by Caleb Scharf
Across the galaxy a small fraction of giant planets also closely orbit their stars, their atmospheres literally white-hot, with clouds and rain of titanium oxides or iron, bands of supersonic jet streams, and hellishly extreme weather. How these worlds came to be is still somewhat mysterious. Planets of this size—hundreds of times the mass of Earth—simply can’t form where we find them. In other words, they have changed location, undergoing a grand form of migration, coerced to come close to their parent stars by the complex gravitational pulls of the proto-planetary disk during childhood. Or they’ve played planetary pinball, jostling with the gravity of the other planets in their family and getting scattered into orbits close to the star.
Elsewhere there are ice giants with layers of steel-like frozen water and hydrocarbon-laced atmospheres. We suspect that there are worlds fantastically rich in carbon, or water. And there are super-Earths—worlds larger than Earth but smaller than Neptune—that have no counterpart in our own solar system, yet orbit about 60 percent of all other stars. Some of these appear to be radically different from any rocky world we are familiar with, such as molten-magma planets encased in great warm blankets of hydrogen gas.
As we chart these new exoplanets and explore our own solar system, we’re upending our preconceptions about Earth’s monopoly as “most important world.” Everywhere we look, and everywhere we go, we see signs of activity and complexity. Saturn’s moon Titan has a thick nitrogen and hydrocarbon atmosphere. While its surface may be a frigid 93 Kelvin on average, Titan nonetheless has seasons. It has summers and winters across its hemispheres, and great cycles of evaporation and condensation that can dry up the moon’s lakes of methane and then rain that hydrocarbon back out. Remote Pluto has a rich landscape of mountains, glaciers, and cryovolcanic change across its frozen surface.
A ROAD ALMOST TAKEN
Most of the planets we study are following paths through the curved space-time of Einstein’s relativistic universe—inside the gravity wells of their stellar parents. For our species these planetary orbits are among the most iconic features of the cosmos, yet in truth they are products of our imagination. They are extrapolations and geometric fantasies that help us get a grip on nature’s complexity.
An orbit is really an approximation, an averaging over time of the motion of stars or planets. Real systems are complicated landscapes of gravitational attractions. Stars pull on stars, planets pull on planets. Planets hang on to moons, and moons pull at one another. At any instant, the forces acting on objects are a summation of all these pulls—the collective action of the many on one.
As a result, planetary systems are chaotic at heart, and over millions and billions of years there can be orbital creep—a phenomenon known as chaotic diffusion. This creep is inherently unpredictable in its details, but we can anticipate what different timelines might look like. Tiny changes in planetary positions or speeds, even a few millimeters here or there, can lead to very different futures a billion years hence. Even the effects of Einstein’s relativity—although subtle at typical planetary speeds and masses—help determine these cosmic histories, because they modify the behavior of moving worlds.
The orbits of the inner rocky planets of the solar system—from outside in: Mars, Earth, Venus, Mercury
The orbits (left to right) of Mercury, Venus, Earth, and Mars
Fire and ice: The Jovian moons Io (above) and Europa (below) both experience gravitational tides.
Tidal forces also play a critical role in the story of planets. These are differential pulls, exerting force unevenly on “near” and “far” sides of the same body as a consequence of gravity’s dependence on distance—the inverse-square law. Over time, tides can turn elliptical orbits into circular orbits, by bleeding off the energy of motion and converting it into the frictional heat of a planetary interior.
Tides have sculpted much of our own solar system’s detail. The lockstep motion of many moons—in which their rotation period is matched to their orbital period—is a product of tides. The hot volcanoes of Io, and the cryovolcanism of Enceladus, Triton, and ancient Pluto, together with the suspected dark interior oceans of Europa, Ganymede, Titan, and many more bodies, are largely the result of tidal forces.
The track of Earth’s orbit and the loop of the Moon’s orbit
Looking across a 109-meter scale. The Moon’s orbit encircles Earth, and closer still is the scale of a geosynchronous orbit, such as that of human-made satellites.
Earth seen from the Moon during a lunar eclipse
And as our journey of scale brings us to our own Moon, our dark, dusty, gray companion, we find that it’s been trapped and coerced by tides into its present configuration, still evolving as its orbit recedes a few centimeters a year and Earth gradually slows its own spinning.
These changes go back a long way. The record of Earth’s ocean tides, seen in sandstone rocks that were coastal beaches 620 million years ago, suggests that back then Earth had a roughly twenty-two-hour day. Combined with our best bet for how the Moon formed—from a massive collision between an earlier version of Earth and a Mars-size proto-planet—we can guess that a young Earth, four and a half billion years ago, spun fast and had a far shorter day. The ocean tides, generated as Earth spun ahead of the Moon’s orbital circuit, would have been huge. Billions of years later those sloshing mounds of water and the kneading of rocky mantles have still not dissipated all the momentum of that original agitated system.
That’s a happy thing, because those tides are some of the tangible links that exist between our daily experience and the dynamics of our place in the universe. You were able to use your flashlight to send a probe into the cosmos because you live on a planet molded by these forces. Earth’s present conditions have created opportunity for organisms like us to exist, and the same conditions provide clues to decoding how we came to be.
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A WORLD WE CALL EARTH
108, 107, 106, 105, 104 meters
From 100,000 kilometers to 10 kilometers
From about 26 percent of the Earth–Moon distance to the height of Mauna Kea from the ocean floor
What is Earth?
The answer depends on whom you ask. For a planetary scientist or a geophysicist, Earth is a deep sphere of iron, blanketed by a droplet of hot rock, topped off by a thin crust of crystallized minerals. For an astronomer, Earth is a minuscule condensation of star-stuff, the heavy-element detritus from very-long-dead previous generations of stellar objects. For a number-crunching statistician, Earth is one data point out of a trillion trillion worlds in the observable universe. It is a modest and slowly evolving outlier in the landscape of planets.
For biologists, Earth has been the incubator for the dynamic phenomenon we call life for nearly all of its existence. During that long passage of time, the face of this world has changed and changed again. Earth has transformed from a fearsome magma ocean to a wet ball of rock, and back and forth from a global hothouse to a global freezer with all shades of climate in between. Sometimes its crinkled surfaces have swarmed with living things. Sometimes it has been nearly barren after colossal extinction events have eliminated entire families of organisms.
Earth is also a mixture of the very old and the very young. The most ancient surface rocks you can hold in your hands have been dated to an age of over 3.5 billion years. Dense little crystals of zircon that you can find in igneous rocks like granite have been dated to an astonishing 4.4 billion years. Despite this age, the planet is active: it still builds new volcanic islands, its deep dynamo drives a strong dipolar magnetic field, and its surface niches still evolve new species of organisms. In these ways Earth is a world where no one instant truly represents its full history or future.
But for you and me, beyond these colorful scientific waypoints, what matters most of all is that Earth is an indelible part of our being.
Earth evokes a wealth of sights, sounds, smells, and tastes: It’s the first time you feel the ground beneath your feet, or the soft ocean waves rollin
g over your floating body. It’s your experience of the Sun rising over the horizon in the serene morning mist. It’s the potent odor of soil and plant life after a summer rainstorm. For some of us, it’s the thrill we feel with the gentle brightening of stars in a blackening evening sky. Earth is also a restless and sometimes treacherous home for us. This is a planet whose winds can buffet us, a world where blizzards and typhoons beat us down and the ground itself can shake us to bits.
Earth’s deep history, from the Hadean eon (over 4 billion years ago) to the Phanerozoic eon (541 million years ago until now).
The Hadean eon, starting 4.6 billion years ago. (above) The start of the Archean eon, 4 billion years ago (top left); early microbial life builds stromatolites, 3 billion years ago (bottom left); global glaciation, a “snowball Earth” around 2.4 billion years ago (top right); the lush Jurassic Earth, 150 million years ago (bottom right)
Exploring a volcanic mid-ocean ridge. A scientific gold mine, but also a worryingly tempting place to exploit for mineral resources
We dig into this world with our hands, and mine it with our tools. We mold its substances into forms that we need, or simply want. Like all living things, we are constantly converting one set of compounds into another, in the air we breathe, the food we eat, and the fuels we burn. Billion-year-old bedrock becomes building material for a home, a school, or a statue. Seams of naturally concentrated metals become our bridges, cars, bicycles, wedding rings, and circuit boards. Refined ores become fuel for nuclear reactors; arduously extracted rare-earth elements help guide electrons and enhance magnets in computers and smartphones.
We’ve become expert at this scavenging—too expert for our own good. We’ve drastically altered the balance of the Earth, and as a consequence now exert enormous pressure on organisms and environments that are part of the very system that supports us.
Of course, we’re not the first species to mess around with the global environment. About two and a half billion years ago, species of microbial life began to dump their waste product, oxygen, into the atmosphere. That chemical pollution signaled a profound shift in the chemical and climatological state of the planet, and in all the life that followed.
Those early photosynthesizing oxygen producers had little choice in the matter—they were simply deploying the metabolic tools that emerged from their evolution. We humans are distinct, and interesting, because we are aware of what we’re doing and usually have some sense of the consequences of our actions.
By the same measure, while Earth is our birthplace and playpen, it’s also completely indifferent to us. There is no special reason why Earth is “just right” for us. After all, we came from it—not the other way around. Whatever we do to the Earth, and whatever we do to life here, the planet will carry on and evolution will keep unfolding, relegating our era to a thin band in some future sedimentary rock.
That’s because (like any planet) Earth is a powerful thermodynamic, chemical, and radiological machine. Multitudes of phenomena are knit into the planetary surface and interior, as well as woven through time. Properties that we take for granted, from climate to fossil fuels, are the consequence of deep cycles and serendipitous events scattered across billions of years. In truth, all that we relish about Earth is one sentence of a much larger story.
ABSORB, CHURN, RADIATE
The connected processes of our planet start revealing themselves as our cosmic journey brings us into familiar territory. As we hover near the 42,000-kilometer altitude of a geosynchronous orbit above the Earth, we get to see an almost complete hemisphere. Visible across this side of the world are some of the primary engines of a rocky planet—the forces that drive the thermal and environmental state of its surface.
On the daytime side of Earth, the solar radiation hitting the top of the atmosphere deposits around 1,300 watts of power per square meter. That’s about the same amount used by an electric kettle. It doesn’t seem like a great deal.
But add up that incoming solar radiation across one whole hemisphere of the Earth, and a total of about 174 petawatts (1015, or a quadrillion, watts) of solar power is hitting the top of the atmosphere. A colossal total of 89 petawatts of that same power is absorbed by the surface of the Earth directly. The rest is reflected by the surface, or absorbed by the atmosphere and reflected by its clouds of condensed water.
The top of Earth’s atmosphere receives a barrage of solar photons.
By human standards this is a fearsome amount of energy. Estimates of current human energy consumption suggest that in a single year we use roughly 1.6 × 1011 megawatt-hours, which means that with 8,760 hours in a year we are using energy at a rate of about 0.018 petawatts. All life on Earth (adding up photosynthetic organisms, water transpiration in plants, and what life gets from chemical and geophysical energy) is estimated to consume energy at a rate of between 0.1 and 5 petawatts. In other words, despite life’s potent footprint on the planet, on a cosmic scale it’s still barely sipping at what the Sun’s photons rain down on us.
The native heat of Earth—from its still-molten interior—is also modest in comparison. All geothermal and geochemical power flowing through the planet’s surface adds up to about 0.047 petawatts.
The gloriously bright daytime side of the Earth is therefore not just a pretty sight. It signals a relentless absorption of electromagnetic radiation. Earth reflects, but it also acts like a giant sponge for photons that would otherwise streak off into the rest of the cosmos. We may be a small world, but we cast a long shadow.
But where does that power go? As with any material object, Earth’s tendency is to constantly lose any excess energy—to reach equilibrium with the surrounding universe. Except it’s coated with atmosphere and oceans that slow down the loss of energy. The planet heats up as a consequence, promoting the shedding of infrared photons into space to redress the imbalance. But much of the energy has to pass through other forms and intermediate steps before escaping. It drives the movement of the atmosphere and oceans and both their chemistries, transforming this planet into an engine of novelty.
We can easily see some of these energy-reprocessing machines in action. Across the Earth we can watch flows of atmosphere and ocean spanning thousands of kilometers. The spinning planet pulls these fluid materials around with it—one gas, one liquid. But solar energy interferes on a grand scale. For example, warm, moist air rises from the equator and tropics, lofting to altitudes of ten to fifteen kilometers before turning north and south. Eventually that air falls at mid-latitudes, creating a planet-spanning north-and-south torus of atmospheric flow. Similar flows exist toward the poles, with different mechanisms at play.
Because air is transported away from the equator, where it is moving the fastest with the spinning planet, its rotation gets out of sync. As a result the atmosphere experiences the Coriolis effect, causing it to move toward the east relative to Earth’s surface.
As a consequence of this shift, high-speed flows of atmosphere called jet streams (both subtropical and polar) form at altitudes of around ten kilometers and can encircle the planet. If you’ve ever flown between North America and Europe, you’ve probably experienced a jet stream that speeds your journey west to east, but slows your return east to west as a headwind.
Jet streams also help divide Earth’s atmosphere, separating cool and warm air. But if a high-latitude jet stream weakens or meanders, frigid air can descend on lower latitudes—causing trouble and grumbling for humans who are in the way.
Yet, in a universe filled with planets, Earth exhibits only modest examples of these phenomena. Elsewhere in our solar system, the gas giant Jupiter spins once every ten hours and has multiple atmospheric jets, writ large as great bands of color wrapping the planet. Saturn, appearing so calm and regal, actually has a polar vortex of atmosphere at its north and south ends, and equatorial high-altitude wind speeds that reach a brisk 1,800 kilometers an hour.
Another consequence of the turbulent behavior of Earth’s solar-driven atmosphere is weather. The mos
t spectacular examples are spawned by the low-pressure regions where massive amounts of warm, moist air loft into the sky: the tropical storms, whether called hurricanes, cyclones, or typhoons, that roll across the planet.
These rotating beasts give us a glimpse of the sheer power being pumped into the planet. As water evaporates from an ocean and re-condenses in a hurricane-forming region, it releases an enormous amount of thermal energy. By many estimates, one day of a single large hurricane involves 200 times as much power as the world’s total electrical production capacity.
Earth doesn’t just get hot and bothered by solar energy; its chemistry is changed. For the past 4.5 billion years ultraviolet light from the Sun has helped break apart stratospheric molecules including water and oxygen, driving vigorous photochemistry in Earth’s atmosphere. Sunlight has also persistently beaten down on surface minerals, inducing chemical and structural changes. And, of course, via the catalytic processes of life, especially photosynthesis, solar energy has affected profound chemical alteration. Some changes take a single day, as a bloom of algae grows in shallow marine environments. Others play out over centuries, as tree roots break ground, or acidic wash from microorganisms alters the rocks and minerals of Earth’s surface.
Above Africa and the Great Rift Valley
In the eye of the storm: A meteorological plane ventures across the eyewall of a hurricane.
HEAD IN THE STARS
From the scale of Earth’s globe to a patch of ten thousand meters, this is a world of texture, dynamism, and variety driven by energy. This planet is the most meaningful connection most of us have to the cosmos. Yet, since the rise of modern humans, most of us could easily go a lifetime without moving more than a short distance above Earth’s surface. That’s still true, with a few special exceptions.
