If you were to look up into a clear
night sky with your naked eye, what
planets would you see? Technically, if
conditions were right and you had very
good eyesight, 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 icy giants and
also the furthest away. It's a wonder we
found it at all. And it is a place of
wonders.
Ginormous storms that rise and fall,
super fast winds that are almost
supersonic.
Icy rings and enigmatic moons hinting at
a violent past. Of all the planets, it's
the one we know the least about. But
what exactly do we know about Neptune?
I'm Alex Mccoan and you're watching
Astramm. Today in this super cut, we're
going to delve into everything you could
want to know about Neptune.
Let's start right at the beginning with
how we found Neptune in the first place.
Because unlike every other planet in the
solar system, Neptune was not discovered
by merely being spotted with a
telescope. Its story is far more
intriguing. Neptune is the only planet
found through mathematical prediction.
It happened in 1846
thanks to Obuban Jean Joseph Leferier, a
French astronomer and mathematician.
Leferier had been examining the rotation
of the then seven planets when he
noticed that something was off about
Uranus. It didn't seem to be moving in
line with the theories of gravity
proposed by Newtonian physics.
The very reasoned that the only
explanation for this was that there must
be a planet out beyond the orbit of
Uranus that was perturbing its motion to
account for this discrepancy.
He did some maths and then declared that
his eighth planet must exist at an exact
point in the night sky. The incredible
thing was after hearing his prediction,
Aar's friend Johan Godfrieded Gala got
out a telescope, searched in that
location and sure enough discovered the
planet Neptune in a single hour. It was
almost exactly where Leier had predicted
that it would be.
Triton, Neptune's biggest moon, was
discovered a few days later. But since
then, Neptune has been a place of
mystery. It orbits at the very edge of
deep space. At certain points of its
year, it can even be further away from
the sun than Pluto. And the only thing
found further out than Neptune are icy
rocks.
The sun is only a 30th of the size in
Neptune's sky. And Neptune receives
roughly 1,000th of the sunlight Earth
does. This dim lighting and vast
distance from us means that Neptune
couldn't be studied from groundbased
telescopes very easily. Neptune is the
eighth and furthest planet from the sun.
It orbits at about 30 astronomical
units, which in other words is 4.5
billion km and means it's 30 times
further from the sun than we are. 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. All in all, our icy
neighbor is certainly difficult to
study. It is hard to spot and the
alternative traveling to it with a
spacecraft is not exactly easy either.
In fact, only one spacecraft has ever
visited Neptune and that craft was only
able to do so by taking advantage of a
rare alignment of the planets that only
happens once every 175 years. That craft
was Voyager 2. It wasn't until 1989 when
Voyager 2 arrived that huge amounts of
information about the planet became
available.
On the 25th of January 1986, Voyager 2
departed Uranus and snapped this
wonderful goodbye shot from 1 million km
as it set off to its final planetary
target, Neptune, the last destination on
Voyager 2's grand tour of the solar
system. And after 3 years of travel at
the speed of 54,000 km per hour, Neptune
finally came into view. Here are the
first closeup images we ever received of
the giant blue planet passing only 5,000
km above its north pole, the closest of
any flybys.
Suddenly, we could see what the planet
looked like, confirmed it had planetary
rings, and discovered a lot of
previously unknown moons. Neptune's
distance from the sun means the average
temperature in Neptune's atmosphere is
very cold, minus 201° C. Its axle 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 2. The big difference
being these four seasons last 40 Earth
years each. At this moment in time, the
Southern Hemisphere is experiencing
spring. During this spring, the Southern
Hemisphere receives more sunlight and
appears brighter. This increase in
brightness is actually quite noticeable,
which is strange as you would have
thought that 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 increased sunlight
levels in the southern hemisphere warm
it up by about 10° C compared to the
rest of the planet. This comparably
higher temperature releases frozen
methane into the stratosphere causing
its increased brightness. Whereas
elsewhere on the planet, it remains
frozen and stays deeper in the
troposphere.
Hydrogen was found to be the most common
element in Neptune's atmosphere making
up 80% of it. Although the high
abundance of methane, sitting at 19% is
what gives the planet its blue
appearance. Voyager 2 measured
extraordinary wind speeds in Neptune's
atmosphere with the equatorial winds
blowing at speeds reaching a staggering
2,160
kmh nearly a supersonic flow. And
interestingly, most wind travels
retrograde to the rotation of the
planet. These remarkable speeds were yet
another surprise and highlighted just
how dynamic and ferocious Neptune's
weather systems are. This planet is home
to colossal storms. When Voyager 2
passed by in 1989, it saw the Great Dark
Spot, a storm about the size of Earth
passing through its atmosphere. This
turbulent storm seemed to be rotating
counterclockwise just like the Great Red
Spot on Jupiter and exhibited winds
reaching up to 2,400
km hour, the strongest recorded in the
solar system. One NASA analyst, Ken
Bolinger, commented on the findings in
1989, saying, "Every day, what you see
is brand new. Nobody's ever seen it.
It's just an incredible feeling. There's
changes going on constantly on Neptune
that happen very, very fast.
Voyager also saw the smaller storm known
as the small dark spot south of its big
sibling. As Voyager 2 approached
Neptune, this smaller storm changed in
shade from dark to light.
While our knowledge of Neptune's weather
systems is limited, it would be more
limited still were it not for Hubble
observing them from time to time. 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 completely disappeared
and other storms have come and gone ever
since. The latest one was seen in 2015.
It has lasted a few years, but it too is
disappearing.
While there is not enough data to
speculate how these vortices develop, it
could be that Neptune is like Jupiter
with bands in the atmosphere. While they
won't be as defined or as many as there
are on Jupiter, the bands on Neptune
would travel at different speeds. This
could cause vortices to appear where the
bands meet. Once the storm has got
going, it can drift around the planet,
even between the bands. But once it
leaves its power source, it begins to
slowly diminish, which is what we have
seen.
Interestingly, Hubble is the only
program that can monitor these weather
changes as in most light wavelengths,
they are very hard to spot. Hubble
though can probe Neptune and Uranus in
the ultraviolet, making these
observations possible.
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
speeds on Uranus are fast, too. It
doesn't compete with Neptune at only 900
kmh.
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.
The answer may lie deep beneath
Neptune's surface.
I mentioned that Neptune is the furthest
planet from the sun. So, you would have
thought it's 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 crashed into Uranus
billions of years ago, which depleted
all of its primordial heat. Astronomers
now theorized that 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.
There is a liquid mantle of water,
ammonia, and methane ices surrounding
the core. And 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,
but there could be a liquid carbon ocean
with solid diamondbergs floating in it
and diamonds raining down through the
mantle like hailstones. This is just a
theory though as technology has only
recently started to recreate such
pressures. Around the core of Neptune is
thought to be 7 million bar or 700 gigap
pascals which is about 7 million times
the pressure of Earth's atmosphere at
the surface.
The two ice giants magneettospheres also
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 axle
tilt. But then it found the same thing
out about Neptune, which has a more
normal axle tilt. So the current theory
is that 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 its
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 actually is. But it's only Uranus
and Neptune that have such a tilted
magneettosphere.
Aurora do exist on Neptune 2, but they
are different from 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 gets these
too, but they are not visible and you
need scientific instruments to know that
they are there. They could be stretching
across the whole sky without you
actually knowing about it. Another
difference with the SAR arcs of Neptune
is that they are not only found around
the poles, but rather are around the mid
latitudes of the planet.
However, although they are faint, we
recently were able to capture images of
them thanks to the powerful near
infrared detection of the James Webb
Space Telescope. In March of 2025, Web
released these shimmering images of
Neptune's mid latitude aurora shining
out across the planet. Interestingly, at
the same time, Web was able to notice a
massive temperature drop in Neptune's
upper atmosphere. Its temperature had
dropped by hundreds of degrees since
Voyager's reading in 1989, almost
halfing during those intervening few
decades. What this means for the
turbulent processes taking place within
Neptune remains open for investigation.
But as a harder to detect in colder
temperatures, this may help explain why
Neptune's aurora remain so elusive.
Zooming out from Neptune a bit, we come
to its ring system. Up until 1986,
scientists suspected the planet might
have rings, but couldn't be certain. It
turns out that like all other gas
giants, Neptune does have a ring system,
although it is very faint as it is not
as dense and is extremely dark in color.
If you have these rings against a black
backdrop of space and also have them be
this far away from the sun, then they
are very hard to see. Intriguingly,
Voyager 2 identified several partial
ring structures or ring arcs within
Neptune's ring system. These arcs raised
questions about the mechanisms
responsible for their formation and
stability since they mainly consisted of
incomplete and dusty rings.
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 Gallay ring which
is very faint and very wide at 2,000 km.
Next is the first bright ring, Leferier.
Although it's bright, it's only 113 km
wide. Next and connected is the lasso
ring, a very faint band 4,000 km across.
On the edge of this ring is the Arago
ring. It is slightly brighter than the
lasso ring and less than 100 km wide.
Lastly is the outermost and the most
research ring, the Adam's ring. It is
only 35 km wide, but is one of the
brightest rings. It is particularly
interesting as it is slightly inclined
and has bright arcs in it. These arcs
have been quite stable since they were
discovered in 1980. But usually
planetary rings are uniform throughout.
These arcs must be material clumping and
clustering up within the ring. But the
reason for this is currently unknown.
Lastly, a visit to Neptune wouldn't be
complete without talking about its
moons. Neptune has 14 known moons which
are named after water deities in Greek
mythology. Seven of these moons are
regular moons or in other words moons
that orbit along Neptune's ecliptic with
very circular orbits or orbits with very
low eccentricity.
After these inner regular moons, we get
to the irregular moons. The first of
which is Triton. I personally think it
is one of the prettiest moons in our
solar system as it has amazing patterns
and this burnt orange color. An
irregular moon is a moon that follows an
inclined eccentric and often retrograde
orbit. This by itself is already where
Triton is set apart from any other
spherical moon in the solar system. It
has an irregular orbit.
Triton orbits clockwise around Neptune
as Neptune rotates counterclockwise
and Triton orbits at a 130° angle to the
ecliptic of the planet. Although it
should be noted that its orbital
eccentricity is close to zero. Its orbit
is almost perfectly circular.
All other large moons in the solar
system are regular moons orbiting the
same direction as the rotation of their
parent planet. What this heavily implies
is that Triton did not form alongside
Neptune, but it is in fact a captured
object, specifically a captured dwarf
planet.
No wonder then, that it is by far the
biggest of Neptune's 14 moons,
comprising 99.5% of the mass found in
Neptune's orbit.
But how big is that in scales we can
relate to? Well, it is the second
largest moon in relation to its parent
planet, second only to Earth and its
moon. While it is smaller than our moon,
it orbits closer to Neptune than our
moon orbits Earth, which means it
appears about the same size in the sky.
It is the seventh largest moon in the
entire solar system. And most
interestingly, it is bigger than Pluto.
Pluto is often considered the king of
the Kyper belt, the biggest object that
we know of that formed there until we
consider that Triton once ruled that
area before Neptune captured it. So
although Triton is a moon of Neptune, it
could be said that it is the biggest and
most massive Kyper belt object.
Further evidence for this was found as
New Horizons passed Pluto in 2015,
suggesting Triton and Pluto share a near
identical composition, which supports
the theory that they share a common
origin. Beyond Triton are six other
irregular moons found much further out.
They are almost certainly captured
objects, too, with unusually eccentric
orbits that take years to complete. They
were probably perturbed into these weird
orbits by the gravity of Triton.
So if Triton was a captured object, how
did that happen? Objects need to lose
momentum to be captured, otherwise they
would have enough momentum to escape.
Well, we can't know for sure, but the
leading theory right now is that Triton
was once part of a binary system,
perhaps like Pluto and Keron. As Neptune
approached Triton and its moon, the
gravity from the encounter would have
caused the binary system to fall apart
with Triton's moon being slingshot away
and Triton losing enough momentum to be
captured in orbit around Neptune.
I mentioned that Triton shares similar
characteristics with Pluto. What exactly
does that entail? Well, they both have a
predominantly nitrogen ice surface with
other ices mixed in like water and
carbon dioxide. It has quite a flat
terrain. Its topography never varies by
more than a kilometer. Although Voyager
2 did see ridges and troughs, plateaus
and ice plains.
What you may find unusual though is that
it has very little in the way of
craters. This implies its surface is
very young and is constantly being
renewed.
Like Pluto, it also has some reddish
patches, which is thought to be methane
having reacted to UV light from the sun,
producing what is known as tholins, an
organic compound that has a supposedly
tarlike consistency.
While organic compounds do not mean life
is present there, organic compounds are
the basic chemicals from which life
forms. Life likely couldn't exist on the
surface of Triton anyway, as it's far
too cold and the sun far too dim to
support any life form that we can
imagine. But what's interesting is what
could be found under Triton's crust.
Under Triton's surface is thought to be
a rocky and metallic interior, which
gives Triton a reasonably high density
for a moon at 2 g per cime cubed.
Because of this and also due to the big
step up in size from the next biggest
moon in the solar system, Titania, it
has more mass than all the moons smaller
than it in the whole solar system
combined. The radioactive decay from the
rocky core could be enough to heat and
power convection in a subsurface ocean
of water, much like what is thought to
be under the surfaces of Europa,
Enceladus, and some other large moons in
the solar system. And just like Europa
and Enceladus, cryovalkcanism is an
active process today on Triton. Liquid
water in the mantle erupts onto the
surface like lava on Earth. This is the
main reason why the surface is so young.
It is being actively renewed by liquid
water erupting and then freezing on
Triton's surface.
Some very young lava planes have been
identified. Sparse and flat regions yet
interestingly with a wall that surrounds
the plane. This is called a plenicia or
in other words a solidified lava lake.
We also can see cauldra which is the
collapse found at the center of a cryo
volcano where lava planes formed from.
It is thought that the water from these
eruptions would have also brought
minerals from the underground oceans
onto the surface. Perhaps even being the
source of the tholins and organic matter
I mentioned earlier. If this is the case
and organic compounds are found in the
subsurface ocean, it means that there's
a possibility that conditions are right
for life to have been able to form
there. We also see long lines permeating
over the surface. These are likely
faults caused either by tectonic
activity or freeze thor weathering
processes.
If we look at some more of the Voyager 2
images of Triton, we can see the results
of some recent eruptions. You'll notice
what looks to be dark deposits on the
surface in cone or funnel-like shapes up
to 150 km long. However, these smaller
eruptions may not originate from the
mantle itself.
Voyager 2 spotted some plumes reaching 8
km high, but these are thought to be
because of a solid greenhouse effect
within the moon's icy crust.
Imagine the surface of Triton consisting
of a clear ice which has settled on dark
deposits like tholins. The sun shines
through the ice, warming the darker,
more absorbent tholins beneath, which
sublimates a pocket of ice under the
surface. As the ices sublimate, the
pressure builds in the air pocket until
the surface above the pocket gives way,
causing an eruption. This eruption also
takes the dark deposits with it,
spreading them out over the surface
again. If this is the case, a very
similar process has been seen on Mars's
poles with carbon dioxide ices and
darker deposits under the ice layer.
This process can only exist because of
one thing. Triton has an atmosphere,
although not as thick as scientists were
initially expecting. Triton's atmosphere
is thin, only 0.014 mibars, about the
equivalent of 80 km up on Earth.
Although, like Pluto, this density
varies through seasonal changes.
Since Voyager 2's observations, Triton's
atmosphere has become denser as the
surface has warmed, evaporating a little
of the nitrogen ices on the surface.
However, when Voyager 2 passed, Triton's
atmosphere was still dense enough to
support weather up to 8 km above its
surface. In this image, you can actually
see clouds on Triton.
And going back to the image of the
plumes, you can see the deposits from
the eruptions all end up facing the same
direction due to a prevailing wind in
that region of the moon.
Like Pluto, Triton's atmosphere is hazy.
The cause of which is thought to be
hydrocarbons in the atmosphere, not yet
broken down into tholins by UV light
from the sun. The constant depositing of
organic compounds through
cryovalkcanism, ice evaporating and
freezing again through seasonal
variations, and a weather rich active
atmosphere makes Triton a very dynamic
world. All of these factors combined
make it one exceptionally unusual moon.
It is more a dwarf planet than a moon.
But what about the others? The next
biggest moon is Proteus, which is a
little irregular in its shape. Normally,
we only see this on smaller objects like
asteroids, but Proteius is actually
bigger at 400 km across than the
spherical moon of Saturn, Mimis.
Why it is not a sphere is explained by
past collisions of things hitting the
moon leaving these massive craters which
you see. The inner regular moons orbit
around the rings, some acting as
shepherd moons. The outer irregular
moons are all likely captured moons.
Some of the irregular moons orbit
prograde and others retrograde. The
outermost moons of Neptune are Samatha
and Neto and are the furthest out
satellite of any planet that we know of
to date. They take a massive 25 years to
orbit Neptune only once. This is because
Neptune has a very large hillphere. The
hillphere being the sphere in which the
planet's gravity overcomes the gravity
of the sun. It has such a large
hillphere 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.
One of the major discoveries Hubble made
about Neptune was the discovery of a new
moon in 2013, which has now been named
Hippoamp.
Now, Hubble has discovered many moons in
its time, especially around Jupiter and
Saturn. But what makes Hippoamp special
is that it could well be a fragment from
the much larger moon of Neptune,
Proteius.
The 400 km wide moon does indeed look
like it had a tumultuous past with giant
impact craters 50 to 100 km in diameter.
One of these collisions likely
fragmented parts of Proteus which then
fell into orbit around Neptune.
Hippocamp is probably the biggest
fragment as it's an irregular 35 km long
object and orbits fairly closely to the
larger Proteus.
Finally, there is one last object that
shares a strange relationship with
Neptune. Not quite a moon, but certainly
something that it captured and
influenced by its gravity. 2020 VN40 is
a trans Neptunian object that orbits the
sun exactly once for every 10 orbits
Neptune completes. For point of
reference, that means it has an orbital
time around the sun of 1,650
years. While Neptune orbits every 165
years, 2020 VN40's orbital plane is
highly inclined, but its precise 10:1
ratio showcases that it is in something
known as an orbital resonance with
Neptune, captured in a delicate
gravitational dance with the larger
planet. This finding helps showcase the
farreaching influence Neptune has on the
wider solar system.
So there you have it. Neptune is a
fascinating world that plays an
important role in the gravitational tug
of war at the extreme edges of our solar
system. Along with Uranus, it's the
planet we know the least about as we
have only visited it once and glimpsed
it with telescopes infrequently.
There's still so much for us to learn
about its weather, its internal
structure, and its moons. Sadly, there
are no confirmed missions planned to
return to this Azure world, and given
NASA's looming financial difficulties,
none are on the horizon. On the other
hand, China is considering a proposal
for a Neptunian probe that could be
launched as soon as 2033.
If it goes ahead, who knows what
discoveries will be uncovered.
I hope we do see a new mission go ahead.
Neptune has so much more story to tell.
And although it is extremely difficult
to access, sometimes the best things in
life are the ones that seem just out of
reach. And for us, there is no planet in
our solar system harder to explore than
this, the final planet, Neptune.
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