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  • 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)

Nobody likes to make a mistake.

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【TED】Where did the Moon come from? A new theory | Sarah T. Stewart

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    林宜悉   に公開 2019 年 03 月 26 日
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