,160kph,
nearly a supersonic flow.
And interestingly,
most wind travels retrograde to the rotation of the planet.
Bands are also formed on the planet, as well as colossal storms.
When Voyager 2 passed the planet by in 1989, it saw the Great Dark Spot, a storm about
the size of Earth passing through its atmosphere.
It also saw the smaller storm called Small Dark Spot, south of its bigger sibling.
As Voyager 2 approached Neptune, this storm changed in shade from dark to light.
When Hubble was launched, astronomers were curious to see the fate of these storms to
see if they were a permanent feature like Jupiter’s Great Red Spot.
But when Hubble was pointed at Neptune in 1999, these storms had disappeared, and storms
have come and gone ever since.
Giant, bright, high altitude clouds also come and go.
But why then doesn’t Uranus, which is very similar in composition and size to Neptune,
also have such a blustery atmosphere?
Don’t get me wrong, wind speed is fast on Uranus too, but it doesn’t compete with
Neptune at 900kph.
Can all this only be due to interactions with the Sun and its seasons?
Something else must be at play here to explain the extremes in weather.
I mentioned that Neptune is the furthest planet from the Sun, so you would have thought it
is also the coldest.
But actually, Uranus is the coldest planet in our solar system.
Neptune radiates heat from within, whereas Uranus radiates hardly any excess heat at
all.
This could be because a large Earth sized body crashing into Uranus billions of years
ago which depleted all of its primordial heat.
Now, the more active weather on Neptune might be due, in part, to this higher internal heat.
What is Neptune actually made of then?
Its internal structure and atmosphere is thought to be very similar to Uranus.
Its atmosphere is composed mainly of 80% hydrogen and 19% helium, with very small amount of
methane.
It’s this methane that gives Neptune its blue colour, although it’s a dark shade
of blue compared to Uranus’ cyan.
Again, like Uranus, there is a liquid mantle of water, ammonia and methane ices surrounding
the core.
Where the core and the mantle meet, the pressure is so great that the methane may break apart,
and diamonds are formed under the pressure.
Likely not diamonds as you or I know it, but there could be a liquid carbon ocean with
solid diamond bergs floating in it and diamonds raining down in the mantle like hailstones!
This is just a theory though as technology is only just starting to approach recreating
such pressures, around the core of Neptune is thought to be 7Mbar, or 700 GPa, about
7,000,000 times the pressure of earth’s atmosphere at the surface.
Even the two ice giant’s magnetospheres share similarities.
Neptune’s magnetic field is offset 47° relative to its rotational axis.
When Voyager 2 discovered this about Uranus, the first theory was that it had something
to do with its unusual axial tilt, but then it found out the same thing about Neptune
which has a more normal axial tilt.
So, the current theory is the magnetic field is either not generated in the core but rather
by an electrically conducting liquid mantle, or that the mantle deflects the magnetic field from the core
which gives it this weird offset in relation to the rotational axis.
Every planet in the solar system hasn’t actually got a perfectly aligned magnetic
field; even Earth’s magnetic north is different from where the North Pole is.
But it’s only Uranus and Neptune that have such a tilted magnetosphere.
Aurorae do exist on Neptune too, but they are unusual to what you might expect as they
are extremely faint due to particles not getting as charged from the Sun, and because of the
direction of the magnetosphere they are mainly type B aurora, or SAR arcs.
Earth does get these too, but they are not visible and you need scientific instruments
to know they are there.
They could be stretching across the whole sky without you actually knowing about it.
The difference with SAR arcs as well is they are not around the poles, but rather around
the mid latitudes.
Zooming out from Neptune now, we come to its ring system.
Like all the other gas giants, Neptune does a ring system, although it is very faint as
it is not very dense and extremely dark in colour.
Have these rings against the black backdrop of space and also be this far away from the
Sun, and they are very hard to see.
There are five known rings in all, and they’re all named after people involved in the discovery
and research of Neptune.
The innermost is the Galle Ring, very faint and quite wide at 2,000km.
Next is the first bright ring, Le Verrier.
Although it is bright, it is only 113km wide.
Next and connected is the Lassell Ring, a very faint band 4,000km across.
On the edge of this ring is the Arago Ring.
It is slightly brighter than the Lassell Ring and less than 100km wide.
Lastly is the outermost and most researched ring, the Adams Ring.
It is only 35km wide but one of the brightest rings.
It is particularly interesting as it is slightly inclined, and it has bright arcs in it.
These arcs have been quite stable since they were first observed in 1980, but usually planetary
rings are uniform throughout.
The arcs must be material clumping and clustering up within the ring, but the reason for this
is currently unknown!
Lastly, I want to talk about the moons.
Neptune has 14 known moons, which are named after water deities in Greek mythology.
The most famous and the biggest by far is the moon Triton which actually contains most
of the mass of all the moons put together.
I personally think it is one of the prettiest moons in our solar system, as it has amazing
patterns and this burnt orange colour.
What is most interesting about Triton is the fact it orbits in retrograde, and also at
an inclination to Neptune’s rotation, which implies it is probably a captured object and
not something that was formed alongside Neptune.
Triton might be the cause of the rings of Neptune, as it would have disrupted the orbits of moons
possibly causing them to collide and break up into the rubble of the rings.
Triton is bigger than Pluto, and also has a tenuous atmosphere.
Voyager 2 even saw faint clouds on its flyby.
The next biggest moon is Proteus, which is actually bigger at 400km across than the spherical
moon of Saturn, Mimas.
Why it is not a sphere is explained by past collisions of things hitting the moon, leaving
these massive craters.
The inner, regular moons orbit around the rings, some acting as shepherd moons.
The outer irregular moons are all likely captured moons.
Some of them orbit prograde and others retrograde.
The outermost moons of Neptune are Psamathe and Neso, and are the furthest out satellite
of any planet to date.
They take a massive 25 years to orbit Neptune only once.
This is because Neptune has a very large Hill sphere, the Hill sphere being the sphere in
which its gravity overcomes the gravity of the Sun.
It has such a large Hill sphere because it’s already so far from the Sun; the Sun’s gravity
has less of an influence around Neptune than at the biggest planet, Jupiter.
Well thank you for watching.
There’s only one more planet to go now in this series, and that is our home planet,
Earth.
I’ll be sad when the series is over as I’ve really enjoyed researching these planets and
presenting to you guys my findings.
Don’t worry though, there’s more series to start, Our Solar System’s Moons, or Our
Solar System’s Dwarf Planets perhaps?
Let me know what you’ll want in the comments.
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See you next time.