Professor Dave here, let’s look at Neptune.
Neptune is the last of the gas giants, and the last of the planets in general.
It is very far from the sun at about 30 astronomical units, about as far from Uranus as Uranus
is from Saturn.
Neptune is quite similar to Uranus, nearly identical in size and mass.
It is deep blue in color, and is thus appropriately named after the Roman god of the sea.
Also similarly to Uranus, Neptune has an atmosphere rich with hydrogen above a layer of water,
methane, and ammonia, around a core of iron and rock.
One difference between the planets is that Neptune has distinct cloud belts, more closely
resembling the surface of Jupiter.
We have even seen a great dark spot, which is a huge atmospheric vortex similar to Jupiter’s
great red spot, although it is not a permanent feature, with similar spots forming and dissipating
every few years.
These features are due to convection currents generated by heat emanating from the interior
of the planet, which in turn cause fast winds of up to 2200 kilometers per hour, the fastest
winds recorded anywhere in the solar system.
Neptune has rings just like the other gas giants, although they are rather narrow, most
likely comprised of debris from collisions between small moons or comets, as they contain
a lot of dust.
Speaking of moons, the Neptunian system has fourteen of them, all named for minor water
deities from Greek mythology.
The largest of these by far is Triton, an icy world which strangely orbits Neptune in
the direction opposite Neptune’s rotation, which we call a retrograde orbit.
While Neptune spins one way, Triton goes the other, and with a significantly tilted orbit.
This indicates that Triton was probably not formed form the same matter that became Neptune,
but rather was captured by Neptune as it moved past.
This event would also have disturbed any other moons in the system at the time, which explains
the irregular orbits of other moons like Nereid, which is highly elliptical, causing Nereid
to travel further from its planet than any other moon in the solar system.
Triton, like Titan, has an atmosphere of predominately nitrogen as well as small amounts of other
gases like methane and carbon monoxide.
In general, Neptune has seven regular moons, which are the ones exhibiting prograde motion
within the equatorial plane of Neptune, just like we have come to expect from moons.
That means the other seven are all irregular moons, including Triton and Nereid, because
of their inclined and often retrograde orbits, indicating that these objects were all captured
by gravity.
Triton is the largest irregular moon in the solar system by at least a factor of ten.
So where did Triton come from?
To answer this question, we have to go a little further and talk about the objects that lie
beyond all the planets, and even some objects in between, so let’s move forward and talk
about the key features of the solar system that are not the planets themselves.