The Zoomable Universe
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1017, 1016, 1015, 1014 meters
From about 10 light-years to 92 light-hours, or 668 astronomical units
From about nebula scale to the size of the inner edge of the Oort Cloud
Five billion years ago, your atoms were about ten million times more widely spread across the cosmos than they are now. The same is true of everyone and everything around you, from this book in your hands to the uneaten cheese in your fridge and the noisy neighbor across the street. The same is also true of every lumpy asteroid, moon, and planet in the solar system, as well as the thermonuclear ball of the Sun itself.
All of us used to be strewn across the interstellar void. It is very likely that this is the first time that the specific atoms nestling in the 1.8-meter-long strand of DNA in any one of your cells have been this close together in the history of the universe.
So how did this come to be?
In simple terms, we all condensed. The fundamental physical properties of the universe conspired to pull together a set of atoms and molecules that previously had been occupying a volume a billion trillion times larger.
Because of their complex interactions (driven by gravity, electromagnetism, quantum mechanics, and subatomic physics), all those microscopic components found their way together to assemble themselves into a star, planets, moons, lumpy asteroids, noisy neighbors, cheese in the fridge, and you and this book.
This gathering of matter makes the scale at which we exist, and the scale of our planets and Sun, somewhat special in the present-day universe. It’s a physical range where coherent yet richly complicated structures form. It is also the scale in the universe at which four fundamental states of matter—solid, liquid, gas, and plasma—can coexist. And because of tinkering little humans, this is now a slice of the cosmos in which states have been created or reproduced that otherwise wouldn’t exist in this milieu: ultra-high-energy states like quark-gluon plasmas in particle colliders, or super-low-energy states like Bose-Einstein condensates in laboratories, and low-temperature superfluids and superconductors, to name but a few.
States of Matter
The components of matter—ions, atoms, molecules—can organize and behave in different ways. We call these “states” (or phases) of matter. What happens at a microscopic level in matter often results in very different behavior at larger scales—from the solidity of ice or iron to the flow of a river to the outlandish behavior of a neutron star.
THE CIRCLE OF ASTROPHYSICS
When our solar system condensed five billion years ago, it wasn’t the first time such a thing had happened. The universe started out small, dense, hot, and expanding 13.8 billion years ago. It was also extremely uniform and boring, but not perfectly so. As a result, the bits of the cosmos with the most imperfections started pulling themselves out of the underlying cosmic expansion.
How do you get out of the expansion of the cosmos? Gravity. Some of these imperfections were places where the density of matter was a little higher than the surrounding average. Given a high enough density, self-gravity (a region’s weight) would resist the expansion of space. That “self-pull” (a result of mass warping space-time around itself) was enough to overcome the underlying current of expansion and allow dark and normal matter to clump together.
But physics is tricky. As a gas of normal matter pulls together—or, rather, falls together—it transforms gravitational potential energy into thermal energy. In other words, it heats up. That sets in motion a competition between gravity working to condense matter and thermal-energy pressure spreading matter apart. In the right circumstances, though, if the matter can cool off fast enough, gravity can pull it into very dense concentrations indeed.
It’s still a mystery what happened in that first hundred million years after the Big Bang, but it seems that a starter generation of very large stars and the beginnings of galactic structures were forming. These early stars were critical for generating the universe’s first heavy elements, and for dumping them back out into interstellar space as the stars aged, inflated, and exploded.
The first stars in the universe
Like a good feedstock for seedlings, that sprinkling of heavy elements opened up new channels for gas to cool—to lose the thermal energy that otherwise slows down new condensations and clumping. Subsequent generations of stars are far more varied in size as a result.
The violent endpoint of some stars as supernovas also triggers the speedy collapse of interstellar material that would otherwise teeter on the brink between condensation and dispersal. An exploding star sends relentless shock waves of gas and particles careening across interstellar space, setting off instabilities that lead to new condensations.
These early clumps of material pioneered a cycle of matter processing that has carried on ever since: Interstellar material gathered up, and some got dense enough to bind into stars (and planet-size bodies). Nuclear fusion in the hot cores of stars then built increasingly heavy elements. Anywhere from millions to billions of years later, those stars spewed a mix of elements back out into the void, usually leaving a stellar ember behind. Those embers are white dwarfs, neutron stars, or even black holes. These super-dense agglomerations are generally endpoints, places where matter is going to stay put for eternity (however long that really is).
Across billions of years, hundreds of billions of stars, and many of these cycles, interstellar space has gotten slowly polluted with these heavy elements. It’s delicate pollution, though. Even now, only tiny amounts of these new elements exist compared to the bulk of hydrogen and helium that the universe started with. Elements like carbon, oxygen, nitrogen, silicon, and all the other heavy atoms around us account for barely more than a percent or two of normal matter. And these mostly exist in the sparse gas of interstellar or intergalactic space.
There is also a sprinkling of cosmic dust. This dust is microscopic, made of grains of silicates or carbon-rich compounds that are smaller than a tenth of a millionth of a meter across. These specks are the flash-frozen remnants of old stellar atmospheres and stellar guts, expelled by supernova explosions.
In the orbital swirl of a galaxy like the Milky Way, the densest accumulations of such star-stuff are visible in the nebulas: the molecular clouds that dot the galactic disk. There are many thousands of nebular regions. Parts of the largest—the Great Nebula—are actively condensing into new stars today, before the orbital drift of material in the galaxy and its gravitational jostling once again disperse these rather dirty nests.
We can reconstruct much of our solar system’s pre-pre-history. Clues come from investigating the very distant cosmos, and the cosmos closer to home. We’ve also found critical insight by peering at microscopic remnants of this extraordinary past—the elements and radioisotopes embedded in ancient meteorites here on Earth.
A star explodes.
All of which brings us to the bedtime story to end all bedtime stories.
PAST TO PRESENT
Once upon a time, over four and a half billion years ago, a modestly clumpy part of a rich nebula in the Milky Way galaxy began to slip inward and condense. In all likelihood it was tipped over its gravitational edge by the forces from a nearby stellar explosion. That’s because in other parts of the nebula, the cycle of stellar birth and death was already far along. There was a litter of new stars and soon-to-be stars. And there were the more massive stars that had lived fast and died young, exploding as supernovas after racing through the chain of nuclear fusion processes that could occur in their cores.
Despite resistance from the pressure of gas in this clump, and the ethereal pressure of interstellar magnetic fields, gravity managed to prevail—causing more and more nebular material to fall together.
The physics of this condensation is rooted in laws like gravity, angular momentum, and thermodynamics but is simultaneously prone to nasty, messy, chaotic behaviors that cause many anxious moments for astrophysicists.
The shape and geometry that the matter takes on as it falls together is key. This mix of gas a
nd dust concentrates in a growing central spherical core, it also flattens out into a giant orbiting disk. The disk is a varied, turbulent thing, puffed up toward its edges. Gas and dust spiral in through the disk, feeding the core. Fresh nebular material also feeds the disk from above and below in a great two-sided funnel that extends hundreds of times farther from the disk’s center than Earth is from the Sun today.
The core is a fast-spinning globe of warming gas. Left to its own devices, this core would spin faster and faster while it grew, until it was disrupted. But in many cases, for a brief few tens of thousands of years, the combination of swirling influx and increasingly star-like core launches a fierce outflow of hot, accelerated matter from the new north and south poles of the central sphere.
Astronomers call this a proto-stellar jet—a beam-like torrent of particles that can race across spans reaching a light-year or more. These jets of matter plow into any nebula that’s in the way, creating shock waves and glowing fronts of hot matter. This outflow helps regulate the core, bleeding off some of the angular momentum that would otherwise tear it apart, and allows it to shrink further and further.
Our stellar nursery, billions of years ago, across 1017 meters
Our Sun today
The “egg” our solar system came from: a dense knot of gas and dust
Closer in to our Sun today
All the while the great disk of surrounding material undergoes its own transformation. Deep inside, beneath layers of varying temperatures and chemical composition, motes form that are made of dust and molecules. Microscopic at first, they are buffeted by the swirling gas. That buffeting can accelerate their growth as they sweep through more and more of their surroundings. These growing aggregates—sometimes like dust bunnies—reach scales of centimeters, meters, and larger. Some grow like a snowball rolling down a hillside. The bigger they get, the more matter they sweep up, and in a runaway process they can grow to hundreds of kilometers across in a few tens of thousands of years. We call these planetesimals and proto-planets.
A proto-star pours jets into space across 1015 meters.
But time passes, and the exposed disk surfaces are blasted by ultraviolet light. That light comes from other young stars and from the hot central proto-stellar core. What it does is evaporate material away.
The modern-day Sun comes into view.
The clearing of the proto-stellar disk across 1014 meters
This combined agglomeration and loss of matter turns the great disk from a thick spread of gas and dust into a sparser collective of solid bodies and dissipating atoms and molecules. Well away from the proto-stellar core, in cooler zones, is a planetary snow line where water ice, a key building block for making giant objects, forms. Here we find the baby worlds of Jupiter, Saturn, Uranus, Neptune, and possibly other, later displaced, giant planetary embryos.
The Sun’s haze of interplanetary (zodiacal) dust and a foreground asteroid remnant of planet formation
Simultaneously, down in the deep gravity well of the core, that spherical condensation of matter is reaching a critical point in its contraction. The inner temperature of this object is starting to edge above a million degrees Kelvin. When it hits ten million degrees it will become a true star, with the first flashes of nuclear fusion in its heart.
The amount of time that has passed since the drifting infall of nebular material is around a hundred million years. Even less time—about a hundred thousand years—has passed since this proto-star was simply a core of gas. By the standards of the cosmos, the birth of a star and its planets is pretty much an overnight affair.
We don’t yet have the full story of the formation of the planets of our solar system, and of Earth itself. Our planet and the other rocky inner worlds emerge toward the end of the birthing process. And these objects get their final indelicate touches from great collisions in which proto-planets smack into proto-planets, again and again.
It’s a rough, stochastic (randomly patterned) process that poses a multitude of still-unanswered questions: Did our solar system once have young planets within the orbit of Mercury? Where did Earth’s water come from, and why are we not even wetter? How did Mars end up so small? Why are our planetary orbits comparatively circular? Did the Moon really form in a giant collision? What planetary bodies actually lurk in the outermost zones of the solar system?
If we can resolve questions like these, we’ll not only better understand our origins, and the way in which we condensed from the universe, but we’ll shed light on how a story like this can play out across trillions of other star systems. And that is key, because as our journey through scale continues, we are about to come right up against one of the biggest puzzles: the puzzle of us versus the cosmos.
Earth accreting from proto-planetary pieces
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PLANETS, PLANETS, PLANETS
1013, 1012, 1011, 1010, 109 meters
From about 9.3 light-hours to a million kilometers
From about twice the current Earth–Pluto distance to about three times the Earth–Moon distance
Go outside on a clear night and wave a flashlight at the sky. Then go and get a good night’s sleep. The next morning, the photons you released into the universe will have traveled a distance of roughly ten billion kilometers (1013 meters).
With your flashlight and just over nine hours of peaceful dreaming, you’ve become a cosmic architect, sending out a photon-tipped yardstick that reaches past the orbital path of Neptune. Even infamous Pluto, that remote sphere of ice and rock, now lies more than two billion kilometers closer to us than the reach of your beamed photons.
Well within those 1013 meters is the orbital terrain occupied by the major planets of our solar system, all the worlds that we’ve spent thousands of years staring at from our hairy little primate heads. Our naked eyes can’t see them all—Neptune and Uranus are beyond our unaided reach—but Jupiter, Saturn, Mars, Venus, and Mercury have always been visible to any terrestrial life-form with good enough eyesight, from bugs to baboons.
But just because our solar system’s major planets orbit on these scales doesn’t mean that other places are set up the same way. Across the galaxy, at these orbital separations we can also find stellar siblings: binary stars, triple star systems, quadruple star systems, stars paired with white dwarfs or black holes, neutron stars paired with neutron stars, and other exotic configurations. Even supermassive black holes’ event horizons—the emperors of cosmic singularities—occupy this range of scales.
Neptune’s orbit as seen from the Trans-Neptunian world Eris
Pluto’s sky, with its moon Charon, and the Sun a dim star more than 4.4 billion kilometers away
Blue Neptune in the skies of its moon Triton. Geysers of nitrogen erupt from the deep-frozen surface.
These are the scales that take us deep into the landscape of the densest condensations of normal matter and energy that the universe produces. And it’s a weird landscape. We’ve already seen that although the interstellar muck that builds stars and planets starts out spread over 1013 to 1014 meters, by the time gravity has pulled those objects together and energy has dispersed the leftovers, what remains is largely empty space.
It is the planets that fill in the gaps, though—drops of iron, rock, water, and gas. And next to living things, planets may be the most diverse and complex objects in the universe. No two planets are identical: not in their orbital paths through space nor in their rotations and orientations, their atmospheres, clouds, hazes, layers, seas or continents, or soupy molten innards. All these properties are invariably variable.
And now, thanks to a series of extraordinary technological and astronomical advances beginning in the late twentieth century, we know that there are at least as many planets as stars in our galaxy, and probably in the universe as a whole. In fact, there are likely many more. The cosmos is teeming with exoplanets.
And when it comes to places that could resemble Earth in size, and possibly in chemical and thermal states as well, our statistical extra
polations tell us that between 15 percent and 40 percent of stars play host to such worlds outside our solar system. That’s an astonishing projection to make. It means that across the entire observable cosmos, there could easily be a billion trillion of these potentially life-friendly rocky globes.
Our nearest exoplanet? Proxima Centauri b may have skies filled with auroras caused by its flaring red-dwarf sun.
Despite that abundance, small rocky planets represent just one of the many possible variations of the planetary form. There are other planets almost large enough to be stars, and there are worlds that are themselves the satellites of other worlds. There are even worlds that have been gravitationally ejected from orbiting their birth stars to wander the frigid backwaters of interstellar space. These “Steppenwolf” planets could conceivably maintain warm oceans of water, snugly trapped and insulated inside a thick outer ice crust and hydrogen atmosphere—interior oases lasting for billions of years after their formation.
There are plenty of gas giants, some with dense iron and rock cores enveloped in massive cloaks of primordial hydrogen and helium. Planets like these can be chilly on the outside, orbiting billions of kilometers from their star, but fiercely hot on the inside. Their internal pressure must also result in states of matter that are quite alien to us feeble surface dwellers. Pressures inside a gas giant like Jupiter are so extreme that hydrogen can become metallic—a state of matter we can barely reproduce in our laboratories, yet one that makes up the greatest fraction of planetary mass in our solar system.
Auroras glow across a terrestrial-type world orbiting a flaring star.