Where did the Moon come from A new theory Sarah T. Stewart
Nobody likes to make a mistake.
And I made a whopping one.
And figuring out what I did wrong
led to a discovery
that completely changes the way
we think about the Earth and Moon.
I’m a planetary scientist,
and my favorite thing to do
is smash planets together.
(Laughter)
In my lab, I can shoot at rocks
using cannons like this one.
(Cannon shot)
(Laughter)
In my experiments, I can generate
the extreme conditions
during planet formation.
And with computer models,
I can collide whole planets together
to make them grow,
or I can destroy them.
(Laughter)
I want to understand
how to make the Earth and the Moon
and why the Earth is so different
from other planets.
The leading idea for the origin
of the Earth and Moon
is called the “giant impact theory.”
The theory states that a Mars-sized body
struck the young Earth,
and the Moon formed from
the debris disk around the planet.
The theory can explain
so many things about the Moon,
but it has a huge flaw:
it predicts that the Moon is mostly made
from the Mars-sized planet,
that the Earth and the Moon
are made from different materials.
But that’s not what we see.
The Earth and the Moon
are actually like identical twins.
The genetic code of planets is written
in the isotopes of the elements.
The Earth and Moon
have identical isotopes.
That means that the Earth and Moon
are made from the same materials.
It’s really strange that the Earth
and the Moon are twins.
All of the planets are made
from different materials,
so they all have different isotopes,
they all have their own genetic code.
No other planetary bodies
have the same genetic relationship.
Only the Earth and Moon are twins.
When I started working
on the origin of the Moon,
there were scientists that wanted to
reject the whole idea of the giant impact.
They didn’t see any way for this theory
to explain the special relationship
between the Earth and the Moon.
We were all trying to think of new ideas.
The problem was,
there weren’t any better ideas.
All of the other ideas
had even bigger flaws.
So we were trying to rescue
the giant impact theory.
A young scientist in my group suggested
that we try changing the spin
of the giant impact.
Maybe making the Earth spin faster
could mix more material
and explain the Moon.
The Mars-sized impactor had been chosen
because it could make the Moon
and make the length of Earth’s day.
People really liked
that part of the model.
But what if something else determined
the length of Earth’s day?
Then there would be many more possible
giant impacts that could make the Moon.
I was curious about what could happen,
so I tried simulating
faster-spinning giant impacts,
and I found that it is possible
to make a disk out of the same
mixture of materials as the planet.
We were pretty excited.
Maybe this was the way
to explain the Moon.
The problem is, we also found
that that’s just not very likely.
Most of the time, the disk
is different from the planet,
and it looked like
making our Moon this way
would be an astronomical coincidence,
and it was just hard
for everyone to accept the idea
that the Moon’s special connection
to Earth was an accident.
The giant impact theory
was still in trouble,
and we were still trying to figure out
how to make the Moon.
Then came the day
when I realized my mistake.
My student and I were looking at the data
from these fast-spinning giant impacts.
On that day, we weren’t actually
thinking about the Moon,
we were looking at the planet.
The planet gets super-hot
and partially vaporized
from the energy of the impact.
But the data didn’t look like a planet.
It looked really strange.
The planet was weirdly
connected to the disk.
I got that super-excited feeling
when something really wrong
might be something really interesting.
In all of my calculations,
I had assumed there was a planet
with a separate disk around it.
Calculating what was in the disk
as how we tested
whether an impact could make the Moon.
But it didn’t look that simple anymore.
We were making the mistake
of thinking that a planet was always
going to look like a planet.
On that day, I knew that a giant impact
was making something completely new.
I’ve had eureka moments.
This was not one of them.
(Laughter)
I really didn’t know what was going on.
I had this strange, new object
in front of me
and the challenge
to try and figure it out.
What do you do when faced
with the unknown?
How do you even start?
We questioned everything:
What is a planet?
When is a planet no longer
a planet anymore?
We played with new ideas.
We had to get rid
of our old way of thinking,
and by playing, I could
throw away all of the data,
all of the rules of the real world,
and free my mind to explore.
And by making a mental space
where I could try out outrageous ideas
and then bring them back
into the real world to test them,
I could learn.
And by playing, we learned so much.
I combined my lab experiments
with computer models
and discovered that
after most giant impacts,
the Earth is so hot, there’s no surface.
There’s just a deep layer of gas
that gets denser and denser with depth.
The Earth would have been like Jupiter.
There’s nothing to stand on.
And that was just part of the problem.
I wanted to understand the whole problem.
I couldn’t let go of the challenge
to figure out what was really going on
in giant impacts.
It took almost two years
of throwing away old ideas
and building new ones
that we understood the data
and knew what it meant for the Moon.
I discovered a new type
of astronomical object.
It’s not a planet.
It’s made from planets.
A planet is a body whose self-gravity
is strong enough to give it
its rounded shape.
It spins around all together.
Make it hotter and spin it faster,
the equator gets bigger and bigger
until it reaches a tipping point.
Push past the tipping point,
and the material at the equator
spreads into a disk.
It’s now broken all the rules
of being a planet.
It can’t spin around together anymore,
its shape keeps changing
as it gets bigger and bigger;
the planet has become something new.
We gave our discovery its name:
synestia.
We named it after the goddess Hestia,
the Greek goddess of the hearth and home,
because we think the Earth became one.
The prefix means “all together,”
to emphasize the connection
between all of the material.
A synestia is what a planet becomes
when heat and spin push it over the limit
of a spheroidal shape.
Would you like to see a synestia?
(Cheers)
In this visualization
of one of my simulations,
the young Earth is already spinning
quickly from a previous giant impact.
Its shape is deformed, but our planet
would be recognizable
by the water on its surface.
The energy from the impact
vaporizes the surface,
the water, the atmosphere,
and mixes all of the gases together
in just a few hours.
We discovered that many
giant impacts make synestias,
but these burning, bright objects
don’t live very long.
They cool down, shrink
and turn back into planets.
While rocky planets
like Earth were growing,
they probably turned into synestias
one or more times.
A synestia gives us a new way to solve
the problem of the origin of the Moon.
We propose that the Moon formed
inside a huge, vaporous synestia.
The Moon grew from magma rain
that condensed out of the rock vapor.
The Moon’s special connection to Earth
is because the Moon formed
inside the Earth
when Earth was a synestia.
The Moon could have orbited
inside the synestia for years,
hidden from view.
The Moon is revealed by the synestia
cooling and shrinking
inside of its orbit.
The synestia turns into planet Earth
only after cooling
for hundreds of years longer.
In our new theory,
the giant impact makes a synestia,
and the synestia divides
into two new bodies,
creating our isotopically
identical Earth and Moon.
Synestias have been created
throughout the universe.
And we only just realized that
by finding them in our imagination:
What else am I missing
in the world around me?
What is hidden from my view
by my own assumptions?
The next time you look at the Moon,
remember:
the things you think you know
may be the opportunity to discover
something truly amazing.
(Applause)