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

877,592 views ・ 2019-03-26

TED


Please double-click on the English subtitles below to play the video.

00:13
Nobody likes to make a mistake.
0
13161
2331
00:16
And I made a whopping one.
1
16462
1847
00:19
And figuring out what I did wrong led to a discovery
2
19534
5298
00:24
that completely changes the way we think about the Earth and Moon.
3
24856
3497
00:29
I'm a planetary scientist,
4
29531
1920
00:31
and my favorite thing to do is smash planets together.
5
31475
3730
00:35
(Laughter)
6
35229
1011
00:36
In my lab, I can shoot at rocks using cannons like this one.
7
36264
5819
00:43
(Cannon shot)
8
43120
1246
00:44
(Laughter)
9
44762
1075
00:46
In my experiments, I can generate the extreme conditions
10
46278
4032
00:50
during planet formation.
11
50334
1626
00:52
And with computer models, I can collide whole planets together
12
52775
4018
00:56
to make them grow,
13
56817
1710
00:58
or I can destroy them.
14
58551
1587
01:00
(Laughter)
15
60162
1077
01:02
I want to understand how to make the Earth and the Moon
16
62354
4424
01:06
and why the Earth is so different from other planets.
17
66802
3240
01:11
The leading idea for the origin of the Earth and Moon
18
71414
3737
01:15
is called the "giant impact theory."
19
75175
1955
01:18
The theory states that a Mars-sized body struck the young Earth,
20
78011
3408
01:21
and the Moon formed from the debris disk around the planet.
21
81443
5325
01:28
The theory can explain so many things about the Moon,
22
88061
3211
01:31
but it has a huge flaw:
23
91296
2021
01:34
it predicts that the Moon is mostly made from the Mars-sized planet,
24
94051
4531
01:38
that the Earth and the Moon are made from different materials.
25
98606
3344
01:42
But that's not what we see.
26
102504
1661
01:45
The Earth and the Moon are actually like identical twins.
27
105154
3717
01:49
The genetic code of planets is written in the isotopes of the elements.
28
109704
4897
01:54
The Earth and Moon have identical isotopes.
29
114625
2869
01:57
That means that the Earth and Moon are made from the same materials.
30
117518
4641
02:03
It's really strange that the Earth and the Moon are twins.
31
123080
3623
02:07
All of the planets are made from different materials,
32
127160
2636
02:09
so they all have different isotopes,
33
129820
2129
02:11
they all have their own genetic code.
34
131973
2055
02:14
No other planetary bodies have the same genetic relationship.
35
134695
4456
02:19
Only the Earth and Moon are twins.
36
139824
2447
02:23
When I started working on the origin of the Moon,
37
143588
2575
02:26
there were scientists that wanted to reject the whole idea of the giant impact.
38
146187
4077
02:30
They didn't see any way for this theory to explain the special relationship
39
150288
4181
02:34
between the Earth and the Moon.
40
154493
1736
02:36
We were all trying to think of new ideas.
41
156729
2554
02:40
The problem was, there weren't any better ideas.
42
160084
2980
02:43
All of the other ideas had even bigger flaws.
43
163768
3534
02:48
So we were trying to rescue the giant impact theory.
44
168437
3622
02:53
A young scientist in my group suggested that we try changing the spin
45
173376
4881
02:58
of the giant impact.
46
178281
1365
02:59
Maybe making the Earth spin faster could mix more material
47
179670
4394
03:04
and explain the Moon.
48
184088
1839
03:06
The Mars-sized impactor had been chosen
49
186623
2683
03:09
because it could make the Moon
50
189330
1743
03:11
and make the length of Earth's day.
51
191097
2941
03:15
People really liked that part of the model.
52
195227
2202
03:18
But what if something else determined the length of Earth's day?
53
198481
3417
03:22
Then there would be many more possible giant impacts that could make the Moon.
54
202484
4722
03:28
I was curious about what could happen,
55
208444
2665
03:31
so I tried simulating faster-spinning giant impacts,
56
211133
4387
03:35
and I found that it is possible
57
215544
2484
03:38
to make a disk out of the same mixture of materials as the planet.
58
218052
3878
03:42
We were pretty excited.
59
222871
2010
03:44
Maybe this was the way to explain the Moon.
60
224905
2672
03:48
The problem is, we also found that that's just not very likely.
61
228950
5304
03:54
Most of the time, the disk is different from the planet,
62
234278
3080
03:57
and it looked like making our Moon this way
63
237382
2837
04:00
would be an astronomical coincidence,
64
240243
3157
04:04
and it was just hard for everyone to accept the idea
65
244243
2984
04:07
that the Moon's special connection to Earth was an accident.
66
247251
3811
04:13
The giant impact theory was still in trouble,
67
253220
2750
04:16
and we were still trying to figure out how to make the Moon.
68
256911
3139
04:21
Then came the day when I realized my mistake.
69
261932
3964
04:27
My student and I were looking at the data from these fast-spinning giant impacts.
70
267471
4939
04:32
On that day, we weren't actually thinking about the Moon,
71
272434
2682
04:35
we were looking at the planet.
72
275140
1655
04:36
The planet gets super-hot and partially vaporized
73
276819
2539
04:39
from the energy of the impact.
74
279382
2166
04:43
But the data didn't look like a planet.
75
283479
1998
04:45
It looked really strange.
76
285501
1423
04:47
The planet was weirdly connected to the disk.
77
287388
2397
04:51
I got that super-excited feeling
78
291433
2787
04:54
when something really wrong might be something really interesting.
79
294244
3701
05:00
In all of my calculations,
80
300011
1328
05:01
I had assumed there was a planet with a separate disk around it.
81
301363
3260
05:04
Calculating what was in the disk as how we tested
82
304647
2425
05:07
whether an impact could make the Moon.
83
307096
2401
05:10
But it didn't look that simple anymore.
84
310828
2178
05:15
We were making the mistake
85
315756
3083
05:18
of thinking that a planet was always going to look like a planet.
86
318863
4281
05:24
On that day, I knew that a giant impact was making something completely new.
87
324190
6919
05:33
I've had eureka moments.
88
333133
1826
05:35
This was not one of them.
89
335655
1619
05:37
(Laughter)
90
337298
1167
05:38
I really didn't know what was going on.
91
338489
2913
05:42
I had this strange, new object in front of me
92
342062
2152
05:44
and the challenge to try and figure it out.
93
344238
2781
05:48
What do you do when faced with the unknown?
94
348473
2914
05:52
How do you even start?
95
352054
1681
05:55
We questioned everything:
96
355058
1769
05:57
What is a planet?
97
357400
1195
05:58
When is a planet no longer a planet anymore?
98
358619
2304
06:01
We played with new ideas.
99
361320
2871
06:04
We had to get rid of our old way of thinking,
100
364969
2184
06:07
and by playing, I could throw away all of the data,
101
367177
3625
06:10
all of the rules of the real world,
102
370826
1913
06:12
and free my mind to explore.
103
372763
2285
06:16
And by making a mental space
104
376711
1926
06:19
where I could try out outrageous ideas
105
379501
2576
06:23
and then bring them back into the real world to test them,
106
383233
3434
06:27
I could learn.
107
387331
1354
06:31
And by playing, we learned so much.
108
391423
2976
06:34
I combined my lab experiments with computer models
109
394880
3458
06:38
and discovered that after most giant impacts,
110
398362
2582
06:40
the Earth is so hot, there's no surface.
111
400968
2162
06:43
There's just a deep layer of gas that gets denser and denser with depth.
112
403154
3590
06:47
The Earth would have been like Jupiter.
113
407289
2003
06:49
There's nothing to stand on.
114
409316
1516
06:52
And that was just part of the problem.
115
412373
2872
06:55
I wanted to understand the whole problem.
116
415269
2374
06:58
I couldn't let go of the challenge to figure out what was really going on
117
418242
3911
07:02
in giant impacts.
118
422177
1469
07:04
It took almost two years
119
424719
1591
07:07
of throwing away old ideas
120
427191
1790
07:09
and building new ones
121
429733
1542
07:11
that we understood the data
122
431941
1884
07:14
and knew what it meant for the Moon.
123
434637
1809
07:17
I discovered a new type of astronomical object.
124
437811
4363
07:23
It's not a planet.
125
443168
1419
07:25
It's made from planets.
126
445087
1495
07:27
A planet is a body whose self-gravity
127
447733
2518
07:30
is strong enough to give it its rounded shape.
128
450275
2570
07:32
It spins around all together.
129
452869
1794
07:35
Make it hotter and spin it faster,
130
455415
2719
07:38
the equator gets bigger and bigger until it reaches a tipping point.
131
458158
3813
07:42
Push past the tipping point,
132
462629
1642
07:44
and the material at the equator spreads into a disk.
133
464295
4097
07:48
It's now broken all the rules of being a planet.
134
468953
2769
07:52
It can't spin around together anymore,
135
472215
2395
07:54
its shape keeps changing as it gets bigger and bigger;
136
474634
2661
07:57
the planet has become something new.
137
477319
1967
08:00
We gave our discovery its name:
138
480771
2910
08:04
synestia.
139
484610
1323
08:06
We named it after the goddess Hestia,
140
486402
2182
08:08
the Greek goddess of the hearth and home,
141
488608
2306
08:10
because we think the Earth became one.
142
490938
2282
08:13
The prefix means "all together,"
143
493244
1551
08:14
to emphasize the connection between all of the material.
144
494819
2797
08:18
A synestia is what a planet becomes
145
498640
3202
08:21
when heat and spin push it over the limit of a spheroidal shape.
146
501866
4450
08:27
Would you like to see a synestia?
147
507691
2490
08:30
(Cheers)
148
510205
2039
08:33
In this visualization of one of my simulations,
149
513554
3932
08:37
the young Earth is already spinning quickly from a previous giant impact.
150
517510
4294
08:42
Its shape is deformed, but our planet would be recognizable
151
522663
3236
08:45
by the water on its surface.
152
525923
1906
08:48
The energy from the impact vaporizes the surface,
153
528692
3857
08:52
the water, the atmosphere,
154
532573
1522
08:54
and mixes all of the gases together in just a few hours.
155
534119
3923
08:59
We discovered that many giant impacts make synestias,
156
539414
3619
09:03
but these burning, bright objects don't live very long.
157
543566
2977
09:06
They cool down, shrink and turn back into planets.
158
546567
3536
09:11
While rocky planets like Earth were growing,
159
551266
2423
09:13
they probably turned into synestias one or more times.
160
553713
3526
09:19
A synestia gives us a new way to solve the problem of the origin of the Moon.
161
559338
5740
09:26
We propose that the Moon formed inside a huge, vaporous synestia.
162
566821
6915
09:34
The Moon grew from magma rain
163
574285
3070
09:37
that condensed out of the rock vapor.
164
577379
2415
09:41
The Moon's special connection to Earth
165
581198
2289
09:43
is because the Moon formed inside the Earth
166
583511
2766
09:46
when Earth was a synestia.
167
586301
1880
09:49
The Moon could have orbited inside the synestia for years,
168
589457
4286
09:53
hidden from view.
169
593767
1617
09:56
The Moon is revealed by the synestia cooling and shrinking
170
596622
4908
10:01
inside of its orbit.
171
601554
1895
10:06
The synestia turns into planet Earth
172
606652
2593
10:09
only after cooling for hundreds of years longer.
173
609269
3414
10:14
In our new theory,
174
614990
1804
10:16
the giant impact makes a synestia,
175
616818
2810
10:19
and the synestia divides into two new bodies,
176
619652
3645
10:23
creating our isotopically identical Earth and Moon.
177
623321
4734
10:29
Synestias have been created throughout the universe.
178
629420
3717
10:34
And we only just realized that by finding them in our imagination:
179
634574
5581
10:40
What else am I missing in the world around me?
180
640179
4903
10:45
What is hidden from my view by my own assumptions?
181
645106
3772
10:51
The next time you look at the Moon,
182
651371
2294
10:53
remember:
183
653689
1659
10:55
the things you think you know
184
655372
1832
10:58
may be the opportunity to discover something truly amazing.
185
658320
4459
11:05
(Applause)
186
665167
4380
About this website

This site will introduce you to YouTube videos that are useful for learning English. You will see English lessons taught by top-notch teachers from around the world. Double-click on the English subtitles displayed on each video page to play the video from there. The subtitles scroll in sync with the video playback. If you have any comments or requests, please contact us using this contact form.

https://forms.gle/WvT1wiN1qDtmnspy7