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

877,228 views ・ 2019-03-26

TED


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00:13
Nobody likes to make a mistake.
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And I made a whopping one.
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And figuring out what I did wrong led to a discovery
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that completely changes the way we think about the Earth and Moon.
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I'm a planetary scientist,
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and my favorite thing to do is smash planets together.
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(Laughter)
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In my lab, I can shoot at rocks using cannons like this one.
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(Cannon shot)
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(Laughter)
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In my experiments, I can generate the extreme conditions
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during planet formation.
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And with computer models, I can collide whole planets together
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to make them grow,
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or I can destroy them.
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(Laughter)
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I want to understand how to make the Earth and the Moon
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and why the Earth is so different from other planets.
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The leading idea for the origin of the Earth and Moon
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is called the "giant impact theory."
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The theory states that a Mars-sized body struck the young Earth,
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and the Moon formed from the debris disk around the planet.
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The theory can explain so many things about the Moon,
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but it has a huge flaw:
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it predicts that the Moon is mostly made from the Mars-sized planet,
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that the Earth and the Moon are made from different materials.
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But that's not what we see.
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The Earth and the Moon are actually like identical twins.
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The genetic code of planets is written in the isotopes of the elements.
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The Earth and Moon have identical isotopes.
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That means that the Earth and Moon are made from the same materials.
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It's really strange that the Earth and the Moon are twins.
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All of the planets are made from different materials,
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so they all have different isotopes,
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they all have their own genetic code.
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No other planetary bodies have the same genetic relationship.
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Only the Earth and Moon are twins.
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When I started working on the origin of the Moon,
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there were scientists that wanted to reject the whole idea of the giant impact.
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They didn't see any way for this theory to explain the special relationship
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between the Earth and the Moon.
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We were all trying to think of new ideas.
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The problem was, there weren't any better ideas.
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All of the other ideas had even bigger flaws.
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So we were trying to rescue the giant impact theory.
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A young scientist in my group suggested that we try changing the spin
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of the giant impact.
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Maybe making the Earth spin faster could mix more material
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and explain the Moon.
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The Mars-sized impactor had been chosen
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because it could make the Moon
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and make the length of Earth's day.
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People really liked that part of the model.
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But what if something else determined the length of Earth's day?
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Then there would be many more possible giant impacts that could make the Moon.
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I was curious about what could happen,
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so I tried simulating faster-spinning giant impacts,
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and I found that it is possible
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to make a disk out of the same mixture of materials as the planet.
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We were pretty excited.
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Maybe this was the way to explain the Moon.
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The problem is, we also found that that's just not very likely.
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Most of the time, the disk is different from the planet,
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and it looked like making our Moon this way
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would be an astronomical coincidence,
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and it was just hard for everyone to accept the idea
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that the Moon's special connection to Earth was an accident.
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The giant impact theory was still in trouble,
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and we were still trying to figure out how to make the Moon.
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Then came the day when I realized my mistake.
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My student and I were looking at the data from these fast-spinning giant impacts.
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On that day, we weren't actually thinking about the Moon,
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we were looking at the planet.
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The planet gets super-hot and partially vaporized
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from the energy of the impact.
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But the data didn't look like a planet.
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It looked really strange.
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The planet was weirdly connected to the disk.
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I got that super-excited feeling
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when something really wrong might be something really interesting.
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In all of my calculations,
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I had assumed there was a planet with a separate disk around it.
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Calculating what was in the disk as how we tested
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whether an impact could make the Moon.
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But it didn't look that simple anymore.
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We were making the mistake
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of thinking that a planet was always going to look like a planet.
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On that day, I knew that a giant impact was making something completely new.
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I've had eureka moments.
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This was not one of them.
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(Laughter)
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I really didn't know what was going on.
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I had this strange, new object in front of me
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and the challenge to try and figure it out.
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What do you do when faced with the unknown?
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How do you even start?
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We questioned everything:
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What is a planet?
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When is a planet no longer a planet anymore?
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We played with new ideas.
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We had to get rid of our old way of thinking,
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and by playing, I could throw away all of the data,
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all of the rules of the real world,
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and free my mind to explore.
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And by making a mental space
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where I could try out outrageous ideas
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and then bring them back into the real world to test them,
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I could learn.
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And by playing, we learned so much.
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I combined my lab experiments with computer models
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and discovered that after most giant impacts,
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the Earth is so hot, there's no surface.
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There's just a deep layer of gas that gets denser and denser with depth.
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The Earth would have been like Jupiter.
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There's nothing to stand on.
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And that was just part of the problem.
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I wanted to understand the whole problem.
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I couldn't let go of the challenge to figure out what was really going on
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in giant impacts.
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It took almost two years
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of throwing away old ideas
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and building new ones
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that we understood the data
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and knew what it meant for the Moon.
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I discovered a new type of astronomical object.
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It's not a planet.
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It's made from planets.
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A planet is a body whose self-gravity
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is strong enough to give it its rounded shape.
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It spins around all together.
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Make it hotter and spin it faster,
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the equator gets bigger and bigger until it reaches a tipping point.
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Push past the tipping point,
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and the material at the equator spreads into a disk.
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It's now broken all the rules of being a planet.
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It can't spin around together anymore,
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its shape keeps changing as it gets bigger and bigger;
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the planet has become something new.
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We gave our discovery its name:
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synestia.
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We named it after the goddess Hestia,
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the Greek goddess of the hearth and home,
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because we think the Earth became one.
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The prefix means "all together,"
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to emphasize the connection between all of the material.
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A synestia is what a planet becomes
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when heat and spin push it over the limit of a spheroidal shape.
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Would you like to see a synestia?
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(Cheers)
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In this visualization of one of my simulations,
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the young Earth is already spinning quickly from a previous giant impact.
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Its shape is deformed, but our planet would be recognizable
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by the water on its surface.
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The energy from the impact vaporizes the surface,
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the water, the atmosphere,
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and mixes all of the gases together in just a few hours.
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We discovered that many giant impacts make synestias,
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but these burning, bright objects don't live very long.
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They cool down, shrink and turn back into planets.
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While rocky planets like Earth were growing,
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they probably turned into synestias one or more times.
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A synestia gives us a new way to solve the problem of the origin of the Moon.
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We propose that the Moon formed inside a huge, vaporous synestia.
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The Moon grew from magma rain
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that condensed out of the rock vapor.
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The Moon's special connection to Earth
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is because the Moon formed inside the Earth
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when Earth was a synestia.
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The Moon could have orbited inside the synestia for years,
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hidden from view.
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The Moon is revealed by the synestia cooling and shrinking
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inside of its orbit.
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The synestia turns into planet Earth
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only after cooling for hundreds of years longer.
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In our new theory,
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the giant impact makes a synestia,
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and the synestia divides into two new bodies,
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creating our isotopically identical Earth and Moon.
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Synestias have been created throughout the universe.
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And we only just realized that by finding them in our imagination:
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What else am I missing in the world around me?
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What is hidden from my view by my own assumptions?
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The next time you look at the Moon,
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remember:
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the things you think you know
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may be the opportunity to discover something truly amazing.
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(Applause)
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