Two days ago on the 4th of July, 2016,
the Juno spacecraft entered orbit
around the planet, Jupiter after a five-year journey from Earth.
Why do we care?
Because Jupiter is the second most important planet
in the universe, at least from our point of view,
and it holds the secrets to the formation of the solar system.
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Jupiter is a monster.
It's by far the largest and most massive thing
in our solar system after the sun.
It may only have 1,000th of the sun's mass,
but it weighs more than all the other planets combined,
two and 1/2 times more.
Jupiter's enormous gravity influences
the orbit of all the planets in the solar system.
In fact, its effect during the first billion years
of the solar system's formation defines the positions
of all planetary orbits.
Today, we're going to see why, without Jupiter, Earth as we
know it would not exist.
But first, let's talk about the planet itself.
Jupiter was probably the first planet
to start forming in our system.
It likely grew as fragments of rock and ice
clung to each other in the disk of debris and gas
surrounding the infant sun, the protoplanetary disk.
It grew to at least 10 and maybe over 40 times
the mass of the Earth before it had enough gravity
to start holding on to the 300-ish Earth
masses of hydrogen and helium that make up most of the planet
today.
That solid core now comprises, at most,
15% of the planet's mass.
The bulk of Jupiter is thought to be
in the form of metallic hydrogen fluid,
a strange theoretical state of hydrogen existing only
at extreme densities.
The conductivity of metallic hydrogen
is thought to result in the enormous electric currents that
produce Jupiter's prodigious magnetic field.
It's 20,000 times the strength of Earth's field,
giving Jupiter the brightest auroras in the solar system.
Jupiter patrols the outer solar system
at an average distance of 5.2 astronomical units.
That's 5.2 times Earth's average distance from the sun.
Its ponderous orbit takes around 12 years.
In the wake of its intense gravitational field,
it drags with it its own mini solar system
of at least 67 moons and a faint ring system.
That gravitational influence extends
through the entire solar system.
Earth's own Milankovitch cycles, periodic changes
in Earth's orbit and spin, are largely
driven by Jupiter's influence and give us
our cycles of glaciation.
Even the sun feels Jupiter's tug.
The center of mass of the sun-Jupiter system
lies just above the solar surface, and both the sun
and Jupiter circle this point.
And then, of course, there's Jupiter role as cosmic Roomba.
Its powerful gravity clears up some
of the most dangerous comets before they
could collide with the Earth.
I'm talking about the rare long period
comets that orbit the sun with periods of hundreds
to many thousands of years.
But Jupiter is less like a loyal hound guarding our door.
It's more like a giant monster stalking
the edge of the garden of the inner solar system.
It keeps away some dangerous intruders,
but is itself not safe by any means.
Although it probably intercepts some
of the most worrying comets that cross its orbit,
it also perturbs the orbits of asteroids in the asteroid belt
and can send them plummeting towards the Earth.
But the havoc caused by Jupiter now
is nothing compared to the earliest
days of the solar system.
Jupiter probably didn't form in its current orbital location.
See, the early solar system was a messy place.
At first, there was the gassy, dusty protoplanetary disk,
and then a mess of asteroids and planetesimals
left over from the formation of the planets.
There were no stable orbits in this sort of environment.
A planet can lose angular momentum to the debris,
causing its orbit to shrink.
The planet migrates inwards in a similar way
to a satellite falling to the Earth due to dragging
against the upper atmosphere.
The planet can even migrate outwards,
stealing angular momentum from the debris.
Astrophysicists have run many computer simulations
to find formation scenarios that lead to the configuration
of planets that we see today.
There's more than one possibility,
because we don't know exactly how the formation of planets
started.
However, almost all scenarios have
Jupiter rampaging through the protoplanetary disk
before settling into its current orbit
or driving other planets to do the same.
We're going to outline a possible scenario
for the first billion years of the solar system.
It combines two of the strongest contenders of formation models.
One possible scenario for the first half billion years
is the Grand Tack Hypothesis.
In this model, Jupiter first formed closer to the sun
at around 3.5 astronomical units.
With these first millions of years of planetary life,
it was still embedded in the thick protoplanetary disk.
Drag from that gas would have sapped angular momentum
from Jupiter, causing it to migrate inwards until it
stalled at around 1.5 AU.
Saturn would have quickly followed until it fell
into orbital resonance with Jupiter,
three of Saturn's orbits to every two of Jupiter's.
This changed the way the planets interacted with the surrounding
disk, and, in fact, they would then have slowly
migrated back outwards until Jupiter
reached its current position.
This change in direction is the tack in the grand tack.
Why propose such a complicated scenario?
Well, computer simulations show that something like this
may be expected given the right starting position for Jupiter.
But it also solves several problems
with the way our inner solar system is currently laid out.
Most simulations predict that a protoplanetary disk
capable of forming Venus and Earth
should also have formed another massive planet, at least half
of Earth's mass in Mars's location.
But Mars is a measly 10% of Earth's mass.
The idea is that Jupiter lumbered
through that part of the protoplanetary disk,
clearing it of a lot of the material
it needed to build another large planet in Mars's location.
Another clue is that other real exoplanetary systems tend
to have super Earths, rocky planets several times
the mass of our own planet.
Our solar system has none.
Jupiter's early rampage into the inner solar system
would have sent such planets spiraling into the sun,
leaving only a limited amount of material
to then build the current crop of terrestrial planets.
The asteroid belt is also an interesting clue.
It's comprised of a mixture of materials
from both the inner and outer solar system,
and that's exactly what you might
expect if Jupiter passed through that region in the early days.
The Grand Tack Hypothesis is still just that-- a hypothesis.
Slightly more accepted, although still unproven,
is the scenario of the Nice Model.
The events it describes starts after the protoplanetary disk
had evaporated, so chronologically,
after the Grand Tack, although the Nice
Model was proposed earlier.
The solar system at this point had all its planets-- eight,
maybe nine of them-- but also a huge amount of left over junk.
However, the Nice Model posits that the orbits of the four gas
giants were much more tightly clustered back then.
Jupiter would be roughly in its current location
after any early destructive wanderings
in the first half billion years, but Saturn, Uranus, and Neptune
were all closer in than their current locations.
Beyond Neptune was a vast sea of planetesimals, many thousands
of tiny, icy worlds.
At some point, Saturn's orbit once again fell into resonance
with Jupiter's, but now two Saturnian
orbits to every one Jovian.
This amplified Jupiter's gravitational influence
on Saturn, causing Saturn's eccentricity to increase.
This, in turn, pushed Uranus and Neptune further out
to their current locations.
As the three outer gas giants plowed through the great field
of planetesimals, they scattered this material
through the solar system.
This is hypothesized to have caused the massive meteor
activity of the late heavy bombardment,
re-liquefying Earth's newly solidified crust.
Oh, and it may also have nearly ejected Planet Nine, which
we may have recently found lurking far, far out
beyond the Kuiper Belt.
So that's where we are.
We have a tiny Mars, no super Earths,
a weird, mixed up asteroid belt, and an early obliteration
of Earth's surface, and, perhaps,
a missing ninth planet.
Thanks, Jupiter.
Now the scenarios I've laid out are by no means confirmed.
Whether the solar system's formation really happened
this way or by some completely different scenario,
we can be sure that Jupiter's influence
on the early solar system is the key to how
it got to its current state.
One giant unknown is where Jupiter first formed.
Answer that, and we can nail down these simulations
to truly understand our planet's origin.
The key to finding Jupiter's birth orbit
is in its internal composition, and that
is one of the things the Juno spacecraft was
sent to discover.
Stay tuned.
We'll keep you up to date right here on Space Time.
Guys, if you think Jupiter is cool,
you have to check out our buddies over at Shanks FX.
They've created the most awesome model
of Jupiter's weird atmosphere.
See how they did it.
Also, an announcement-- I'm going to be traveling,
so we'll have to skip next week's release.
Sorry.
We'll be back with Astro News and the Challenge Answer
the following week.
We recently talked about the origin of quantum theory
with Max Planck's derivation of an equation
to describe the black body spectrum.
Let's see what you guys had to say.
So many of you, including Neil deGrasse Tyson,
point out that the sun is actually white and not yellow.
Well, the sun usually appears white to our eyes
and so to our brains, and since the entire concept of color
only exists in our brains, it's reasonable to say
that the sun is truly white.
But this observed whiteness is a result of the fact
that our color sensitivity is limited.
The sun does emit a bit more light in the yellowy green part
of the electromagnetic spectrum compared
to the blue and the red parts on either side.
This is due to its black body spectrum
peaking in that yellowy green region.
A more color sensitive eye or electronic detector
would see the sun as yellowish green.
Our eyes can't register the difference in brightness
across the visible part of the spectrum,
so we register the color as white.
A few of you rightly know that quantum mechanics
isn't needed to resolve Zeno's paradox of Achilles
and the tortoise.
So I said that the assumption of infinite divisibility of space
was a problem with the scenario in this paradox,
that the real resolution is that there are
different types of infinity.
Zeno complains that to catch a tortoise,
Achilles needs to complete an infinite number of tasks,
closing the remaining gap every time
he reaches the tortoise's last position.
However, it doesn't need to take an infinite amount of time
to complete an infinite number of tasks,
as long as the time taken for those tasks gets smaller.
For example, if the time taken to catch the tortoise is,
say, half a minute at first, and then the time
halves at every step, then you can write that total time
series as 1/2 plus a 1/4 plus 1/8
plus a 1/16 of a minute, et cetera.
That's what we call a convergent series,
because it doesn't add up to infinity,
even if you do an infinite number of these additions.
In this case, it adds up to one-- well, one minute.
That's how long Achilles takes to complete the infinite steps
and overtake the tortoise.
Many of you noticed that the tortoise
we used in Zeno's paradox is actually a turtle,
and not just any turtle, but Great A'Tuin, the Giant Star
Turtle, bearer of Discworld.
Well spotted.
As a physicist, I usually approximate everything
as a sphere, including planets, so
approximating a tortoise as a turtle is actually pretty good.
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