Category
📺
TVTranscript
00:00This is space. But not as you know it. Trillions of icy rocks. A dark, mystical star.
00:13Giant magnetic bubbles, each one more than a hundred million kilometers across.
00:19And a cosmic war zone, where fields of deadly radiation collide.
00:26This isn't a distant part of the galaxy. This is the edge of our own solar system.
00:33And what's going on here determines whether we live or die.
00:40Over the last 30 years, our understanding of the universe has exploded.
01:03We've discovered billions of stars in our galaxy, and billions of new galaxies across the universe.
01:13We've seen black holes shooting out cosmic jets, exploding stars, and new planets in distant solar systems.
01:26But when it comes to our own cosmic postcode, we know remarkably little.
01:32When we study space, the closest thing to us is our solar system. So that's what we should understand the best.
01:38But it turns out we're just getting started. We don't understand everything in our solar system.
01:42The solar system is our home, our local neighborhood. But it may surprise you to know that we don't really understand how far it extends.
01:50When I was a kid, I was taught that where the planets were was the solar system.
01:55So that the farthest planet at the time, Pluto, was where the solar system stopped.
02:00And it turns out that's completely wrong.
02:03If you think that the solar system stops where the planets stop, where Neptune and Pluto are, you're missing the big picture, literally.
02:1790% of our solar system lies beyond the planets.
02:22Out here, the sun could be mistaken for just another star.
02:27In this dark region of the solar system, giants lurk.
02:33Their very existence only confirmed by sightings of a few of them that strayed into the inner solar system.
02:40They are the comets.
02:44It was proposed that comets came from a mysterious disk out beyond the orbit of Neptune.
02:50But no one could prove it.
02:53The Kuiper Belt was theoretical. It was thought to exist.
02:56But it was also thought that it would be too hard to observe.
02:59You're talking about objects which may be 100 miles across, but they're billions of miles away.
03:05Trying to find something that's much, much smaller would be incredibly difficult, and no one really thought you could do it.
03:12In 1986, a team of astronomers based at Hawaii's Mauna Kea Observatory decided to prove them all wrong.
03:23I'm a bit of a hoarder.
03:25These are the notes that we took while we were observing.
03:29I used to write lots of things in a notebook to try to keep track of everything that was happening on the mountaintop.
03:37Here's the name of the telescope, the 88-inch telescope on Mauna Kea, also known as the Rust Bucket.
03:45Visitors to that telescope always think it's the ugliest thing they've ever seen.
03:48It's painted brown. It looks like it's totally rusty. Parts of it are rusty.
03:51So it's a horrible-looking telescope, but we use it for all of our work.
03:55Jewett's goal was to locate an object beyond the planets, an object that inhabited the ghostly Kuiper Belt.
04:03It was like searching for a rusty penny dropped somewhere in the Pacific Ocean.
04:07For six years, they found nothing.
04:10We spent many, many years looking for objects in the outer solar system.
04:14Lots of pain, frankly. Lots of disappointment.
04:17You have to really have some respect for the scale of objects.
04:21Yes, we can see stars that are hundreds or thousands of light-years away, but they're these giant glowing objects.
04:26How do you see something that's only 100 miles across?
04:29And it's dark. It's not giving off any visible light of its own.
04:32That's why finding these things have been difficult.
04:36Kuiper Belt objects are made mostly of ice.
04:42But they don't sparkle like icebergs.
04:49We're in a volcanic country, so some of these icebergs are actually dark, covered in ash.
04:54An ash-covered iceberg makes a good analogy for a Kuiper Belt object.
04:58So I really wanted to get close to this one, but my guides just told me that this one's too big.
05:02We can't get close. It might flip over.
05:06All right, we found a really cool one. It's really dark.
05:09Let's get over here.
05:11So, you see how black this thing is on the outside?
05:14Well, comets are also black.
05:17But actually, they're even blacker than this.
05:20We think that comets typically are blacker than asphalt, blacker than tar.
05:26The ash that covers these icebergs comes from a volcano.
05:31But the ink-black skin that surrounds comets
05:34seems to originate from its surface being cooked by the sun.
05:39So where does this black come from? It's not ash.
05:42Well, we now know that comets have a lot of organic molecules.
05:45That means molecules with carbon.
05:47And when these things get blasted by ultraviolet radiation and cosmic rays,
05:51they kind of get charred and turn black.
05:54So it's a lot like you had a piece of meat,
05:56and you leave it on the barbecue too long, and it gets charred black.
05:59It's the same thing going on with comets.
06:03With their jet-black coating,
06:05comets and other Kuiper Belt objects blend into the dark background of space.
06:11The problem with studying these objects
06:13is that many of them are very, very dark in colour.
06:16They're very, very small, and they don't emit their own light.
06:19So they're just very difficult to see.
06:21It requires a very large telescope to do so.
06:25After a six-year search,
06:27DeWitt and his team spotted something.
06:31And the solar system would never be able to see it.
06:34So here's the log sheet, the notes that we keep while we're observing.
06:3929th of August, 1992.
06:43We started as normal, looking at the images as they came out of the telescope,
06:47and found this first object.
06:50It was a single, tiny dot,
06:52moving very slowly across the sky.
06:56It was a single, tiny dot,
06:58moving very slowly across the sky.
07:01It's path showed it was orbiting the Sun
07:03from much further out than any of the planets.
07:06An orbit that placed it in the mysterious Kuiper Belt.
07:11It was moving incredibly slowly across the sky,
07:14and therefore was incredibly far away.
07:16And we knew, just from the slow speed,
07:18we knew immediately it was the most distant object
07:21ever seen in the solar system up to that point.
07:26Here's this note, SMO, Slow Motion,
07:29Here's this note, SMO, Slow Moving Object, question mark.
07:32So that's the discovery of the Kuiper Belt right there,
07:35with those three tiny little letters.
07:38In an instant, the Kuiper Belt was no longer just theory.
07:43It was real.
07:46It was groundbreaking, because now we have this
07:49whole part of the solar system that we had predicted,
07:52but had no evidence for.
07:54Boom. Evidence.
07:57The discovery of the first Kuiper Belt object
07:59was a clear indication that there is this
08:01new population of objects out there.
08:03Basically, overnight, we doubled the diameter
08:06of the solar system.
08:09The team later identified several other Kuiper Belt objects.
08:13They calculated there must be billions
08:15orbiting the Sun in a vast disk.
08:19The discovery of the first Kuiper Belt object
08:21was a fantastic achievement.
08:24It was observational proof that there was
08:27an entire population of objects out in the solar system
08:30that we thought might exist, and then found out truly did.
08:34It would be as big as discovering another full-fledged planet.
08:39The Kuiper Belt extends 50 times farther away from the Sun
08:43than the orbit of Earth.
08:46A vast ring...
08:49populated with mountain-sized lumps of black ice.
08:55The material in the Kuiper Belt really is
08:57left-over building blocks of the planets.
08:59This stuff should have formed into larger planets
09:01like the Earth or Jupiter,
09:03but some of it actually got thrown out,
09:05and it stayed that way, small little chunks of stuff,
09:08remnants from the formation of the planets.
09:13The discovery of the Kuiper Belt
09:15instantly inflated the size of our solar system,
09:18but deflated the status of one member.
09:23Because some of the objects in the belt
09:25were bigger than our smallest planet,
09:27Pluto was demoted to a dwarf planet.
09:34I have actually really enjoyed the controversy
09:36about what do you call Pluto?
09:38Is it a planet? Is it a dwarf planet? Is it something in between?
09:40It shows people still care.
09:42Pluto is still fascinating,
09:44an important part of our solar system,
09:46but now we see it in the context of its nearest neighbors.
09:49Let's not get angry that Pluto is no longer labeled a planet.
09:52Let's get interested in the fact that Pluto is a dwarf planet
09:55and can teach us lots of things about all of its new friends
09:58that we're discovering year after year after year.
10:01Now Pluto has over 1,400 brothers and sisters called Plutinos.
10:07If Pluto is a planet, all the Plutinos are too.
10:10So I have a daughter.
10:12It's painful for her to try to remember all those names.
10:18The discovery of the Kuiper Belt opened up the solar system.
10:22But it was only the start.
10:24What has been discovered farther out is even more remarkable.
10:31The Kuiper Belt extends about 50 times farther out from the sun
10:35than the Earth's distance from the sun.
10:37It's huge.
10:39If the edge of the Kuiper Belt is the edge of the solar system,
10:42you're thinking way too small.
10:45Far beyond the Kuiper Belt,
10:47the sun has deployed a deflector shield around the solar system.
10:52Our star is waging a cosmic war
10:55against the biggest guns in the galaxy.
10:58And our lives depend on it winning.
11:10For a long time,
11:12we assumed the solar system ended where the planets ended.
11:16But the discovery of the Kuiper Belt in the 90s
11:19doubled the reach of the sun's influence
11:22and led scientists to ask,
11:24could it reach further?
11:26We now know that it does, much further.
11:30And we wouldn't be here if it didn't.
11:33If you put a Geiger counter in this room, it would click.
11:36If you put a Geiger counter up in space, it would click a lot more.
11:40Geiger counters detect a deadly form of radiation
11:43called cosmic rays.
11:45Cosmic rays are essentially subatomic particles
11:48like the nuclei of helium atoms and protons and electrons
11:52that are zipping through space at almost the speed of light.
11:55And these can travel across great distances,
11:57hundreds, thousands of light years.
11:59And it turns out they hit us here on Earth.
12:03Cosmic rays are blasted out of exploding stars deep in the galaxy.
12:08And they're incompatible with life.
12:11They can rip out electrons.
12:13They can, in fact, change the chemical configuration
12:16of cells causing mutations.
12:19They can ultimately kill you.
12:22If the Earth were continually blasted by cosmic rays
12:25without any protection,
12:27it's not clear that advanced forms of life could survive.
12:31Very few of the cosmic rays aimed at us actually hit us.
12:3614 million kilometers away,
12:38a deflector shield is protecting us
12:40from the full onslaught of this deadly radiation.
12:44The shield is powered by the sun,
12:48which fights the cosmic rays
12:50by sending out its own stream of super-fast particles
12:53known as the solar wind.
12:57When you think of wind, you think of the air of the Earth,
13:01you know, moving through your hair.
13:03The solar wind is nothing like that.
13:05It's actually a stream of high-energy particles coming off the sun,
13:08things like hydrogen.
13:10And it's substantial. It's billions of tons every second.
13:13Some parts of the solar wind are going as fast as about 2 million miles an hour.
13:17It can pack quite a wallop.
13:20The wallop blows by us here on Earth
13:23as the planet's magnetic field deflects it.
13:29The solar wind doesn't just stop at the Earth.
13:32It's out in space. It keeps going.
13:34It blows past the inner planets.
13:36It blows past the outer planets, Jupiter, Saturn, Uranus and Neptune,
13:39and keeps going.
13:41The solar wind rushes 5.5 billion kilometers past the Kuiper belt,
13:45carrying the sun's magnetic field with it,
13:48creating a huge defensive shield around the solar system
13:52called the heliosphere.
13:54The solar wind is blowing a bubble around our solar system.
13:57The sun is like a big mother
14:00that literally creates a bubble of charged particles
14:04and associated magnetic fields
14:06that protect us from cosmic rays coming from outside the solar system.
14:10The incoming cosmic rays hit the outgoing solar wind,
14:14and most of the harmful rays are deflected into space.
14:18Without this shield,
14:20our complex life might not have had the chance to evolve on Earth.
14:23Like everything else in nature,
14:25the solar wind is kind of a two-edged sword.
14:27It can harm us here on Earth,
14:29but it also protects us from this galactic material.
14:32There are so many instances in the universe
14:34where something that you think at first would be very dangerous and destructive
14:37can even be protective.
14:39And the solar wind is one such thing.
14:41Yes, there are these high-energy particles coming from the sun,
14:44but the effect of the solar wind is to protect our entire solar system
14:48from even more dangerous kinds of particles.
14:51The protective bubble battles its way through the dust and gas
14:55as the solar system slowly orbits the center of the galaxy.
14:59It's plowing through it like a boat moving through water,
15:02and there's a wave where the solar wind has slowed to a stop
15:06as it's slamming into the material outside of the solar system,
15:09into interstellar space.
15:12And it was here that NASA's Interstellar Boundary Explorer, or IBEX,
15:17found a completely strange and unexpected phenomenon.
15:21The IBEX mission actually found a giant streamer of hydrogen gas,
15:25the largest thing we've ever seen in our solar system.
15:28And this may be due to the interaction of our solar system's magnetic field
15:32with the larger magnetic field of the galaxy.
15:38Scientists think that the interaction between the sun's solar wind racing out
15:42and the interstellar medium of the galaxy pushing in
15:46traps the hydrogen particles into a giant ribbon.
15:55The ribbon is a million kilometers wide and three billion kilometers long,
16:03making it the biggest object in our solar system.
16:09It's fundamentally so exciting to learn about these extended structures
16:13at the edge of the solar system.
16:15Actually, no one expected this. No one ordered this.
16:17And so by seeing things that we weren't expecting before,
16:20that's how we really learn, and that's how science progresses.
16:23Twenty-five years ago, we thought that Pluto was the edge
16:26of at least the planets in our solar system.
16:28But now we know that there are many more objects like Pluto out there.
16:31So not only has the edge of the solar system moved out,
16:34but we've become much more aware of how much more we have to learn.
16:39The latest eyes in the sky
16:42peer into the most distant regions of our solar system.
16:47We are just learning about the vast array of objects
16:51in the outer part of our solar system.
16:53As our technology has increased,
16:55we keep discovering more and more exotic objects.
16:57And our picture of what the solar system really is has changed
17:00as a result of our ability to improve our eyes.
17:05But it's old technology that's actually taking us to the edge.
17:12When NASA launched Voyager 1 in 1977,
17:16they thought it might work for five years.
17:20Thirty-seven years later, the little space probe that could
17:23is still sending us mind-boggling information.
17:28In August of 2012, 18 billion kilometres from Earth,
17:33it left the heliosphere and entered the utterly mysterious heliopause,
17:38the junction between the furthest reach of the solar wind
17:42and interstellar space.
17:45That heliopause is sort of where those two boundaries meet.
17:48It's sort of where our neighbourhood kind of stops
17:51and sort of the general neighbourhood begins.
17:54I was excited when the news came back that Voyager was in the heliopause.
17:58This is an entire region of the solar system
18:00about which we know almost nothing.
18:02And for the first time in human history,
18:04we had a robot probe sitting in the middle of it,
18:07measuring what was going on around it.
18:10It was groundbreaking.
18:12For the first time we can say, even at a limited extent,
18:15that humanity is an interstellar species.
18:17We are now out between the stars.
18:21When this veteran probe took a look back at the solar system,
18:25it had quite a surprise for us.
18:29Astronomers had thought the heliosphere was shaped a bit like a comet,
18:34with a giant tail extending back hundreds of billions of kilometres.
18:40But Voyager 1 found something new and extraordinary.
18:45Not a comet-shaped heliosphere with a giant tail,
18:49but a crescent moon shape with two short tails.
18:54It's a really important observation because it teaches us something new.
18:57It teaches us what the fundamental shape of the heliosphere is.
19:00A long time ago, we just had the simplest picture,
19:02but now we have some detail, we know it's got some structure,
19:05and we can start with that and push forward
19:07and really begin a deep understanding of the region around the sun.
19:11The tails are actually jets of energised solar wind.
19:17What causes these jets is fundamentally the tension,
19:19or the interaction between the background magnetic field of the galaxy
19:23and those streaming, streaming fast charged particles that are coming from the sun.
19:30Voyager also sent back shocking news about the very edge of the sun's influence.
19:37The heliopause may not be a clear barrier.
19:39It's not just a hard edge, it's not just something really sharp.
19:42It's actually sort of a fuzzy extended region.
19:44It's hundreds of millions of miles wide.
19:48And right in the middle, Voyager detected something truly bizarre.
19:53A sphere of giant magnetic bubbles.
19:56We expected that the solar magnetic field would just sort of disperse at that point.
20:01But that's not really what happened.
20:02What they found is that the magnetic field lines from the sun have tangled up,
20:06creating sort of a frothy, foamy magnetic structure with bubbles.
20:10That was quite a surprise and a very exciting one.
20:15As the sun's magnetic field ripples out into deep space,
20:19it twists and tangles.
20:23The lines crisscross and reconnect, forming giant magnetic bubbles.
20:29Each foamy sphere bubble thing is about 100 million miles across, which is tremendous.
20:36That's the distance from the earth to the sun.
20:38And there are thousands of these surrounding the solar system.
20:42No one is sure what these magnetic bubbles do.
20:46But it's possible that this cosmic bubble wrap
20:49is the first line of defense against galactic radiation.
20:53Helping protect the solar system from the cosmic onslaught.
20:57Think about how much technology has changed since Voyager was launched and today.
21:02No one had phones in their pockets.
21:04I mean, no one could even make long-distance phone calls across the ocean
21:07without it being a tremendous ordeal.
21:09But yet, with this old technology, we're now learning about the very edge of our solar system.
21:13Just imagine what we're going to learn over the next 50 years.
21:18Voyager is now beyond the heliopause.
21:21But has it really left the solar system?
21:24Not really. There is still plenty of solar system out there for Voyager to pass through.
21:30And it will still be going for many centuries, many millennia,
21:33before it leaves the solar system proper.
21:38Voyager may be outside the range of the solar wind.
21:42But it's not beyond the sun's gravity.
21:46As we go out, we run into the Kuiper Belt, beyond Pluto's orbit, right?
21:50This is very, very far away.
21:52And then even beyond that, we have the heliopause, the area of the sun's magnetic field.
21:56So you might think that you're at the end of the solar system.
21:59But guess what? There's still stuff out there that's gravitationally bound to our solar system.
22:07Every once in a while, one of these objects falls towards the sun.
22:12And sometimes, Earth gets in the way.
22:27Deep in the outer reaches of the solar system, far beyond the reach of the solar wind, we are blind.
22:36Even the most powerful telescopes are unable to discern what lurks in the darkness.
22:43But sometimes, mountain-sized rocks appear from beyond.
22:49These potentially life-threatening wanderers are the long-period comets.
22:56There's basically two kinds of comets.
22:58There's comets that are called short-period comets, and the other kind is called long-period comets.
23:04Short-period comets come from the Kuiper Belt, the disk of space junk just beyond the planet.
23:11They travel around the solar system in relatively small circles,
23:15returning at regular intervals of 200 years or less.
23:22Long-period comets are more mysterious.
23:27The long-period comets can come back around something like every thousand years or even a million years.
23:33And unlike the short-period comets that come in on sort of circles,
23:36the long-period ones come in on really plunging orbits,
23:39and they can come in from all over the place in all different directions.
23:45These comets are on oblique, elliptical orbits.
23:48Starting at the edge of the solar system, it can take thousands of years for them to reach us.
23:53By the time they do, they've picked up a lot of speed.
23:57These things can move as fast as 100 miles a second when they get close to the sun.
24:01So just imagine that, 100 miles in a second.
24:07Comets travel fast because they're falling, dragged in by the sun's immense gravity.
24:14And the longer and farther something falls, the faster it goes.
24:25Physicists like to talk about gravity wells.
24:27And the sun is at the bottom of a deep gravity well.
24:30The skater at the top of the ramp, before he falls down into the pool, is going slow.
24:35By the time he gets to the bottom, he's going as fast as he's going to go.
24:38It's the same way with the sun.
24:39The comet is plunging in to the very depths of the sun's potential well.
24:43When it gets very close, it's going extremely fast.
24:46And as it comes back out, it's going slow, just like a skater at the top of this pool.
24:52Their orbits are a clue to the comet's mysterious origin.
25:00The long-period comets, we think, are coming from someplace much, much farther away.
25:04And it's more like a spherical cloud of very distant things that are now plunging down into the sun every now and again.
25:13When a comet comes in, the sun heats it up.
25:15The ice turns into a gas, and the comet loses mass.
25:18Over time, these comets will disappear.
25:21And yet they keep coming.
25:23That means that there must be a reservoir of them out there.
25:27The question is, where are they coming from?
25:31One of the first explanations for these strange comets is maybe they're not from our solar system.
25:36Maybe the galaxy is full of these icy chunks that occasionally rain down on us.
25:40But then we thought, maybe they come from another part of the solar system.
25:45Perhaps at the edge of the solar system, there's debris left over from the very first days of its creation.
25:51A hypothetical dumping ground known as the Oort Cloud.
25:56It was theorized that there must be a reservoir of them, a giant cloud surrounding the solar system,
26:01with millions, billions, maybe even trillions of these icy bodies.
26:06But we've never seen it.
26:09So this Oort Cloud, this theorized population of comets, is that. It's theoretical.
26:14It's never been directly observed.
26:16The objects in the Oort Cloud are so far away and so small and dark
26:20that we've never observed something actually out in the Oort Cloud.
26:23Something has to fall in.
26:25So you see things falling in from all of these different angles.
26:29From there, you can deduce what must be out there.
26:36It turns out that deducing what the Oort Cloud might look like is a staggeringly complicated task,
26:41requiring one of the world's most powerful number crunchers.
26:46Luckily, Kevin Walsh has just the machine for the job.
26:49Here we are in the guts of a modern supercomputer.
26:51On these racks are thousands of processors running day and night,
26:54analyzing computer models to understand the evolution of our solar system
26:58and what's out there at the edge.
27:01Meet the Janus supercomputer in Boulder, Colorado.
27:07In supercomputers like these, there are thousands of processors,
27:10tens of thousands of computing cores.
27:13You can do over 100 trillion calculations per second in a machine like this.
27:17And we need this because we're trying to understand the evolution of hundreds of millions of comets
27:21over billions of years.
27:23Compared to what I could do with a pen and paper, it's no contest.
27:31Using the supercomputer, astronomers can begin to piece together
27:35a picture of the giant invisible Oort Cloud.
27:39And they start with what we can see,
27:41the comets that race in toward the sun.
27:44We never see the Oort Cloud until a comet comes by,
27:47so we need to estimate how frequently one of them is going to be perturbed
27:51in just the right way that will make this close passage by the sun
27:55so that we can observe it.
27:57We can then work backwards and say,
27:59well, we see about one every 10 years of about this size,
28:02and then say, therefore, there must be billions and billions of these
28:06making up that Oort Cloud.
28:08The calculations indicate the Oort Cloud is home to 2 trillion icy objects
28:13that extend out far beyond the heliopause.
28:20It's hard to grasp just how far out the Oort Cloud extends,
28:25but the edge of it probably is 10,000 times farther away from the sun
28:31than the Earth is, a trillion miles.
28:34It's so big, it is so vast, so much larger than the solar system of planets
28:39that if you removed all the planets from the sun,
28:42the Oort Cloud wouldn't even notice.
28:45So how exactly did the cloud's icy bodies get out here?
28:49A fun way to think of the Oort Cloud is that these are pieces of planets
28:52that never got used.
28:54They really are leftovers.
28:56The Oort Cloud was built by a group of scientists
28:59The Oort Cloud actually didn't come with the original package, if you will.
29:03The Oort Cloud formed when they tried but miserably failed
29:06to join the party and become those giant planets,
29:09and instead got thrown all different directions,
29:12and they now exist halfway to the next star.
29:15They're a big giant spherical distribution.
29:18The sun burst into life 4.6 billion years ago.
29:22A cloud of dust and gas circled the infant star.
29:26Then gravity started to suck in gas, dust, ice, and rocks
29:31to form the infant planets.
29:34The planets formed out of building blocks, smaller chunks of material,
29:38rock, ice,
29:41stuff that came together over time.
29:45The frenzy of swirling, colliding matter left behind debris
29:49as rocky rubble in the asteroid belt,
29:52and ice in the Kuiper belt.
29:55Then Neptune and Uranus scattered the debris out further.
30:01But think about what happens when you have bodies falling together,
30:04all kinds of small rocks under the influence of gravity.
30:07Some of that will get slung shot around the planet
30:10and thrown out into space.
30:12Most of them got thrown out of the solar system forever,
30:15but the ones that didn't go on these big, long, massive,
30:19But the ones that didn't go on these big, long,
30:22million-year looping orbits, and that's the Oort Cloud.
30:25Trillions of icy rocks were ejected.
30:28Some fell into the sun, some flew into outer space,
30:32and some formed the Oort Cloud.
30:35The Oort Cloud comets are dinosaur bones of solar system formation.
30:38Contained in them are the ingredients that went up to make our planets.
30:41They are a time capsule of what the solar system was like
30:444.5 billion years ago.
30:48Most of the Oort Cloud objects will stay in this icy cloud forever,
30:52orbiting the distant sun.
30:55But a few get nudged loose,
30:58and the sun's gravity pulls them inwards,
31:01like moths to a flame.
31:03And such a comet can sneak up on you, and you won't even know it's there.
31:07These comets can be many miles across,
31:10and they're moving extremely rapidly,
31:13sometimes as much as 100 miles per second.
31:16This makes them pretty dangerous.
31:19How dangerous? Ask a dinosaur.
31:22Some people think that the massive impact
31:25that killed off the dinosaurs
31:28may have been caused by a huge comet.
31:31These comets are potentially much more dangerous
31:34than the asteroids usually blamed for extinctions on Earth.
31:37Comets are falling from so far away
31:40that, in general, they're moving faster than asteroids,
31:43and their impact energies are that much larger.
31:46Even something as small as a mile or two across
31:49could cause a lot of hurt.
31:52The impact energies of these comets
31:55are so powerful,
31:58that something as small as a mile or two across
32:01could cause a lot of hurt.
32:04But a lot of these Oort Cloud comets
32:07that come into the inner solar system are very large.
32:10And you can imagine if one of these things is 30 or 50 miles across,
32:13this would be a global event.
32:16Understanding the Oort Cloud better
32:19could help us predict if one of these comets is on the way.
32:23Some have suggested that the Oort Cloud hides
32:26the solar system's best-kept secret.
32:29Something that's responsible for hurling
32:32these high-speed comets at us.
32:35A sinister twin to our own sun.
32:38A second star called Nemesis.
32:52Our theoretical knowledge of the solar system
32:55extends 14 trillion kilometers past the planets.
32:58But at this distance, we're working in the dark.
33:01Carl Sagan said that we're in the age of Magellan.
33:04We're not in the age of Columbus.
33:07What does that mean?
33:10That means that we know the basic outlines of our solar system.
33:13We know the planets. We know there's a Kuiper Belt and an Oort Cloud.
33:16But we don't know the details.
33:19We're really filling in the map right now.
33:22We've worked out that there are around 2 trillion objects in the Oort Cloud.
33:25A sphere of junk left over from the formation of the solar system.
33:28And that every now and then,
33:31one of those objects comes hurtling in towards us.
33:34A big question is,
33:37why would one suddenly decide to come in?
33:40What is perturbing them? What's poking them?
33:43So that they fall in and approach the inner solar system.
33:46Well, there are a lot of different ideas of what it could be.
33:49There may be clues in the series of catastrophes
33:52that nearly wiped out life on Earth.
33:55There appeared to be, in the fossil record,
33:58a periodic series of mass extinctions roughly every 26 million years.
34:01It seemed that something was jostling the Oort Cloud periodically,
34:04spraying comets towards us every 26 million years.
34:07But what could it be?
34:10When we look out at stars,
34:13we notice that most of them occur in binary pairs.
34:16In some cases, they can be very far away.
34:19They can even take millions of years to orbit each other.
34:22So the question seemed sort of natural.
34:25Does the Sun have a companion that we haven't found yet because it's so far away?
34:28It's fascinating to think about whether our Sun
34:31may have a brother or sister currently.
34:34Because we know that more than half the stars in our galaxy, 100 billion strong,
34:37are in binaries or trinaries or quaternary systems.
34:40A lone star is a little exceptional.
34:43If the Sun does have a twin,
34:46it could be responsible for disturbing the Oort Cloud.
34:49What would happen is that every 26 million years,
34:52this binary star partner would pass through the Oort Cloud,
34:55upset orbits and send comets streaming in all directions,
34:58and some of them would come into the inner solar system,
35:01strike the Earth, causing a mass extinction event.
35:05And the name of this really dangerous binary star partner of the Sun was Nemesis.
35:16The Nemesis theory has its problems.
35:19Certainly, the gravity of a mystery star passing through the Oort Cloud
35:22could periodically send comets raining down towards the Sun and Earth.
35:28But if our Sun has a companion star,
35:31it should be visible.
35:34So you think, of course, this Nemesis star should be very easy to see
35:37because, after all, stars are bright.
35:40But the idea here is maybe Nemesis is a dwarf star,
35:43maybe it's a brown dwarf.
35:46So it would be really small, and therefore it would not be very bright.
35:49A brown dwarf isn't a star or a planet.
35:52It's something in between,
35:55a failed star that never got started.
35:58Stars ignite when gravity crushes vast clouds of hydrogen gas together,
36:02generating huge pressures and temperatures.
36:05If there's not quite enough gas,
36:08then there's not quite enough gravity to light a nuclear fire.
36:13So it doesn't become a star.
36:16On the other hand, it's much bigger than Jupiter,
36:19which is only one one-thousandth the mass of our Sun.
36:22These objects heat up, but never go boom,
36:26like stars do.
36:29They're difficult to see because they're very dim,
36:32but they could contain a significant amount of mass.
36:35A brown dwarf that periodically skirted the edge of the solar system
36:38and nudged comets towards us
36:41would be too dim to see,
36:44at least not without night vision.
36:50With infrared telescopes, you can pick up the heat of something,
36:53even if it's not giving off any light.
36:56So something like a brown dwarf,
36:59even though it's not glowing in visible light,
37:02should be giving off a lot of heat.
37:05There have been missions which have surveyed the whole sky,
37:08and if there had been a four-billion-year-old brown dwarf
37:11anywhere near the Sun, it would have been spotted.
37:14So far, no Nemesis has been found.
37:17So if there's no Nemesis star out there,
37:21Another idea is there could be a relatively massive planet,
37:24Earth-sized, for example, orbiting way out past Neptune.
37:29Detecting a planet that far away is much more difficult
37:32than detecting a dwarf star.
37:35The farther a planet is from the Sun,
37:38the dimmer it is and the harder it is to see.
37:41So you could have a planet the mass of the Earth
37:44several billion miles out from the Sun,
37:47basically invisible to our telescopes now.
37:55There is still a chance that something the size of the Earth
37:58might be out there, far away in the Oort cloud, disrupting things.
38:03That's the last gasp of the Nemesis theory.
38:08We may never know what's nudging Oort cloud objects out of orbit,
38:12and it's not the only mystery out there.
38:16We still don't know where the solar system ends.
38:19Finding the edge of the solar system
38:22helps us with the bigger problem of understanding the solar system as a whole.
38:25How do you know when you have the whole solar system
38:28until you've found the edge?
38:31But perhaps the biggest mystery of all is that our solar system,
38:34with a medium-sized star at its center,
38:37seems to contain far too many objects in its outer reaches,
38:40which raises the prospect that many of these icy lumps
38:43originated from elsewhere in the galaxy.
38:46Theoretically, we can calculate how many comets should have formed
38:49when the solar system itself formed.
38:52And then we can look at the number of these comets coming in from the Oort cloud
38:55and try to figure out how many are actually out there.
38:58It turns out the two numbers are way off.
39:01The number that formed with the Sun is much, much smaller
39:04than the number of comets we infer are out there.
39:07There's simply too much material in the Oort cloud
39:10to belong to one average-sized star.
39:13The only way we can balance these two numbers
39:16is if the Sun, when it was young, was a thief.
39:21Five billion years ago, a supernova exploded,
39:24sending a shockwave through a vast cloud of dust and gas.
39:29It compressed the cloud,
39:32creating regions of dense gas.
39:35A star nursery.
39:39Hundreds of pockets of gas collapsed under their own gravity,
39:42giving birth to hundreds of sibling stars.
39:46We think our Sun formed with brothers and sisters in clusters,
39:49and the Sun, we think, formed in a cluster with 400, 500 other stars.
39:53If you can picture these stars forming,
39:56and they're very small compared to the distance between them,
39:59but their own cloud of comets is very large.
40:02So it's possible to steal comets from each other.
40:05The Sun was smart, and it walked away when it was ahead.
40:08It got a bunch of comets from other stars and then went off on its own way.
40:12Some of the stuff orbiting us right now far away
40:15could have come from those other young stars.
40:19This cosmic sibling rivalry may still be happening.
40:23Our Sun could be stealing objects from nearby stars,
40:26while they are stealing from our Oort cloud.
40:30The nearest star that our Oort cloud butts right up against
40:33the solar system of is Alpha Centauri.
40:35There's probably another Oort cloud around Alpha Centauri.
40:38Maybe there are objects that even switch between the two stars.
40:41So halfway in between these two stars
40:44is where we find the true edge of our solar system.
40:47What happens at the edge could spell disaster.
40:52Alpha Centauri could have a real effect on life here on Earth.
40:55It could be that that star and the objects around it in its Oort cloud
40:59have some gravitational effect on our Oort cloud.
41:02Something falls in that may have been due to the influence of our nearest star.
41:07So it's hard to know where our solar system ends
41:10and a new one begins.
41:12We humans love hard edges.
41:15We like boundaries.
41:17But the solar system, it's not like that.
41:19It just fades away.
41:21It blends into the galaxy at large.
41:24The Sun does not live alone and nor does our Oort cloud.
41:27We're interacting continuously
41:29and in that way we're connected to the much larger environment of the Milky Way.
41:34This no man's land could help us travel to the stars.
41:38Because where our solar system ends
41:41we might find stepping stones
41:44that could be the launch pad for a new interstellar journey.
41:51One interesting thing about knowing now the structure of our solar system
41:55is that there are many more objects that we can use for resources
42:00as we venture out into interstellar space.
42:04There's rock out there, there's ice.
42:06We might be able to replenish our spacecraft as we go past.
42:09Maybe something as distant and tenuous as the Oort cloud
42:12will be our next jumping off place to truly explore the galaxy.
42:17We have much to learn about the solar system
42:20with many new surprises to discover.
42:24Every time we open a new window on the universe we're surprised
42:27and that's what excites me.
42:29Every morning I wake up and I'm surprised if I'm not surprised.
42:32And as new technologies open up
42:34so too will our understanding of what's out there.
42:37We're building better telescopes.
42:39We're sending probes out into space.
42:41We have so much more to learn and that's the real exciting part.
42:45Our new eyes to the universe
42:47are showing our solar system in a whole new light.
42:51Slowly we're piecing together the complex connections
42:54that bind it to our lives and to the universe beyond.
42:59Every new discovery we make about the universe
43:02has told us that no solar system is an island,
43:05that materiality is an island,
43:08that no solar system is an island,
43:10that material comes from outside and comes inside
43:13and from inside goes to the outside.
43:15And so as we probe the outer edges of our solar system
43:18we're changing what we mean by solar system.
43:22We are taking the first baby steps off the planet
43:25and starting to look around our environment
43:27and that's pretty neat.
43:30The sun is like one voice in a vast chorus
43:33of hundreds of billions of other stars
43:35and their particular solar system.
43:39And I find it very poetic
43:41that in fact we are part of a much larger population out there.
43:47To think that we found everything in our solar system,
43:50to assume we know everything, is not the way science works.
43:53Science works by pushing the frontiers
43:56and we continue to do so
43:58and in ten years if we do this program again
44:01I wouldn't be surprised if we talk about new things.