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.