A Short History of Planet Earth
How did Earth form? To find out, let’s spin back in time—way back. There was no earth or sun or moon. Our solar system – our sun and the eight planets that rotate around it – didn’t exist at all. There was just a big solar nebula. A nebular is a huge cosmic cloud of gas and dust.
Then, about 4.6 years billion years ago, some of that dust and gas pulled together into a fiery ball. That became our sun. The rest of the dust and gas from the solar nebula clumped into planets. One of those planets was Earth.
Dust to Crust
The particles of dust that made up Earth began to form layers. The heaviest elements sank to the center. The lighter elements rose to the top.
Earth was a hot, inhospitable place at that time. Volcanoes erupted molten rock, called lava, across the surface. The atmosphere—the layers of gases that surround Earth—didn’t exist yet. Without an atmosphere to burn them up, asteroids and comets pelted the planet’s surface. Over time, gases erupted from the volcanoes began to form a primitive, toxic atmosphere.
The young planet continued to cool and form. The heaviest elements kept sinking. Lighter elements kept rising. Within a quick one million years or so after that cloud of dust first pulled together, Earth had taken on the layered structure it has today, 4.5 billion years later.
A simple way to think about the structure is to compare Earth to a hard-boiled egg. At the center, like the yolk, is the core, with inner and outer layers. The inner core, as you might expect, is the densest and hottest layer of all. It is a solid ball made mostly of iron and nickel. The outer core is also made of iron and nickel. But, unlike the inner core, it is liquid. That liquid moves in huge, turbulent currents that generate Earth’s magnetic field.
Next up: the mantle. Like the white of that hard-boiled egg, it’s also the thickest layer. The mantle is dense, hot, and has the consistency of salt water taffy. includes heavy elements and also lighter elements such as magnesium and silicon. Like the outer core, huge currents slowly churn away inside the mantle.
Finally, topping it all is Earth’s crust. Just like the eggshell, the crust is incredibly thin compared to the rest of the planet. Yet, that thin crust is our home sweet home, a tapestry of soaring mountains, deep oceans, smoldering volcanoes, and dark canyons. Oxygen, silicon, aluminum, and calcium are just some of the elements that make up the crust. The crust is broken into thick tectonic plates that slowly move, pushed around by the mantle currents beneath.
Where did Earth’s water come from? That’s an important question, because after all, Earth is a very watery planet. There’s a reason it’s called the Blue Planet! And without water there would be no life…and no us.
Scientists think that much of Earth’s water arrived the hard way, by crashing into the planet’s surface inside icy meteorites during those first million or two years. There is also evidence that a lot of water may have come from within the solar nebula itself. That means a lot of Earth’s water was here from the very beginning.
No one knows exactly how life began on earth. We do know that the first living creatures appeared at least 3.5 billion years ago. The first life was tiny microbes. Over time, those single-celled organisms evolved into plants and animals.
Life changed the atmosphere. Blue-green algae converted carbon dioxide into oxygen. Without that oxygen, humans wouldn’t be able to breathe the air. Water is also essential to life on earth. Water covers 71% of our planet. When the first astronauts sent photos of the earth from space, the planet looked very blue because of all that water.
Earth is still changing. Volcanoes, plate tectonics, and erosion constantly re-shape the crust. Those changes happen on geologic time: so slow we can’t really see them. But over hundreds of millions of years, entire continents have come and gone.
Our Goldilocks Planet
Earth is the only planet in our solar system that can support life. There may be other planets in the Milky Way or other galaxies that support life, but we have not discovered them yet.
Earth happens to be in just the right spot in the solar system. If we were closer to the sun, like Mercury or Venus, it might be too hot for life to survive. If we were farther away from the sun, like Saturn or Jupiter, our planet could be covered in ice all the time and too cold for life.
When astronomers search for planets in other solar systems, they think of what makes Earth so cozy and livable. They have a nickname for such a world: a Goldilocks planet. Not too hot. Not too cold. Just right for water and life. So far, Earth is the only Goldilocks planet we know about. Lucky us!
Written by Laura McCamy
Edited by Beth Geiger, MS Geology
Illustrated by Renee Barthelemy