Professor Dave again, I wanna tell you about
the solar system.
We now know a lot about how stars and galaxies
form, including our own Milky Way galaxy,
so now it’s time to talk a little more about
our location within this structure.
If we zoom in on this portion of the Orion
arm, fairly distant from galactic center,
we see a yellow main sequence star.
This star is not special as far as stars go.
It’s not very big, at precisely one solar
mass, serving as the definition for the unit.
In fact, it’s rather on the small side as
far as stars go.
The only thing that makes this star special
is that it’s ours.
We live on a planet that orbits this star,
and we call it “the sun”.
The sun, which is a population one star, formed
around 4.6 billion years ago from a cloud
of gas and dust, which was rich in heavy elements
that were introduced to interstellar space
when older population three and two stars
spewed out their contents during supernovas.
This cloud, with all of its hydrogen, as well
as silicates, iron, water, and other substances,
began to spin and flatten into a disk, just
like galaxies do, but in this case, we call
it a protoplanetary disk.
The bulk of this cloud came together at the
center, gravity squeezing with such force
that fusion began, powering the newly formed
sun.
Meanwhile, the remaining material was distributed
at varying distances away from the axis of rotation.
Then, these tiny dust particles began to collide
and stick together.
These larger pieces continued to collide with
others to form rocks, and then larger objects
called planetesimals, increasing their gravitational
influence as they grew, eventually attracting
other planetesimals.
Over hundreds of thousands of years, all of
these clumps of rock and ice continued to
collide and accumulate, with every impact
generating lots of heat, keeping things relatively molten.
As most of the surrounding debris at each
particular orbital distance was cleared, these
objects became massive enough to take on a
spherical shape, under the influence of their
own gravity.
These are the inner rocky planets we know today.
For the gas and dust that made up the outer
regions, this also collected into spheres
due to gravity, though beyond just iron and
rock, these also contained lots of ice, given
the colder temperatures present at this greater
distance from the sun.
Once large enough, they could also attract
much of the gas in their vicinity, so these
became the gaseous planets we call the gas giants.
Planetesimals that didn’t make it into a
planet accumulated into moons, or remained
in the form of smaller objects like asteroids
and comets, and other loose matter collected
into rings around the larger planets.
And thus, the solar system was born.
Things have not been completely static since
this formation.
Objects are jostled around all the time, like
a few billion years ago when Jupiter and Saturn
lined up in such a fashion that Neptune’s
orbit altered, in turn sending small objects
in the outer solar system towards the inner
planets, raining down on them in an event
called the Late Heavy Bombardment.
But for now, let’s focus on the solar system
as we see it today.
Later in the series, we will devote an entire
chapter to each major body in the solar system,
but let’s just briefly describe some of
these objects right now.
First, there is the sun.
This is a typical G star, a yellow main-sequence
star, as we mentioned.
It is a population one star as it is relatively
new, having formed from a cloud of gas and
dust which contained significant amounts of
material that had been ejected from the death
of other, older stars.
Its photosphere, or outermost, visible layer,
burns at around 6000 Kelvin, while the hot
inner core burns at around 15 million Kelvin,
unfathomably dense because of the crushing
gravity, but still remaining a plasma due
to the heat.
This is a state of matter beyond the gaseous
state, where it is too hot for electrons to
be coordinated to atoms, so it’s just a
soup of nuclei and free electrons whizzing about.
In between the core and the photosphere is
a radiative zone, where photons radiate outwards,
absorbed and emitted in random directions
for a hundred thousand years before they eventually
find the surface and can move through space.
There is also the convection zone, where material
is far enough from the core that it has an
opportunity to cool as it rises, which allows
it to sink back down, where it then heats
back up, continuing in a cyclical manner.
It may come as a surprise that the sun also
has an atmosphere.
The lower atmosphere is called the chromosphere,
which is relatively cool, and the outer atmosphere
is called the corona, which strangely jumps
back up to a million Kelvin.
We still don’t fully understand how this
material can be so hot, though it is widely
agreed that the sun’s magnetic field is
somehow involved, while some propose that
it is acoustic energy that is responsible.
Along the surface of the sun we can find dark
patches called sunspots, which is where the
magnetic field lines loop out of the sun,
preventing the rise of gas in particular locations,
and thus generating cooler, darker areas.
In addition, there are plumes of plasma called
prominences, which are also generated by the
magnetic field.
The field can also produce solar flares, which
are eruptions of hot plasma in the chromosphere.
Beyond flares, activity in the corona results
in the solar wind, a constant stream of plasma,
or high-energy charged particles, flying through
space in all directions.
This solar wind travels for incredible distances,
the limit of which marks the end of the heliosphere.
This boundary for the solar wind is the limit
of the sun’s influence on its surroundings,
which means it qualifies as the boundary of
the solar system itself, lying extremely far
beyond the outermost planet.
Relative to the Milky Way, the solar system
is minuscule.
If the galaxy were shrunk down to the size
of the earth, the solar system would be the
size of a pancake.
But to now look at the solar system as a whole
and all the objects in it, we see that relative
to all the planets, the sun is absolutely
immense.
Its diameter is more than a hundred times
greater than that of the earth, which means
it would take over a million earths to fill
up the sun.
The sun makes up about 99.86 percent of the
mass of the solar system, so it really calls
the shots, and most of the planets maintain
near-circular orbits around it.
Let’s quickly mention these planets, in
their order from the sun.
Bear in mind that any illustration of the
solar system is nowhere near to scale, as
the planets are incredibly far apart.
But we can use this to simply familiarize
ourselves with their names and approximate
relative sizes, as well as their appearances
and general features.
First is Mercury, the barren planet closest
to the sun.
Venus is next, hot, heavily volcanic, a hellish world.
Then, the third rock from the sun, our little
earth, home to everyone you’ve ever met,
and every place you’ve ever gone.
Just a bit further is Mars, the red planet,
where humans will likely set foot very soon,
making history in the process.
Next is the asteroid belt, which is not shown,
but will be discussed later.
And that marks the end of the inner, rocky
planets.
The first of the gas giants is Jupiter, the
largest of the planets.
After that comes Saturn, with its breathtaking
system of rings.
Then comes the icy planet Uranus, and lastly
the outermost planet Neptune.
Four small balls of rock, and four big balls of gas.
Most of these planets have moons, and there
are lots of other objects in the solar system
that are worth discussing, but we will get
to all of them a little later in the series.
So we can see how at first glance, it may
seem impossible for objects as complex as
Earth and the other planets to form spontaneously.
But even with just a rudimentary understanding
of only two astronomical processes, solar
system formation becomes quite intuitive,
or even obvious.
The first of these processes is the fusion
of heavy elements inside high-mass stars which
are then ejected into the interstellar medium
during a supernova.
The second is the accretion of interstellar
gas and dust to form a protoplanetary disk,
which then slowly accumulates into large spherical
objects at varying orbital radii purely by gravity.
Now the spontaneous formation of the solar
system makes just as much sense as any other
knowledge we can infer from our immediate
surroundings.
We also come to the incredible realization
that every single atom on earth, and therefore
every single atom in your body, other than
hydrogen, was fused inside a long dead star.
All of your carbon, oxygen, nitrogen, phosphorus,
and all the trace metals that make you what
you are, were ejected during the death of
one or more stars, which then floated through
space until it became part of the disk of
matter that originally formed the solar system.
As Carl Sagan so poetically put it, we are
“star stuff”.
This fact only enhances the deep longing we
feel when we gaze up into the night sky.
In a very literal sense, it’s where we came
from, and perhaps we will go back one day.
