The Solar System Sleepy Science Club

[00:00:00]Good evening and welcome. However you have found your way here tonight and wherever in the world you happen to be, I am so very glad that you've come.

[00:00:12]Perhaps you are already tucked beneath the blankets with a lamp turned low.

[00:00:17]Perhaps you're resting your eyes at the end of a long journey home or lying in the quiet dark of a room where the day has finally gratefully ended. Wherever you are, there is nothing at all you need to do for the next little while.

[00:00:33]You don't have to concentrate. You don't have to remember a single word. You need only breathe slowly. Let your body grow heavy and warm and allow the sound of a calm voice to carry you gently along.

[00:00:47]Before we set out, I'd love to know one small thing. Which corner of the world are you settling down in tonight?

[00:00:56]These quiet recordings seem to find their way to the most wonderful scattering of places. A high window above a glittering city, a still house at the end of a country lane, a narrow bed in a room where it is deep in the night, and another where the morning is only just beginning to glow.

[00:01:18]If you feel like it, you might leave a gentle word somewhere below to say hello and to tell us where you're listening from.

[00:01:27]There is something deeply comforting in the thought of so many people spread right across this small and turning world all quietly lying down beneath the very same sky. And if these slow and gentle wanderings bring you a little peace at the end of the day, then a soft tap of the like button and a quiet press of subscribe help more than you might easily imagine. A channel such as this one is a small and tender thing, and it can only grow when a few kind souls decide to linger a while and stay. Each like is a quiet nod to the unseen machinery that decides what gets shown, telling it that this calm little harbor is worth offering to someone else who lies awake and longs for rest. Each subscription is a soft and generous vote that simply says, "Please keep making these." It asks nothing of you but a moment. It costs you nothing at all. And here on this end of things, it means the whole world. So, thank you truly and warmly whether tonight is your very first visit or one of many you've paid.

[00:02:44]Now, let me tell you where we are wandering this evening. Tonight, we are going on a journey. A slow, unhurried, altogether gentle tour of our own cosmic neighborhood, the little family of worlds that circles our sun. We call it the solar system. and it is the closest thing we have to a home address in all the vastness of space. We'll begin at the warm center at the sun itself and then we'll drift outward ever so slowly from world to world past scorched and crated rocks. Past a planet wrapped in poison cloud. Past our own blue home and its pale companion. past a red desert world and out through a field of drifting stone. And then onto the great giant planets with their rings and their swarms of moons.

[00:03:40]And beyond even them, all the way out to the frozen edges of the sun's realm, where the light grows faint and the cold grows deep and comets sleep in the dark.

[00:03:54]I promise there will be nothing difficult here. No mathematics, no long names to memorize, no test waiting at the end of the night. If anything, I'd like this to feel less like a lesson and more like being read a bedtime story, a slow, wandering tale about the strange and beautiful places that quietly share our little corner of space. And if you happen to drift off to sleep somewhere along the way, long before we reach that final frozen edge, please don't feel that you've missed a thing. That is exactly what all of this is for. It would be the kindest possible way to spend the night. So, make yourself as comfortable as you possibly can. Settle your head deeper into the pillow. Let your shoulders sink and close your eyes whenever they feel ready to close. And let's begin our slow and peaceful voyage out among the worlds, a calming tour of the solar system, our small and wondrous home in [clears throat] the dark.

[00:05:01]Before we visit any single world, let's take a moment to picture the whole of it. The shape of the little kingdom we are about to tour. Because to wander from planet to planet without a sense of the map is a bit like walking through a great house in the dark. So let's turn on a soft light just for a moment and see the whole of our home laid out before us.

[00:05:26]At the very center of everything sits the sun. One ordinary star, a great glowing ball of light and warmth. And here is the first thing that surprises almost everyone. The sun is not merely the center of the solar system. It very nearly is the solar system. If you could gather up all the material in our entire cosmic neighborhood, every planet, every moon, every asteroid and comet and grain of dust and weigh it all together, you would find that the sun alone makes up more than 99 12% of it. Everything else, all the worlds we are about to visit, every planet you have ever heard of are together just the smallest leftover crumbs scattered around that one enormous fire. We live quite literally in the crumbs of the sun's making. And those crumbs are held in place by an invisible patient hand. The gentle pull of the sun's gravity reaching out across the emptiness and keeping every planet swinging around it in a great slow circle.

[00:06:41]The planets do not fall into the sun and they do not fly off into the dark. They simply drift around it endlessly.

[00:06:51]Each one tracing a wide and nearly circular path. All of them lying in roughly the same flat plane, like the grooves on an old record or ripples spreading on a still pond. If you could float high above the solar system and look down, you would see a set of nested rings, the planets sweeping around their central fire in the same unhurried direction, patient and orderly, the way they have for billions of years. But here is the part that is hardest to hold in the mind and the most important to feel. How astonishingly empty it all is.

[00:07:33]When we see the solar system drawn in a book, the planets are crowded together, bright and close, one after another. But that is only because the truth cannot fit on a page. In reality, these worlds are separated by tremendous yawning gulfs of empty space. Let me offer you a small model to rest on. Imagine the sun shrunk [clears throat] down to the size of a large beach ball set at one end of a long field. On that scale, the Earth would be a tiny peppercorn, and it would sit some 26 paces away, and the outermost planets would be hundreds of paces further still. Tiny specks lost in the grass with nothing but silent, empty darkness in between. The solar system is not a crowded place. It is a vast and quiet emptiness with a handful of small, precious worlds drifting through it. far far apart.

[00:08:38]It's worth pausing too on where all of this came from because the whole neat arrangement is no accident.

[00:08:47]Long ago, about 4 1/2 billion years in the past, there was no sun and no planets at all. Only a vast, slowly turning cloud of gas and dust drifting in the dark. Over time, gravity drew that cloud together, pulling most of it into a dense, hot center that would become the sun. And the leftover material swirling around that newborn star in a great flattened disc slowly gathered itself into clumps and the clumps into worlds. The rocky inner planets formed close to the warmth where only rock and metal could endure. The great gas giants formed far out in the cold where ice and gas could gather in abundance. So the whole solar system, sun and planets alike, was born together out of a single spinning cloud. Which is why the planets all lie in the same flat plane and all circle the same way like the last slow swirl settling in a stirred cup of tea. And that is the shape of our little kingdom. One great fire at the center and a scattering of lonely worlds circling it in the dark.

[00:10:05]Bound together by nothing more than the soft and constant pull of gravity.

[00:10:11]Tonight we are going to visit them one by one, drifting outward from the warmth at the center toward the cold and distant edge. So let's begin our tour where all the light and warmth begins at the great hearth of the solar system, the sun itself. Let us begin then at the heart of it all. At the sun, the great fire around which everything else quietly turns. We see it every day, so familiar that we hardly notice it. Just the bright thing we shade our eyes against. But tonight, let's look at it with fresh and drowsy wonder. Because our sun is a truly magnificent thing.

[00:10:57]First, its size, which is almost impossible to feel. The sun is enormous.

[00:11:05]It is so large that you could fit well over a million Earths inside it with room left to spare. If the Earth were a small marble, the Sun would be a great sphere as tall as a two-story house. It is by far the largest object in our entire solar system. And yet, out among the stars, it is really rather ordinary, a middling star among countless others, only close enough to us that it fills our whole sky with light. That is worth remembering softly. Our great and mighty sun is just one of the everyday stars scattered across the night brought near.

[00:11:47]And what is it exactly? The sun is not solid. You could not stand upon it or land a ship there. It is a vast churning ball of extremely hot gas, so hot that it glows with blinding light. And deep within its core, under unimaginable pressure and heat, something wonderful is happening quietly and endlessly. The very smallest pieces of matter are being squeezed together so fiercely that they merge. And in that merging, they release a flood of energy. A gentle, ceaseless process that has been pouring out light and warmth for billions of years. That is the source of every sunbeam that has ever fallen on your face, every warm afternoon, every sunrise you have ever seen. It all begins in the deep hidden heart of the sun.

[00:12:46]Now that light does not reach us instantly. The sun sits about 150 million km away. And even light, the fastest thing there is, takes about 8 minutes to cross that gulf.

[00:13:01]So when you step outside on a bright morning and feel the warmth on your skin, that warmth is already 8 minutes old. It left the surface of the sun and traveled all the way across the emptiness of space just to reach you.

[00:13:19]There is something rather tender in that if you think about it slowly, that the light warming you now began its journey before you even stepped outside.

[00:13:30]The sun is not quite as calm and unchanging as it appears either. Its glowing surface churns and boils constantly, and from time to time it is dotted with darker, cooler patches that we call sunspots, some of them larger than the entire Earth, which come and go over the years. And the sun does not simply sit still and shine. It breathes out continuously a faint stream of tiny particles that flows outward in every direction, washing over all the planets out to the very edge of the solar system. When that gentle stream reaches the Earth and is guided down toward our poles by our planet's magnetic field, it paints the night sky with shimmering curtains of green and red light. the auroras, the northern and southern lights. So even those quiet dancing lights over the far north and the far south are really the touch of the distant sun reaching all the way across space to brush against our air.

[00:14:40]The sun has been shining like this for about 4 and a half billion years since it first formed from a great collapsing cloud of gas and dust. And it is not even halfway through its long life. It has fuel enough to keep shining steady and warm for billions of years yet to come. So there is nothing to fear in the sun. It is not going anywhere. It is the great constant hearth of our little home. The fire at the center of the room, patiently pouring out the light and warmth that makes every other world we will visit tonight possible. And it is that light we will follow now outward into the cold. For circling closest to this great fire, scorched and battered and small, is the first of our worlds, a little gray planet racing around the sun, so near to the flames that it can barely be seen in the glare.

[00:15:41]Let us drift outward to meet it. The very first world out from the sun is a small gray cratered planet named Mercury. It is the smallest of all the planets, only a little larger than our own moon, and it lives its whole life pressed close to the great fire at the center of everything. Of all the worlds we will visit tonight, it is the nearest to the sun and [clears throat] in many ways the most extreme. Because it sits so close to the sun, you might imagine Mercury to be the hottest planet of all and its sunlit side is indeed fearsomely hot. Hot enough in the full glare of the sun to melt many metals. But here is the strange and lonely thing about Mercury.

[00:16:30]It has almost no air. It is a bare rock with no real atmosphere to speak of, no blanket of gas to hold onto its warmth or to spread it around.

[00:16:43]And so the moment you pass into its shadow, into the long dark night, all that heat simply escapes into space and the temperature plummets to well below the coldest place on Earth. On Mercury, the difference between the sunlit day and the shadowed night is greater than on any other planet. A searing, blistering day and a bitterly frozen night with nothing in between. It is a world of harsh extremes.

[00:17:16]All because it has no gentle blanket of air to soften them.

[00:17:21]If you could stand on its surface, and let's only imagine it from the safety of your warm bed, you would find a landscape that looks a great deal like our own moon.

[00:17:33]Mercury is covered almost everywhere.

[00:17:37]in craters. The round scars left by countless collisions over billions of years with asteroids and comets and drifting stones. With no wind and no rain to wear them away, those craters remain sharp and ancient exactly as they were made. There is one truly enormous one. A vast impact basin so wide it could swallow whole countries left by some catastrophic collision in the deep past.

[00:18:09]Mercury wears the whole long history of the solar systems violence written across its gray and silent face. And yet for all its harshness, this scorched little world holds a gentle secret. Deep within the shadows of certain craters near its poles, in places where the sun's light never ever reaches, it is thought that there may be ice, patches of frozen water, hiding in the permanent dark on the planet closest to the sun. Imagine that. On this seared and blasted rock, roasting under a sun three times larger in its sky than in ours, there are pockets of cold so deep and so lasting that water has stayed frozen there, perhaps for billions of years. Even the most extreme places keep their quiet surprises.

[00:19:04]Mercury is a difficult world to catch sight of, precisely because it clings so close to the sun. It never wanders far from the sun's glare in our sky. And so it can only ever be glimpsed low on the horizon briefly in the soft light just after sunset or just before dawn. A shy little world always half hidden in the brightness. It is fitting perhaps that the ancient peoples named it after their swiftfooted messenger, forever darting close beside the sun and never straying far. The quickest of all the planets to race around the sky.

[00:19:44]Mercury also moves in a curious rhythm.

[00:19:47]It races around the sun [clears throat] very quickly, completing a full lap in only about 88 of our days.

[00:19:56]The fastest orbit of any planet. And yet, it turns on its own axis very slowly. so slowly that a single day on Mercury, one full cycle of sunrise to sunrise, lasts far longer than its year. It is a strange, unhurried spin for such a fast little traveler.

[00:20:19]But then everything about Mercury is a study in contrasts, fast and slow, scorching and freezing, close to the great fire, yet hiding pockets of ancient ice. Let us leave it now, this small gray wanderer, and drift a little further out to a world that looks from a distance deceptively beautiful and turns out to be the most hostile place of all.

[00:20:50]The second world out from the sun is one you have almost certainly seen, though you may not have known it. If you have ever noticed a single brilliant steady point of light low in the sky just after sunset or just before dawn brighter than any star shining with a calm silvery glow.

[00:21:14]Then you have seen the planet Venus. For thousands of years people have watched it and loved it, calling it the evening star and the morning star. It is one of the loveliest sites in all the sky and it hides one of the most astonishing and terrible secrets in the entire solar system.

[00:21:36]From a distance, Venus looks serene and beautiful. A smooth, pale, creamy globe wrapped from pole to pole in soft, unbroken cloud. And in size, it is almost a twin of our own Earth. nearly the same width, nearly the same weight.

[00:21:56]For a long time, before we could see beneath those clouds, people imagined it might be a warm and misty sister world, perhaps even a lush and living one. But the truth, when at last we uncovered it, could hardly have been more different.

[00:22:15]For those beautiful clouds are not made of water. They are made of sulfuric acid. And beneath them lies a world transformed into a furnace.

[00:22:27]Venus is the hottest planet in the entire solar system, hotter even than little Mercury, which sits far closer to the sun. And the reason why is a lesson written across the sky. Venus is wrapped in an incredibly thick, heavy blanket of gas. An atmosphere so dense and so choked with heat trapping carbon dioxide that the warmth of the sun once it seeps in can almost never escape again.

[00:22:57]And so the heat builds and builds and builds over and over until the surface reaches a temperature hot enough to melt lead. Everywhere day and night, pole to pole, Venus roasts at that same searing heat. There is no cool place, no shaded valley, no gentle night. It is all of it, a furnace beneath the cloud. And the heat is only the beginning. That thick atmosphere presses down on the surface with a crushing weight. A pressure so great that standing on Venus would feel like being deep, deep beneath the ocean.

[00:23:38]The weight of the whole heavy sky bearing down on you at once. It would flatten you in an instant. The sky itself glows a dull, sullen orange, thick and hazy. And somewhere high above, the clouds of acid drift on winds that race around the whole planet. It is by almost any measure the most hostile world we know. A place of scorching heat, crushing weight, and poisoned air, all hidden beneath that soft and beautiful veil. There is a quiet lesson in Venus, too, if we care to hear it.

[00:24:18]For Venus and the Earth were born as near twins, sister worlds of nearly the same size, made of much the same material, formed side by side around the same young son. And yet one became a living blue jewel and the other a lifeless furnace.

[00:24:38]The difference came down in large part to that runaway blanket of heat trapping gas. The way Venus's atmosphere came to trap more and more of the sun's warmth until the heat spiraled entirely out of control, and any gentle oceans it may once have had boiled away into the sky forever. It is a sobering thing to look up at that bright and lovely evening star and to remember that it is in its way. A cautionary tale written across the heavens. A reminder of how delicate the balance is that keeps a world cool and gentle and kind to life.

[00:25:23]Venus turns in its own strange way, too.

[00:25:27]It spins so slowly that a single day there lasts longer than its entire year.

[00:25:33]And stranger still, it spins backwards in the opposite direction to nearly every other planet. So that on Venus, were you ever able to see through the cloud, the sun would rise in the west and set in the east. No one is quite certain why. Perhaps long ago, some tremendous collision knocked it turning the wrong way. And so Venus stands as a kind of quiet warning drifting there in the sky. A world so like our own in size and make and yet transformed by its runaway blanket of heat trapping air into a place where nothing we know could ever survive. It is a beautiful thing to look up at that steady silver light in the dusk. But it is a fierce and lonely furnace beneath. Let us leave it now and drift onward to a world softer and kinder than any we have met so far. For the next planet out is our own.

[00:26:39]The third world out from the sun is one you know very well indeed. In fact, you are resting upon it right now. It is our own earth. And I'd like us to pause here for a while and try to see it not as the ordinary ground beneath our feet, but as one more world among the others, seen freshly as a traveler passing through might see it for the very first time.

[00:27:06]Imagine you were arriving from far out in space, drifting inward past the scorched gray rock of Mercury, past the poisoned furnace of Venus, and then you came upon the Earth. After all that harshness, what a sight it would be. A world of deep shining blue wrapped in swirls of soft white cloud with patches of green and gold and brown and glittering seas that catch the light of the sun. It would look against the black of space like a single perfect jewel alive with color glowing with warmth.

[00:27:49]Astronauts who have actually seen it with their own eyes hanging there in the dark often describe it as the most beautiful and moving sight of their lives. And from far enough away, from out beyond the distant planets, our whole world shrinks to nothing more than a tiny point of pale blue light, a single soft pixel in the vast dark. And yet that little dot is everything. Every person who has ever lived, every story ever told, every one of us, all held on that one small moat of blue.

[00:28:29]What makes the Earth so different from the harsh worlds on either side of it? A few gentle, precious things all coming together at once. First, we sit at just the right distance from the sun. Not so close that our oceans boil away as they would nearer in, and not so far that they freeze solid. We rest in a gentle temperate zone where water can exist as a liquid flowing in rivers and gathering in seas. And that liquid water, as far as we know, made all the difference.

[00:29:05]Second, we have an atmosphere, but a soft and balanced one. a thin, breathable veil of air that warms us just enough and shields us from the worst of the sun's harsh rays without ever becoming the smothering furnace of Venus. And third, and most wonderful of all, our world is alive. As far as we yet know, in all the vast solar system, and perhaps far beyond it, the Earth is the only place where there is life, the only world with forests and coral reefs, with bird song and whale song, with creatures that swim and crawl and fly, and with beings like you who can lie awake at night and wonder about the stars. That is a truly staggering thing to rest on, gently in the quiet.

[00:29:59]Every other world we visit tonight is, as far as we know, silent and empty.

[00:30:06]Beautiful, yes, and strange and wondrous, but without a single living thing. And here on this one small blue world, wrapped in its thin veil of air, life bloomed. Not just life, but life that grew curious enough to look up and out and to send little machines drifting to all the other worlds just to see what was there.

[00:30:33]We are the way this small planet has come to know its neighbors. And it is worth remembering softly how thin and delicate the living layer of our world truly is. The atmosphere that holds all our weather, all our air, everything we breathe. It is a shockingly thin skin, no thicker in proportion than the peel on an apple. Everything you have ever known, every living thing that has ever existed has lived its whole life within that fragile, precious film of blue. It is a fragile, beautiful thing, our home.

[00:31:13]And tonight, as we drift outward from it to visit the other worlds, I hope you'll carry a little of that wonder with you.

[00:31:22]The wonder of resting right now, safe and warm, on the one living jewel in all the dark.

[00:31:30]But before we leave the earth behind, there is one more world close by that we must visit. For the Earth does not travel alone. No tour of our neighborhood would be complete without a visit to the Earth's own faithful companion, the moon. It is the one other world in all the solar system that human beings have actually set foot upon. And it hangs there in our sky every night.

[00:31:57]So familiar and so beloved that we sometimes forget what an extraordinary thing it is.

[00:32:04]The moon is a world in its own right. A great ball of gray rock about a quarter the width of the Earth circling us patiently at a distance of some 380,000 km.

[00:32:18]That may sound close, but it is farther than it looks. You could line up every other planet in the solar system side by side in the gap between the Earth and the Moon, and they would all fit with room to spare. Our nearest neighbor floats in a wider gulf than we usually imagine. Where did it come from?

[00:32:41]The leading story is a dramatic one. Set in the earliest days of the solar system when the young Earth was still forming.

[00:32:50]It's thought that long ago, another young world, a body perhaps the size of Mars, collided with the early Earth in a tremendous glancing blow. The impact flung an enormous cloud of molten rock and debris up into space where it swirled and gathered and slowly over time drew itself together under its own gravity cooling into the moon we know.

[00:33:18]So the moon may be in a sense a piece of the early earth thrown into the sky and reshaped into a companion.

[00:33:28]It has circled us faithfully ever since.

[00:33:31]If you stood on the moon, you would find a still and silent world. There is no air there. And so there is no wind, no weather, no sound at all, only a deep and perfect quiet. The sky above is black even in the daytime and dusted with unblinking stars. The ground is a soft gray powder marked all over with craters and with the footprints of the few astronauts who walked there, which with no wind to disturb them, will remain exactly as they were left for millions of years to come. And there, hanging in that black sky, you would see the Earth, our whole blue world, distant and small and impossibly beautiful, the loveliest thing in the moon's silent heavens.

[00:34:26]The moon also keeps one gentle secret of motion. It is locked in a slow embrace with the earth, turning on its own axis at exactly the same rate that it circles us, so that it always shows us the very same face. The familiar pattern of light and dark that you see on the moon tonight is the same pattern your grandparents saw and their grandparents before them all the way back.

[00:34:53]The far side of the moon, the side that always faces away, was never seen by any human eye until we sent machines around to look. There is something quietly touching in that, that our nearest companion has always turned the same face toward us, like an old friend who never looks away. And the moon does something for us, too. Softly and constantly, its gentle gravity reaches across that gulf and tugs on our oceans, drawing them up and letting them fall, giving us the slow, patient rhythm of the tides, the rising and falling of the seas twice each day, all around the world, guided by the pull of that pale companion in the sky. It is a calming thought as you drift towards sleep that the tides breathing in and out along every shore tonight are being gently led by the moon. But now let us leave the earth and its faithful companion behind and travel further from the sun's warmth out to a cold and rusty world that has captured human imagination like no other. The red planet Mars.

[00:36:08]The fourth world out from the sun is a small, cold, rusty red planet named Mars. Of all the worlds beyond our own, it is the one we have dreamed about the most. The one we have sent the most little robotic explorers to, the one we imagine someday walking upon. And it is easy to see why. For of all the planets, Mars is in some quiet ways the most like home. First, that famous color. Mars glows a soft warm red in our night sky.

[00:36:45]And up close, its surface really is red, a rusty orange brown, the color of dried clay or autumn leaves. And the reason is beautifully simple. The soil of Mars is quite literally rusty. It is rich in iron and over billions of years that iron has rusted just as an old iron nail rust red staining the whole planet the color of rust from pole to pole.

[00:37:15]Mars is a great cold rusted desert. Its dunes and plains stretching red beneath a dusty pale sky.

[00:37:24]It is a smaller world than ours, only about half the width of the Earth. And it is a cold one, far enough from the sun that even at the height of a Martian summer's day, the warmth barely reaches the freezing point, and the long nights grow bitterly cold.

[00:37:42]Its air is thin, a wisp of an atmosphere, far too thin and too starved of oxygen for us to breathe. Mostly the same heat trapping gas that smothers Venus, only here spread so thinly that it warms almost nothing at all. Mars is a quiet, frigid, windswept desert of a world. And yet, it holds some of the most spectacular scenery anywhere in the solar system. Rising from its plains is the tallest volcano known on any planet.

[00:38:17]A colossal, gently sloping mountain so vast that it stands about two and a half times the height of the tallest mountain on Earth. So wide it would cover an entire country. And slashed across the planet's face is a canyon system so immense that it makes our own grandest canyons look like tiny cracks. A great wound in the surface stretching as long as an entire continent and in places deep enough to swallow mountains whole.

[00:38:49]Mars is a small world, but it wears some of the grandest landscapes anywhere among the planets. But the thing that draws us most to Mars is the water. For today, Mars is dry, a frozen desert with no rivers, no seas, no rain. And yet everywhere across its surface, we find the unmistakable marks of water that flowed long, long ago. Winding valleys that look for all the world like dried up rivereds.

[00:39:24]Broad basins that seem to have held ancient lakes, deltas where rivers once spread into standing water. Billions of years ago, it seems Mars was a warmer, wetter, gentler place. with flowing rivers and standing seas beneath a thicker sky. A world perhaps not so very unlike the early earth. And then slowly over the ages it lost its thick air and its warmth and its water froze and vanished and it became the cold red desert we see today.

[00:40:01]Which leaves one of the most tender questions in all of science hanging quietly over it.

[00:40:08]In those ancient warmer days when Mars had rivers and lakes, did anything, however small and simple, ever live there? We do not yet know.

[00:40:20]Little rovers trundle patiently across its dust even now, sifting the rusty soil, searching for the faintest whisper of an answer.

[00:40:31]No other world apart from our own have we studied so lovingly and so closely.

[00:40:38]For decades now, we have sent a small fleet of robotic explorers to Mars.

[00:40:44]orbiters circling high above, patiently mapping every dune and crater, and rovers rolling slowly across its rusty plains, sampling the soil, photographing the pink sky, drilling into ancient rock. These patient little machines have become our eyes and our hands on another world, trundling for years across that cold red desert, sending home quiet postcards from a place no human foot has ever touched. And many people dream that one day human explorers might follow, that someone might stand upon those red sands and watch the small pale sun rise over the rim of an ancient crater.

[00:41:32]Whether or not that day ever comes, there is something rather moving in how tenderly we have reached out to this one cold and distant neighbor.

[00:41:43]Mars keeps a little frozen water still, locked away in bright white ice caps at each of its poles, which grow and shrink with the Martian seasons and mixed into the frozen ground. It has two tiny moons, little more than captured lumps of rock, tumbling close above its surface.

[00:42:06]And every so often, great dust storms rise up and can spread until they cloak the entire planet in a haze of red, blotting out the surface for weeks at a time. It is a lonely, beautiful, haunting world, this red desert. So like our own and yet so cold and still and silent. Let us leave it now and cross a truly great gulf of emptiness out to a strange and scattered region that marks the boundary between the small inner worlds and the giants beyond. As we leave Mars behind and travel outward, we cross a wide and curious region of the solar system.

[00:42:51]A great broad band circling the sun that is scattered with drifting chunks of rock and metal, tumbling silently through the dark. We call it the asteroid belt and it marks a kind of boundary, a frontier between the small rocky inner worlds we have just visited and the enormous giant planets that lie beyond.

[00:43:16]The asteroids are in a sense the rubble of the solar system. Countless leftover pieces of rock and metal ranging from tiny pebbles to great tumbling boulders to a few bodies hundreds of kilome across. They are thought to be the scattered building blocks of a planet that never was. In the earliest days of the solar system in this region, small pieces of rock might have gathered together as they did elsewhere and grown into a world of their own. But here they never quite managed it. The enormous gravity of the giant planet just beyond kept stirring the pot, tugging and jostling these pieces so that they could never settle down and come together into a single planet. And so they remain to this day. A great ring of leftover rubble, drifting forever between the worlds, the unfinished pieces of a planet that never got to be born.

[00:44:20]Now here, I must gently correct a picture you may carry in your mind because films and stories nearly always get it wrong. When we hear asteroid belt, we tend to imagine a dense, crowded, treacherous field of tumbling rocks packed so tightly that a ship would have to weave and dodge to survive. But the truth is almost the opposite, and it is far more peaceful.

[00:44:49]The asteroid belt is enormous, a vast wide ring spread across a tremendous gulf of space. And the asteroids within it are on average separated by such huge distances that they are typically hundreds of thousands, even millions of kilometers apart. If you were standing on one asteroid, the nearest one would very likely be so far away that you could not even see it.

[00:45:19]In fact, the little spacecraft we have sent out through the belt on their way to the distant planets have passed through it again and again without ever coming close to hitting a thing. It is not a crowded dangerous field. It is a vast and lonely emptiness with a scattering of quiet stones drifting very very far apart.

[00:45:44]There is one body in the belt large enough and round enough to be counted as a small world in its own right. A dwarf planet, the largest object among all the asteroids, making up a good fraction of the entire belt's material all on its own. It is a round, gray, icy, rocky little world. And there is a real chance that it holds a great deal of frozen water. Perhaps even hidden pockets of liquid water somewhere beneath its surface.

[00:46:18]Another quiet reminder that water so precious to us is scattered all through the solar system, waiting in the cold and the dark.

[00:46:29]So the asteroid belt is a gentle place really for all its rugged name. A broad hushed ring of ancient rubble, the leftover crumbs from the building of the worlds, drifting far apart in the dark.

[00:46:44]It is the great frontier of our tour, the last of the small and rocky places.

[00:46:51]Beyond it, everything changes. For as we cross that final stretch of emptiness, we come at last to the realm of the giants. world so vast that all the little planets we have visited so far could vanish inside them without a trace. Let us drift onward then to meet the king of them all. Beyond the asteroid belt, across a truly immense gulf of empty space, we come at last to the largest and mightiest world in all the solar system, the giant planet Jupiter. After the small rocky familiar worlds of the inner solar system, Jupiter is something else entirely. It is a world of a completely different kind and it sheer scale is difficult to hold in the mind.

[00:47:41]Jupiter is enormous. It is so large that you could fit well over a thousand Earths inside it. It is more than twice as heavy as all the other planets in the solar system combined.

[00:47:55]Every other world put together would not add up to half of Jupiter alone. If the Earth were a small grape, Jupiter would be a great beach ball. It is quite simply the king of the planets, a colossus that dwarfs everything else that circles the sun. [clears throat] And it is a completely different kind of world from the ones we've seen.

[00:48:20]Jupiter is not a ball of rock. It is what we call a gas giant. A vast, deep world made mostly of the lightest gases.

[00:48:30]The same sort of material the sun itself is made from. It has no solid surface at all. Nowhere you could ever stand. If you tried to descend into Jupiter, you would simply fall through thickening layers of cloud and gas, growing hotter and denser around you, with no ground ever rising up to meet you. It is a world all the way down of swirling gas and crushing pressure with perhaps only a strange dense core hidden far beneath at its very heart.

[00:49:05]What you would see from a distance is one of the most beautiful sights among all the planets. A great striped globe wrapped in bands of cloud in soft creams and golds and warm browns streaming around the planet in stripes driven by ferocious winds. These bands are endless storms and currents in Jupiter's thick atmosphere, sliding past one another in opposite directions, curling and swirling. And set among them is Jupiter's most famous feature of all. A colossal deep red storm. A great swirling oval larger than our entire Earth. It is a hurac of unimaginable size. A storm that has been raging without pause for centuries, spinning there, calm from a distance and violent up close longer than anyone has been alive to watch it. a single storm larger than a world that has blown for hundreds of years. But Jupiter is not just a single world. It is more like a little kingdom of its own. For it is circled by a vast host of moons, a great swarm of them, around a hundred known, and quite possibly more still to be found. Most are small, but four of them are true worlds in their own right. large enough and wondrous enough to deserve a mention of their own. One is the most volcanic world in the entire solar system. Its surface forever bursting with erupting mountains of fire and sulfur. Another is wrapped in a smooth shell of ice. And beneath that ice, it is thought there lies a deep, dark worldwide ocean of liquid water. more water perhaps than in all the seas of Earth, hidden in the cold and dark. It is one of the most promising places in all the solar system to search for life beyond our own. A third of these moons is the largest moon anywhere around any planet. So big that it is actually larger than the planet Mercury. And the fourth is an ancient cratered quiet world pockmarked all over with the scars of long ago.

[00:51:32]Jupiter's sheer size gives it a kind of quiet importance for the rest of us too.

[00:51:37]Its enormous gravity acts almost like a great guardian in the outer solar system. its pull helping to catch or to fling away many of the wandering comets and stray rocks that might otherwise come tumbling in toward the inner worlds. In a sense, this giant may have helped to keep our own small world a little safer over the long ages, sweeping up or deflecting some of what might otherwise have struck us.

[00:52:08]And Jupiter is wrapped in a magnetic field of tremendous power, far stronger than the Earth's, which cloaks the whole planet in an invisible cocoon, and lights its poles with shimmering auroras of their own, vast glowing curtains of light, dancing over the poles of the giant, larger across than our entire planet.

[00:52:32]So Jupiter reigns out there vast and striped and stormy with a swarm of worlds circling around it like a solar system in miniature. It is the great giant, the king. And yet for all its size, it is not the most beautiful of the giants. That title belongs to the world we drift toward next. A planet so lovely that when people first saw it clearly, they could scarcely believe their eyes.

[00:53:02]Beyond Jupiter, further out into the cold and the dark, we come to what is for many people the single most beautiful object in all the solar system, the ringed planet, Saturn. If you have ever looked at Saturn through even a small telescope and seen it hanging there, tiny and perfect, wrapped in its shining rings, you will never forget it. It looks almost too lovely to be real, like a jewel or a child's drawing of what a planet ought to be.

[00:53:37]And it is real, drifting quietly out there in the dark. Exactly that beautiful. This very night, Saturn is like Jupiter, an enormous gas giant. The second largest world in the solar system, another vast ball of swirling gas with no solid surface, wrapped in soft, pale bands of gold and cream. It is so large you could fit hundreds of Earths inside it. And here is a delightful little fact to rest on.

[00:54:08]Saturn is so puffy, so light for its great size that it is actually less dense than water. If you could find an ocean vast enough to hold it, Saturn would float. This gigantic planet, wider than 100 Earths, would bob on the water like a cork.

[00:54:29]But of course, it is the rings that make Saturn what it is. From a distance, they look like solid, smooth, glowing hoops of light encircling the planet in perfect delicate bands. But they are not solid at all. Saturn's rings are made of countless billions of separate pieces of ice and rock, all circling the planet together, side by side. pieces ranging in size from tiny grains of frost, no bigger than specks of dust, to great tumbling chunks as big as houses or mountains. Mostly they are ice, glittering, sparkling water ice, which is why they shine so brightly, catching the distant sunlight like a field of freshly fallen snow. And they are astonishingly wide, these rings, stretching out from the planet across a distance. You could fit many Earths [clears throat] within side by side. And yet they are astonishingly thin. For all their width, the rings are in most places only a few tens of meters thick, thinner in proportion than a sheet of paper held out at arms length. A vast shining delicate disc of countless icy fragments wider than worlds and thinner than a whisper. All sweeping silently around the planet together.

[00:55:59]No one is entirely sure how they came to be. Perhaps the shattered remains of an old moon torn apart long ago and spread into a glittering ring. Like Jupiter, Saturn is a world surrounded by a great family of moons. Indeed, it has more moons than any other planet in the solar system. A number that has grown into the hundreds as we keep finding more and more of them, small and large, tumbling in its neighborhood. And two of them are among the most fascinating places we know. The largest is a great moon bigger than the planet Mercury. And it is the only moon in the entire solar system with a thick substantial atmosphere of its own wrapped in a hazy orange sky so dense you cannot see its surface from above. And beneath that veil, this frozen moon holds something astonishing.

[00:57:01]It is the only other place we know of anywhere with liquid on its surface. Not water, for it is far too cold for that, but rivers, lakes, and seas of liquid methane and ethane fed by a gentle methane rain that falls from its orange clouds. Picture that softly. a distant frozen world where it rains and rivers run down to standing seas beneath a hazy amber sky.

[00:57:35]And there is another moon, a small and brilliant white one that hides perhaps the most wonderful secret of all.

[00:57:44]Beneath its icy shell, this little moon holds an ocean of liquid water. and through great cracks near its south pole. It sprays that water out into space in enormous glittering geysers, fountaining hundreds of kilome up into the dark. A tiny frozen moon quietly spouting a hidden sea into space. For many years, a single faithful spacecraft circled Saturn, studying it patiently from up close, sweeping again and again through its neighborhood, past its rings and its moons, and sending home tens of thousands of images of breathtaking beauty. It was that little explorer that first saw the great geysers spraying from the small icy moon and that peered down through the orange haze to glimpse the methane seas below. And it taught us something rather poignant about the rings themselves that they may not last forever. The rings appear to be slowly, gently raining down into the planet, drawn in by Saturn's gravity, a little at a time, which means these glorious, glittering hoops may be a passing thing, gracing the planet for only a stretch of its long life. We may simply be fortunate enough to live in the age when Saturn wears its rings. Far in the future, they may thin and fade and be gone. which makes their beauty, if anything, all the more precious to behold tonight.

[00:59:22]So Saturn is [clears throat] far more than a single ring jewel. It is a whole realm of wonders, the glittering rings, the hundreds of moons, the raining world of methane seas, the little fountain of a hidden ocean. Let us leave it now, this loveliest of planets, and travel onward into the deep cold and the growing dark to a pair of distant giants that almost no one ever thinks about and that we have visited only once. Far, far beyond Saturn, out in the deep and frigid darkness, we come to a world so distant and so faint that no one even knew it existed until a couple of centuries ago.

[01:00:08]the pale, quiet blue green planet Uranus. It is a world we have seen up close only a single time when one lone spacecraft swept silently past it decades ago, and it remains one of the most mysterious and least visited places in all the solar system.

[01:00:28]Uranus is a giant world, though smaller than Jupiter and Saturn, large enough still that dozens of Earths could fit inside it. But it is a giant of a different kind, one that astronomers call an ice giant.

[01:00:45]For while it too is wrapped in deep layers of gas, it is thought to hold beneath those gases a vast quantity of slushy icy material, water, and other frozen substances squeezed and heated deep within. And it is very, very cold.

[01:01:05]Out at that great distance from the sun, Uranus has the coldest atmosphere of any planet in the solar system. A frigid, still hazy world where the temperature falls further than almost anywhere we know.

[01:01:21]Its color is soft and lovely, a pale, serene blue green, gentle and almost featureless, like a smooth ball of frosted glass.

[01:01:33]That calm color comes from a gas called methane high in its atmosphere, which soaks up the red light of the sun and leaves only the cool blues and greens to be reflected back to us. where Jupiter and Saturn are wrapped in bold and stormy bands, Uranus, at least when we glimpsed it, looked almost perfectly smooth and quiet, a tranquil, hazy blue green orb, drifting silently in the dark.

[01:02:04]Uranus holds a small distinction in the story of our discoveries, too. The five planets nearest us, the bright ones, easily seen with the naked eye, have been known since the most ancient times.

[01:02:19]Every early people watch them and name them and wo them into their stories. But Uranus is too faint and too slow to be noticed as anything more than a dim star. And so it went unrecognized for the whole of human history until a couple of centuries ago, an astronomer sweeping the sky with a telescope realized that one faint point of light was in fact a small pale disc slowly moving. A whole new world never before known. In a single evening, the known solar system doubled in size. It must have been an extraordinary feeling to be the first person in all of history to knowingly discover a planet.

[01:03:08]But Uranus hides one truly bizarre and wonderful secret. Something that sets it apart from every other planet. Every other world in the solar system spins more or less upright, like a top turning on a table, tilted only a little from straight up and down. But Uranus is tipped right over onto its side. It rolls around the sun, lying down, its poles pointing nearly straight out toward the sun and away from it, tumbling along its orbit like a ball rolling across the ground rather than a top spinning upon it.

[01:03:47]No one is entirely certain why. The leading guess is that long ago, some tremendous collision knocked it clean over. And this strange sideways tilt gives Uranus the most extreme seasons imaginable. As it rolls slowly around the sun over its very long year, each pole in turn is pointed almost directly at the sun, bathed in decades of continuous daylight, while the other pole is plunged into decades of unbroken night. Imagine a winter that lasts for 20 of our years in darkness that never lifts, followed by a summer just as long in sunlight that never sets. That is the strange rhythm of the world on its side.

[01:04:35]Uranus, too, has rings, faint, dark, narrow ones, far more delicate than Saturn's glittering hoops, and a family of moons of its own. Many of them named rather charmingly after characters from old plays and poems. But mostly Uranus remains a mystery to us. A pale and distant world glimpsed only once in passing and never since.

[01:05:06]There is something peaceful in that thought. that out there right now, this quiet blue green giant is rolling slowly around the sun on its side, just as it has for billions of years, largely unseen and unbothered, keeping its secrets in the cold and the dark. Let us drift onward to the last of the great planets, the final giant and the farthest, a deep and windy world of the deepest blue. We come now to the most distant of the major planets, the last great world before the frozen frontier, the deep blue giant, Neptune.

[01:05:48]It is the farthest planet from the sun, drifting out at a truly tremendous distance. So far that the sun, seen from Neptune, would be little more than a very bright star. And the daylight there would be a deep and dusky twilight, hundreds of times fainter than the noon we know. It is a world of profound cold and profound distance. And like its neighbor Uranus, we have visited it only once. when that same lone spacecraft swept past it long ago and then sailed on into the dark.

[01:06:26]Neptune is Uranus's near twin in size and kind.

[01:06:30]Another ice giant, another world of gas wrapped around a deep, slushy, icy interior, large enough to hold many dozens of Earths. But where Uranus is pale and calm, Neptune is a deep, rich, vivid blue, the loveliest blue of any planet, a color so deep it looks almost like a great sapphire hanging in the black. It too gets its blue from the methane in its air. Though no one is quite sure why Neptune's blue is so much deeper and more brilliant than the pale tint of Uranus.

[01:07:09]It is simply one of its quiet mysteries.

[01:07:12]And unlike the tranquil face of Uranus, Neptune is a wild and stormy world.

[01:07:19]When our spacecraft flew past, it saw great dark storms moving across the blue. Vast oval spots like Jupiter's great red storm, but dark against the deep blue, appearing and fading over the years. And Neptune holds a record that is almost hard to believe given how cold and distant and far from the sun's warmth it is. Neptune has the fastest winds in the entire solar system. Winds race around that far, frigid world at speeds that would dwarf any hurricane on Earth, screaming through its atmosphere faster than the speed of sound, faster than a jet aircraft. the swiftest winds blowing anywhere among all the planets.

[01:08:08]It is a strange and wonderful puzzle that the most distant, most sunless, most frozen of the giant worlds should also be the windiest, the most violently restless.

[01:08:22]Out there in the deep cold and the dark, Neptune rages on. Neptune holds a special little place in the story of how we came to know the sky. For it is the one planet that was found not by chance but by pure thought.

[01:08:40]Long ago, astronomers watching the planet Uranus noticed that it was not quite moving as it should. Something unseen seemed to be tugging gently at it, pulling it ever so slightly off its expected path. And so using nothing but careful mathematics, an astronomer worked out where a hidden undiscovered planet would have to be to explain that gentle tug. He named a patch of sky and asked others to look. And there, almost exactly where the numbers had said it must be, they found Neptune glowing faintly in the dark. It was discovered in a sense on the tip of a pencil, glimpsed in the mind before it was ever seen through a telescope.

[01:09:28]There is something wonderful in that, that a world so distant could be found first as the answer to a puzzle, worked out quietly on paper here at home.

[01:09:40]Neptune has a family of moons, and one of them is truly remarkable.

[01:09:46]It is a large icy pinkish moon that does something no other large moon in the solar system does. It circles Neptune backwards, orbiting in the opposite direction to the planet's own spin. This strange reversed orbit is a powerful clue that this moon did not form alongside Neptune as most moons form alongside their planets.

[01:10:13]Instead, it seems to be a wanderer, an icy world that drifted in from the frozen regions beyond, from the very frontier we are about to visit, and was captured long ago by Neptune's gravity, and pulled into orbit around it. And it is a strange and active little world in its own right. Cold enough that much of its surface is frozen solid and yet dotted with geysers that spout dark plumes of icy material up into the thin air. Even out here at the farthest edge of the planets, the worlds are restless and alive with motion. And so with Neptune, we reach the end of the great planet. We have traveled all the way from the scorched gray rock closest to the sun, out past our own blue home, past the red desert and the drifting rubble, past the mighty king and the ring jewel and the two distant blue giants to this final deep windy world at the far edge of the planetary realm. But the solar system does not end here. For beyond Neptune lies a whole other kingdom. A vast, cold, dim frontier of small, frozen worlds, drifting far out in the dark. Let us drift onward into that frozen frontier and meet its most famous resident. Far beyond Neptune, out in the deep cold, at the edge of the sun's realm, lies a vast and dimly lit frontier, a great ring of small, frozen worlds drifting slowly through the dark, so far from the sun that it is little more than a bright star in their black skies.

[01:12:06]This distant region is home to countless icy bodies and to the most famous small world of them all, a little frozen planet named Pluto.

[01:12:18]For a long time, Pluto was counted as the ninth and outermost planet of the solar system. The last stop on the tour, discovered long ago as a faint moving speck in the dark. Generations of children learned its name as the final planet. But as our telescopes improved, astronomers began to find that Pluto was not alone out there. That it was just one of many, many small icy worlds drifting in that same distant region.

[01:12:48]Some of them nearly as large as Pluto itself.

[01:12:52]And so some years ago, after much debate, astronomers gently gave Pluto a new title. They decided it was best understood not as a full planet, but as a dwarf planet, one of the larger members of that great frozen crowd at the edge of the solar system. Some people felt a little sad at Pluto's change of title, as though a familiar friend had been demoted. But I like to think of it another way. Pluto was not made smaller. Rather, we discovered that it was the first known member of a whole new, vast, and wondrous kingdom of frozen worlds that we hadn't even known was there. And for a long time, Pluto was just a faint and fuzzy dot, too small and too distant for even our finest telescopes to see clearly. We knew almost nothing of what its surface looked like. But then, not so many years ago, a small spacecraft after a journey of nine long years across the solar system finally swept past Pluto. And for the first time, we saw this distant little world up close. And it took our breath away. Far from being a dull gray ball of ice, Pluto turned out to be a place of astonishing beauty and variety.

[01:14:19]There were rugged mountains made of rockh hard water ice standing tall against the black sky. There were strange smooth plains of frozen nitrogen. There was a thin hazy blue sky. And most beloved of all, across one whole face of the planet, there stretched an enormous pale smooth region shaped almost exactly like a great heart. A vast plane of gleaming frozen ices, so striking and so tender a shape that people the world over fell in love with it at once. A tiny distant frozen world 4 billion miles from the sun wearing a great bright heart upon its face. Who could have guessed?

[01:15:08]Pluto has a companion too, a large moon so big in comparison to Pluto itself that the two of them almost make a little double world, each circling the other around a point in the empty space between them, forever locked face to face. turning together in the dark.

[01:15:28]And out there in that same frozen frontier drift other small worlds like Pluto, other dwarf planets, other icy bodies, some with moons of their own, some with strange elongated shapes, all of them slowly circling the distant sun in a realm so cold and so dim and so far away that we have only just begun to explore it. It is the attic of the solar system, if you like. A vast, cold, dark storm of frozen leftovers from the very beginning, drifting quietly at the edge of everything, largely unseen and unvisited, keeping the solar system's oldest secrets in the deep freeze of space. And yet, even this frozen frontier is not quite the true edge of the sun's kingdom. For beyond even these dim and distant worlds, far, far out in the dark, there lies one final realm, the most distant and mysterious of all, the very last outpost of the sun's long reach. Let us drift out to it now softly, to the true edge of everything, where the comets sleep. We come now to the very edge of the sun's great kingdom, the farthest and most mysterious frontier of all. But before we reach it, let us talk about its most beautiful travelers, the comets. Those rare and lovely visitors that every so often drift in from the deep cold and grace our skies with a glowing tail.

[01:17:11]A comet, at heart, is a rather humble thing. a small lump of ice and dust, a sort of dirty cosmic snowball, perhaps only a few kilometers across. For most of its existence, a comet is dark and frozen and [clears throat] utterly invisible, drifting silently through the deep cold of the outer solar system, far from the sun's warmth. But every so often, a comet's long, slow path carries it inward, falling toward the sun. And as it draws nearer to that great warmth, something magical happens. The ice on its surface begins to warm and to turn straight from solid ice into gas, streaming off the comet in a great glowing cloud.

[01:18:03]And the light and the gentle push of the sun blow that gas and dust outward away from the sun into a long luminous streaming tail that can stretch across millions of kilometers of space. And so a humble frozen snowball becomes for a few brief weeks one of the most breathtaking sights in all the heavens.

[01:18:28]A glowing visitor with a shining tail hanging in the night sky, just as it has appeared to wandering people all through human history.

[01:18:39]Then its brief visit over. It swings back around the sun and falls away again into the deep, growing cold and dark once more, not to return sometimes for hundreds or thousands of years. Some comets are faithful, returning visitors, sweeping in past the sun on long but regular journeys, then falling away again, only to come back like clockwork after many years.

[01:19:11]There is one particularly famous comet that returns to our skies roughly every 75 years or so. Close enough to the span of a human life that a person might, if they are lucky, see it once, and perhaps, if they live long and well twice. For centuries, people watched it come and go without understanding it.

[01:19:34]sometimes with wonder and sometimes with fear until at last an astronomer realized it was one and the same comet returning again and again and correctly foretold the year of its next appearance.

[01:19:50]Now we greeted almost as an old friend, a great glowing traveler, keeping its slow and patient appointment with our sky generation after generation.

[01:20:02]But where do the comets come from? Where do they spend all those long cold centuries between their brief bright visits? Some come from that frozen frontier just beyond Neptune that we have already visited. But others, the ones that take thousands of years to return, the ones that come falling in from truly staggering distances, come from somewhere much, much farther out.

[01:20:29]They come from what may be the largest and most distant structure in the entire solar system. A place so remote that we have never seen it and know it only by the comets it sends us. It is thought that surrounding the entire solar system, surrounding the sun and all [clears throat] its planets and all its frozen worlds, there lies a vast faint spherical shell of icy bodies. Countless billions of frozen lumps drifting slowly in the deepest cold and the deepest dark at the very outermost edge of the sun's reach. It is not a flat ring like everything else we have visited but a great hollow sphere wrapping the whole solar system on every side like a shell around an egg. And it lies impossibly far away. This great outer shell is thought to reach outward and for an almost unimaginable distance.

[01:21:30]So far that its most distant members may drift nearly a quarter of the way to the very nearest star.

[01:21:37]If the distance from the Earth to the Sun were shrunk to a single step, this outer shell would begin thousands upon thousands of steps away and reach outward for thousands more.

[01:21:50]It marks the true and final edge of the sun's kingdom. The place where the sun's gentle gravity finally grows so faint that it can just barely hold onto these last, most distant, most frozen of its children.

[01:22:06]Out there in the deepest dark, in cold beyond all imagining, these countless icy worlds drift in perfect silence. Most of them never once falling inward, never warming, never growing a tail, simply drifting slow and cold and patient at the utmost edge of everything the sun can call its own. And that at last is the true boundary of our solar system. Not a wall, not a line, but a vast and gentle fading out there in the deep dark where the light and the warmth and the pull of our sun finally give way to the emptiness between the stars. We have traveled all the way from the great central fire to this farthest, faintest edge. And now, our long journey done, there is only one thing left to do. to turn around and drift gently home.

[01:23:08]And so we come to the end of our long slow tour out to the very edge of the sun's realm and back again.

[01:23:16]Let us pause here in the quiet and gather up all that we have seen softly, drowsily, the way you might page back through a book of photographs at the close of a long and happy day.

[01:23:30]We began at the great warm hearth of the sun, that enormous fire that holds everything else in its gentle grip. We drifted past scorched little Mercury with its blistering days and freezing nights, and its secret pockets of hidden ice. We passed the veiled furnace of Venus, so beautiful from afar, so fierce beneath its clouds. We paused at our own blue jewel of a home, the one living world we know, wrapped in its thin and precious skin of air, and at its faithful gray companion, the moon, that leads our tides and lights our nights.

[01:24:12]We wandered the red deserts of Mars and his dry and ancient riverbeds.

[01:24:18]We crossed the quiet, drifting rubble of the asteroid belt.

[01:24:23]We met the mighty king Jupiter with its centuries old storm and its swarm of worlds and the ringed jewel Saturn with its glittering hoops of ice and its raining seas of methane.

[01:24:38]We glimpsed the two distant blue giants, Uranus rolling on its side and windy Neptune in the deep dark. And out past them all, we drifted through the frozen frontier to little Pluto with his great bright heart and onto the vast and distant shell of sleeping comets that marks the utmost edge of everything the sun can hold. What a place it is, our solar system. What a strange and various and beautiful little kingdom spun out around one ordinary star. And here is the thought I'd like to leave with you as you drift now towards sleep. In all of that, all those worlds, all that fire and ice and storm and stone, all those vast and lonely gulfs of dark, there is, as far as we know, only one small world where someone can lie quietly in the dark, warm and safe beneath a blanket, and hear a gentle voice describe it all.

[01:25:43]And that world is the one you are resting on right now. Of all the planets and moons we have visited tonight, every one of them silent and empty and cold, it is our own blue earth alone that holds life and warmth and you that makes our small home a rare and precious thing indeed. a single glowing island of life, a drift in a vast and quiet sea of worlds.

[01:26:13]So there is no need to feel small or lost among all that we have seen tonight.

[01:26:20]Yes, the solar system is vast and the worlds within it are strange and far apart, but it is also in the end our neighborhood. The little cluster of worlds that shares our sun, our warmth, our light. It is home. And you are resting right now in the safest, warmest, most wondrous corner of it on the one world in all that dark where someone is here to notice how beautiful it all is. Thank you so very much for wandering all this way with me tonight.

[01:26:55]out among the worlds and home again.

[01:26:58]Wherever you are on this one small living planet, whatever tomorrow may bring, I hope you feel just now held gently within something vast and quiet and beautiful, safe at the warm heart of your own small corner of space. The sun will keep its long, patient watch, and the planets will keep their slow and silent circling all through the night, just as they always have. There is nothing at all to do now and nowhere at all to go. Just rest here on our small blue world beneath the turning sky.

[01:27:38]Sleep well.