How the Universe Works - S01E07 - Solar Systems
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00:00Our solar system, eight planets and over 300 moons, circling the sun like clockwork.
00:13But it didn't start that way.
00:15Our solar system has a long history of violence.
00:21The solar system we see today is really just the final survivors of the early chaos.
00:27And in the future, that chaos will return.
00:30The entire house of cards that is our solar system will completely fall apart.
00:36From start to finish, this is how solar systems work.
00:56There are billions of stars in the Milky Way galaxy.
01:00One of them is our sun.
01:07And around the sun orbits a system of planets and moons, a solar system.
01:19Our solar system is clearly a precious planetary system.
01:24And it begs the question, are there other planetary systems like ours orbiting other
01:31stars?
01:32To find out, Marcy scans the skies with the Keck, one of the world's largest optical
01:38telescopes.
01:39Perched at 14,000 feet on top of Mauna Kea in Hawaii, it hunts for new, distant solar
01:46systems.
01:52And the marvelous reality is that our own Milky Way galaxy contains some 200 billion
01:58stars or so.
02:00And many of those stars have their own planetary systems.
02:07Our solar system with its eight major planets is not alone.
02:11There are other brethren planetary systems out there by the billions.
02:18Of course, astronomers hope to find another solar system with a planet like Earth.
02:23And they're off to a good start.
02:25So far, Marcy and other astronomers have discovered over 360 stars with orbiting planets.
02:33One of the exciting discoveries that we've made is that stars tend to be orbited not
02:38just by one planet, but usually two, three, four, or a multitude of planets.
02:43Planets come in families, not unlike the family of planets we enjoy here around our
02:49own sun.
02:53For the first time, scientists can study them in some detail.
02:57We can actually observe how planets heat up as they go around their sun.
03:04For example, we actually saw that one planet got hotter and colder as it orbited its star.
03:09And we realized that we were actually seeing the night side of the planet and then the
03:13day side of the planet.
03:14That was a temperature difference.
03:18We were observing sunrise and sunset on a planet in another solar system.
03:27But that planet is nothing like Earth.
03:30And most of these newly discovered solar systems are nothing like our own.
03:37Solar planets are huge, much bigger than Jupiter.
03:44Some follow wild orbits.
03:47Some orbit in the opposite direction.
03:49And some shoot billions of miles out into space, then dive back toward their star.
03:57A few orbit so close to the star, their surfaces vaporize.
04:03It's bizarre at the least, if not completely frightening.
04:09Planetary systems offer a wide diversity of different architectures, sizes, masses of
04:16the planets and so on, rendering our solar system just one type of a planetary system
04:22out of thousands.
04:24It could be that each and every solar system is a one of a kind.
04:29But they all have one thing in common.
04:32Each one begins with a star.
04:37First a star is born in a cloud of dust and gas called a nebula.
04:42This is the Eagle Nebula.
04:46These are the pillars of creation.
04:52And this is the Horsehead Nebula, an enormous star nursery.
05:02What scientists have been trying to figure out is what triggers the star making process.
05:10One possibility is that a nearby supernova explosion took place.
05:22Something rammed into this otherwise innocuous molecular cloud, smushing it, smashing it,
05:31compressing it down so that gravity could take over.
05:42Once gravity takes over, the cloud begins to shrink, sucking in more and more gas into
05:48a giant spinning disk.
05:52Gravity at the center crushes everything into a dense, super hot ball that gets hotter and hotter.
06:07Suddenly, atoms in the gas begin to fuse and the star ignites.
06:21The leftover dust and debris forms a disk spinning around the new star.
06:28It contains the seeds of planets, moons, comets, and asteroids.
06:39In 2001, the Hubble Space Telescope was scanning the Orion Nebula and took this image of a
06:47young star surrounded by one of these disks.
06:51It's a picture of a solar system being born.
06:56Whenever I look at these beautiful pictures of nebulae, the thing that really gets me
06:59is that these are baby pictures of our own solar system.
07:03We looked like that once.
07:05These fuzzy images have opened the door to understanding how planetary systems form.
07:12We have this marvelous first ever tool by which we can take pictures of planets caught
07:18in the act of formation.
07:21It's quite a marvelous opportunity for us to see the planets around other stars forming,
07:28thereby giving us a glimpse as to how our own solar system must surely have formed.
07:37Scientists understood where stars come from, but not how planets grow from the disk of
07:42gas and dust.
07:44The answer was discovered by accident aboard the International Space Station.
07:51Astronaut Don Pettit was experimenting with grains of sugar and salt in the weightlessness
07:56of space.
07:59Stanley Love was watching for mission control when Pettit stumbled onto the process of how
08:05planets form from cosmic dust.
08:09One of Don's Saturday morning science projects was to take the bags that we store drinks
08:15in and he put other stuff in it, like salt and sugar, and there was one bag that he just
08:20left the coffee powder in.
08:22Then he inflated the bags and with these particles in them, noticed that the particles would
08:28just clump up immediately.
08:30They make a little dust bunny.
08:31We'll be spending some time watching that.
08:34I'd say, Don, this is incredible.
08:36You've just solved a 40-year-old problem in planetary science.
08:41Astronaut Pettit had discovered something big.
08:46In the zero gravity of space, particles of dust don't float apart, they clump together.
08:53This is how mighty planets are made from cosmic dust.
08:58The dust particles would collide and stick and grow into ever larger dust particles and
09:04eventually rocks and eventually boulders.
09:09The bigger the boulder, the more gravity it has.
09:13It begins to eat up everything around it and grows bigger.
09:23It becomes larger, heavier, and consumes bigger and bigger rocks.
09:32Eventually, some of these rocks grow into planets.
09:47This is what happened in our solar system 4.6 billion years ago.
10:00There were about 100 young planets, all orbiting the new sun.
10:11But the consequences were inevitable.
10:22At the beginning, solar systems are violent.
10:27Ours was no different.
10:31It began with about 100 small new planets.
10:36So how did it go from 100 small planets to the eight major planets of today?
10:43We got the answer by studying the evolution of other solar systems.
10:49We see solar systems forming planets, and all of a sudden they have these giant disks
10:54around them.
10:55Those disks must be from huge collisions.
11:04If planets are smashing together in other systems, they probably smashed together in
11:09our own.
11:16We now know that all solar systems do this before they settle down.
11:24It's the way they're built.
11:33The nice, neat, orderly solar system that we see today has not always been the case.
11:38In the early days, a few million years basically after the planet started forming, there were
11:43dozens, maybe even hundreds of these young planets that were bouncing around the solar
11:48system.
11:55They would smash into each other.
11:57Sometimes they would collect and get to be bigger planets.
12:01Sometimes they would smash each other and turn into little bits.
12:09There was heavy traffic in the new solar system.
12:13Objects of all sizes.
12:16They were bound to collide.
12:20Some of the planets grew larger, and so did the collisions.
12:25I like to try to imagine what it would have been like to actually stand on the early Earth
12:28and look up into the night sky.
12:30Things would have looked different.
12:37Planet hit planet.
12:41Only the largest survived.
12:43The rest were smashed to pieces.
12:52Something very large struck the young planet Mercury.
12:58It blew the crust off and left behind just the iron core.
13:11And the young planet Earth did not escape either.
13:14A planet-sized object slammed into the Earth off-center and blew a huge amount of the Earth's
13:20crust into space.
13:35The debris circled around the Earth and eventually coalesced to become the Moon.
13:50This demolition derby raged for 500 million years.
14:05What we see now, Mars and Earth and Mercury and Venus, these planets in the inner solar
14:10system, they're the survivors.
14:12They're the ones who lived through these giant impacts.
14:16Debris from smashed infant planets ended up in the asteroid belt, a junkyard of rocky
14:23leftover planet parts.
14:30Most of the big impacts happened in the inner solar system.
14:36But one of the outer planets, Uranus, was also hit and knocked on its side, a mystery
14:48since the outer planets formed mostly from gas and largely escaped the violence of the
14:53inner solar system.
14:55These rocky cores formed, the gas accumulated around them.
15:00This process actually happened very rapidly in astronomical terms in only about a million
15:06years.
15:10And those are the giant planets we see today.
15:20Beyond the gas giants, Jupiter and Saturn are Uranus and Neptune.
15:28These two are made of gas and ice.
15:42And beyond them lies the Kuiper belt, a band of orbiting icy rocks and dwarf planets.
15:52We used to think that one Kuiper belt object, Pluto, was the ninth planet.
16:01We've since decided that Pluto is in fact a dwarf planet, one of many orbiting more
16:07than three billion miles from the sun.
16:12There are millions of these things out there.
16:17They're so far away and so faint that they're hard to see.
16:21These are left over from the formation of the solar system itself.
16:29The Kuiper belt marks the edge of the sun's influence.
16:33There is no warmth and not much light way out here.
16:41But the Kuiper belt is not the end of our solar system.
16:45A shell of trillions of icy objects called the Oort cloud is even further out.
16:54The Oort cloud is so far away, light from the sun takes a full year to reach it.
17:07From the cold outer edge to the hot star at the center, our solar system seems stable.
17:17Everything appears orderly and in its proper place.
17:24But something isn't right.
17:28Uranus and Neptune are in the wrong place.
17:41The planets of the solar system grew from a giant disk of dust and gas, the four inner
17:48rocky planets close to the sun, and the giant gas planets farther out.
17:56But Uranus and Neptune seem out of place.
18:04There wasn't enough stuff this far from the sun to make such big planets.
18:11So what are they doing out here?
18:14That led us to a theory where Uranus and Neptune formed very close to the sun and were actually
18:20violently pushed outward.
18:26So what could shove two massive planets clear across the solar system?
18:31We believe that Jupiter and Saturn got into this funny configuration where Jupiter went
18:36around the sun exactly twice every time Saturn went around once.
18:42And that configuration allows the planets to kick each other more as they pass one another.
18:48And that caused the whole system to go nuts.
18:54The combined gravity of Jupiter and Saturn yanked hard on Uranus and Neptune and pulled
19:02them away from the sun.
19:05As they moved outward, the two planets plowed through asteroids and other debris left over
19:11from the formation of the other planets.
19:27This sent billions of chunks of rock flying in all directions.
19:38Some rocks formed the asteroid belt.
19:43But most were thrown out to create the vast Kuiper Belt.
19:51The analogy I like to use is think of a bowling match.
19:54The bowling balls go down and the pins just go kaplooey.
19:58That's what happened in the outer part of the solar system.
20:03The gravitational push from Jupiter and Saturn was so strong, it may have reversed the position
20:08of the two planets.
20:11Looks like it's possible that Uranus and Neptune actually formed in the opposite order.
20:16Neptune was closer to the sun than Uranus, but these gravitational interactions actually
20:21swapped their positions.
20:28It was the blizzard of rocks that Uranus and Neptune ran into that acted like a brake and
20:33slowed them into the orbits they keep today.
20:39The idea of planets changing orbits may sound crazy, but scientists have seen it happen
20:45in other solar systems.
20:48So now they think it's just the way all solar systems work.
20:54When we look out into the galaxy and look at planets around other stars, we see lots
21:00of evidence of those kind of events happening elsewhere.
21:05In one far-off system, scientists have spotted something completely off the charts, a planet
21:12as big as Jupiter, but it's not acting like the Jupiter we know.
21:19Some of these giant planets are found orbiting very close to their host star, taking only
21:25days, a few days, to go around the host star.
21:30Obviously, such close-in Jupiters are blowtorched by the star, raising the temperature of the
21:37planet up to 1,000 or 2,000 degrees Celsius.
21:43There's no way a gas giant could have formed this close in.
21:47It's way too hot.
21:48The only explanation is that it must have formed out there and then moved in here.
22:02The same thing could have happened in our own solar system.
22:08Scientists have found large amounts of the element lithium on the surface of the sun.
22:18Lithium doesn't normally exist in stars, but it is found in gas planets.
22:27Maybe there was another gas giant in our own solar system that spiraled in and crashed
22:33into the sun.
22:34That would explain how the lithium got there.
22:48Something very violent happened.
22:50Could it have been one of these Jupiter-sized planets getting thrown in toward the sun long
22:58ago?
22:59In the beginning, solar systems are violent and messy.
23:04But over time, they settle down and become more stable.
23:08But stability is an illusion.
23:11Any planet in the solar system is always in danger of total annihilation.
23:23There are all kinds of solar systems in the Milky Way galaxy.
23:27Most seem strange compared to our own.
23:31Some planets follow crazy orbits.
23:35Some smash into each other.
23:43Others dive into their stars.
23:54So why are the orbits of our own planets so regular and stable?
23:59Well, that's because all the planets have motion left over from the formation of the
24:03solar system.
24:06When the nebula collapsed around the sun as the sun was forming, there was an intrinsic
24:10motion, and that gave our planet a velocity.
24:14Literally, we are falling freely toward the sun at all times, but we're going so fast
24:20we keep missing it.
24:21That's what an orbit is.
24:27Think of a merry-go-round.
24:28The faster it spins, the farther and farther you're thrown from the center.
24:34When it slows down, you lose momentum and fall back inwards.
24:42It's something like that with planets.
24:46The disk that gave birth to the planets was spinning, and the momentum left over from
24:52that keeps everything going around to this day.
24:58Moving at 66,000 miles an hour, the Earth takes one year to orbit the sun.
25:03Planets farther from the sun have bigger orbits, move slower, and take longer.
25:11Saturn orbits the sun once every 29 years.
25:17Neptune takes 164 years.
25:22Each planet stays on a precise path around the sun, and for us, that's a good thing.
25:31Our solar system has a somewhat fortunate spacing of the planets with nearly circular
25:38orbits, which keeps the whole house of cards from falling apart, crumbling, scattering
25:44to the wind.
25:51If our solar system did not have nice, neat, stable, nearly circular orbits, the Earth
25:57wouldn't be here, and we wouldn't be here talking about it.
26:05Most planets are on safe, stable orbits, but billions of comets and asteroids are not.
26:19Many come streaking into the inner solar system, and when they do, watch out.
26:36Meteor crater, which we see here today, formed as a result of a 150-foot rocky iron object
26:43coming in and slamming into the Earth roughly 50,000 years ago.
26:49Some of the objects coming our way can be much bigger.
26:54Look at the moon.
26:55It's covered with large impact craters.
27:00The Earth has been hit, too, a lot, but the craters have eroded.
27:12We know that a huge asteroid smashed into the Earth off the coast of Mexico 65 million
27:17years ago.
27:20It was going 45,000 miles an hour, and when it hit, it released more energy than five
27:26billion Hiroshima bombs.
27:56It wiped out 70% of life on Earth.
28:13A few more impacts like that could destroy all life on Earth.
28:18But believe it or not, Earth has a giant bodyguard.
28:25Jupiter is more than just another pretty face through the telescope.
28:27It's actually really important for life on Earth.
28:31Jupiter's gravity is so huge, and it's just in the right place in the solar system, that
28:35it protects the Earth from comets that come from deep in the solar system and swing by
28:40the sun and could possibly hit the Earth.
28:44Jupiter plays the role of the biggest baseball bat in the solar system.
28:49As these comets come by, most of them get knocked out of the solar system by Jupiter.
28:57In 1994, comet Shoemaker-Levy 9 raced toward the inner solar system, but it never got past
29:07Jupiter.
29:12Astronomers watched as Jupiter tore it to pieces and dragged its remains down to the
29:17planet's surface.
29:19We have seen comets smashing to Jupiter, creating fireballs that were bigger than the Earth.
29:32They were the biggest explosions ever seen in our solar system.
29:40Had that comet hit us, it would have resurfaced the planet.
29:43It would have been the end of life as we know it.
29:45If Jupiter wasn't there, we believe that the impact rate on the Earth would be something
29:50like a thousand times more than we see today.
30:01Lucky for us, Earth has the perfect orbit.
30:06Jupiter protects us from asteroids and comets.
30:12We're close enough to the sun for liquid water, but not so close that it boils away.
30:20It's just the right combination for life.
30:26Question is, if our solar system could create the perfect conditions, could other solar
30:30systems do it too?
30:34Planet hunters have spotted a solar system 20 light years away, and it has a planet just
30:40the right size in just the right place.
30:49Astronomers around the world are looking for new planets in distant solar systems.
30:57So far, they've discovered more than 420.
31:07Astronomers are huge gas giants like Jupiter.
31:14But they're either very close to the star or much farther away.
31:28Then in 2005, astronomers made an exciting discovery.
31:35They detected a solar system with rocky planets like our own.
31:43These planets orbit a star called Gliese 581.
31:49This star, Gliese 581, and its four planets is frankly quite bizarre relative to our solar
31:55system.
31:58The four planets we know of all orbit very close to the host star, all four of them orbiting
32:03closer than the planet Mercury, our closest planet, orbits the sun.
32:13But Gliese 581 is a small star.
32:17It doesn't burn as brightly or give off as much heat as our sun.
32:21So the planets can orbit much closer without being vaporized.
32:28Now we know of four planets going around this star, and a few of them are quite interesting.
32:33There's one that's only about twice the mass of Earth.
32:37Now that particular one is very close to the star.
32:39It's probably very hot, too hot for life.
32:42But there's another one about eight times the mass of the Earth, which is getting far
32:46enough away from the star that it might be in the habitable zone.
32:51Like Earth, this planet orbits at a distance where water is a liquid.
32:59And where there's liquid water, there could be oceans and life.
33:18In March 2009, NASA launched the Kepler Space Telescope.
33:23Its mission, to search for planets similar to our own in new solar systems.
33:32We may find planets that have methane atmospheres, that have ammonia atmospheres.
33:48We may find planets that are covered in heavy organics, a tar-like material.
33:58We may find some that are covered by water.
34:04I think one of the glorious quests here in the next decade or two is to learn the full
34:10diversity of the family of Earth-like planets that may be out there in the universe.
34:20With Kepler, astronomers expect to discover hundreds, possibly thousands, of new solar
34:25systems.
34:26Think about our own Milky Way galaxy.
34:33The galaxy has roughly 500 billion to a trillion stars.
34:39A fairly large percentage of that have planets.
34:44Now think about how many galaxies we know of.
34:48We certainly haven't found all the galaxies in the universe yet, but the ones we can take
34:52a picture of are actually about 60 billion galaxies.
35:00When you look up at the night sky tonight, simply in the path of your sight, even if
35:07you can't see it, there are billions of solar systems all around you.
35:15And there could be a solar system with a planet just like Earth.
35:20If it happened once, it could happen again.
35:31Solar systems don't last forever.
35:35Planets fall apart.
35:37Planets collide.
35:39It might happen to us, but even if it doesn't, in another 5 billion years, a catastrophe
35:46will end our solar system as we know it.
35:59Nothing lasts forever, not even solar systems.
36:02Ours may seem stable now, but actually, it's very slowly coming apart.
36:15If the solar system was chaotic in the past, that doesn't mean it's all settled down now.
36:21There is still a possibility of a little bit of chaos in the future.
36:25In the future, the gravitational pull of the planets on each other will gradually disrupt
36:31their orbits.
36:34Perhaps over the billions of years, the planets will jostle each other in this gravitational
36:39way so that eventually, two of the planets will come close to each other.
36:49When that happens, and it will, those two planets will engage in a sort of a do-si-do,
36:56clinging one or the other of them, maybe both, into wild orbits, perhaps ejecting one
37:02or both of them from the solar system.
37:07Mars could be thrown out of the solar system, and Mercury might crash into the Earth.
37:25The entire house of cards that is our solar system would completely fall apart.
37:33Solar systems begin and end with a lot of collisions and destruction.
37:38But don't panic yet.
37:42This is going to take billions of years, but over the lifetime of the solar system, these
37:46are eventualities that could come to pass.
37:51But one way or another, our solar system is doomed.
37:59Like all solar systems, the end will come when the star at the center dies.
38:08In five billion years, our own star will run out of fuel and become a red giant.
38:17It will heat up, swell, and engulf the inner planets.
38:28The Earth's surface will be scorched.
38:34The seas will evaporate, and the land will melt.
38:46Soon the sun will become about as big as where the Earth's orbit is.
38:49So a likely scenario for the end of the world is that we're going to be inside the sun for a while.
39:07The Earth's going to get swallowed right up into the sun, and it's going to be toast.
39:11Vapor, literally.
39:15After a while, the red giant will fall apart, too, leaving behind a tiny corpse of a star
39:22called a white dwarf.
39:34It'll be about the size of the Earth, and it will cool off over many millions or billions
39:38of years.
39:44That will be the real end of our solar system.
39:53From the Earth, this dead, rocky planet that used to harbor an enormously vibrant civilization,
40:00we will look out, and there will be this fairly faint dot, which is our sun, now a white dwarf,
40:09a dying, almost dead star.
40:15The remains of the inner planets will continue to orbit the white dwarf.
40:25But the giant outer planets will live on, untouched.
40:33They will have warmed up during the red giant phase of the sun.
40:37But once the sun is a white dwarf, those giant planets will survive just as well, holding
40:43onto their hydrogen and helium, albeit colder than they used to be, because that white dwarf
40:49will no longer be warming them up.
40:57Even though this is five billion years in the future for our solar system, it may already
41:04have happened to many other systems throughout the universe.
41:12Our solar system emerged from chaos to eventually support life.
41:18We were lucky.
41:20We've just the right amount of planets, in the right place, at the right distance from
41:24each other, all orbiting the right type of star.
41:30It could have been a very different story.
41:34There are so many things that are fortunate about our solar system, starting with the sun.
41:38The sun is a very stable, easy star, a perfect thing for life to evolve around.
41:44That's probably not a coincidence that we're here.
41:48An extraordinary chain of events, over billions of years, have made our solar system
41:54the perfect place for life to evolve.
42:00What we see today is not the way things have always been, and not the way things will always
42:08be.
42:10We're not unique, but it is just the way things worked out.
42:16The Earth has to be in the right place.
42:18The planets have to be in the right place.
42:20The giant planets have to be in the right place to protect us from impacts.
42:26All that has to be right in order to get life on Earth.
42:34Ours is the only planetary system we know that supports life.
42:38As solar systems go, does that make us extraordinary, or perfectly normal?
42:44We don't know.
42:46But every week, we're discovering new solar systems, with new planets.
42:52It could be just a matter of time before we discover we're not alone.