字幕表 動画を再生する
-
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)