Earth’s Place in the Universe

The Origin of Planet Earth

The blue planet

How Did The 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.

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.

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.

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.

earth layers

What is the Earth made of?

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.

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.

Written by Laura McCamy

Edited by Beth Geiger, MS Geology