Sarah T. Stewart: Where Did the Moon Come From? (Transcript)

Sarah T. Stewart

Sarah T. Stewart-Mukhopadhyay is an American planetary scientist known for studying planet formation, planetary geology, and materials science. She is a professor at the University of California, Davis in the Earth and Planetary Sciences Department.

Sarah T. Stewart – TED Talk TRANSCRIPT

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. In my lab, I can shoot at rocks using cannons like this one.

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.

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 disc 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.

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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 disc 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 disc 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.

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But the data didn’t look like a planet. It looked really strange. The planet was weirdly connected to the disc. 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 disc around it. Calculating what was in the disc was 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.

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.

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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 disc. 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? 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.

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