We live on Earth's hard, rocky surface, breathe the air that surrounds it, drink the water that falls from the sky, and eat the food that grows in the soil. But Earth was not always a welcome place for life.
Billions of years ago, the dust and gas that would become our solar system existed only as an enormous cloud. At some point, a disturbance in that dust cloud set forth a string of events that led to the formation of life as we know it.
Explosion, then Collapse of a Star
Scientists think the disturbance was caused by the explosion and collapse of a distant star. This supernova caused waves in space that compressed, or squeezed, the dust cloud. It began to contract under its own gravity. As it contracted, it formed a spinning disc of gas and dust, known as a solar nebula. The faster the cloud spun, the more the dust and gas became concentrated at the center, causing the nebula to spin even faster.
Over time, the gravity at the center of the cloud became so intense that hydrogen atoms began to move more rapidly and violently. Charged hydrogen atoms called protons began fusing, forming helium and releasing massive amounts of energy. This led to the formation of the sun, the star that is the center of our solar system. It happened roughly 4.6 billion years ago.
The formation of the sun consumed more than 99 percent of the matter in the nebula. The remaining material began to clump together into various masses. The cloud was still spinning, and clumps of matter continued to crash into each other. Eventually, some of those clusters of matter grew large enough to have a gravitational pull, which shaped them into the planets and dwarf planets that make up our solar system today.
A Planet Quad Led by Earth
Earth is one of the four inner, terrestrial planets in our solar system. Just like the other inner planets—Mercury, Venus, and Mars—it is relatively small and rocky. Early in the history of the solar system, rocky material was the only substance that could exist close to the sun and withstand its heat.
At its beginning, Earth was unrecognizable from its modern form. It was extremely hot, to the point that the planet likely consisted almost entirely of melted magma. The planet cooled over the course of a few hundred million years, forming oceans of liquid water. Heavy elements, like iron and nickel, began sinking toward the center of the planet. As this occurred, the planet separated into layers. Lighter material formed the outer layer, while denser, heavier material sank to the center.
Scientists think Earth formed in three different stages. The first stage is known as accretion. Particles within the solar system crashed into each other and stuck together forming larger and larger bodies.
During the next stage, a protoplanet crashed into the very young planet Earth. This collision is thought to have occurred more than 4.5 billion years ago. It might have resulted in the formation of Earth's moon.
The final stage of development saw the bombardment of the planet with asteroids. Scientists think the asteroids that slammed into Earth, the moon, and other inner planets contained a significant amount of water in their minerals. The asteroids hit the surface of Earth at a great speed, shattered, and melted. Experiments suggest nearly 30 percent of the water in the asteroids could have remained on Earth.
Planet Earth Takes Shape
Earth's early atmosphere was most likely composed of hydrogen and helium. As the planet changed, and the crust began to form, volcanic eruptions occurred frequently. These volcanoes pumped water vapor, ammonia, and carbon dioxide into the atmosphere around Earth. Slowly, the oceans began to take shape, and eventually, early life evolved in those oceans.
Around 2.7 billion years ago, oxygen produced by photosynthesizing bacteria began to build up in the atmosphere. Over a few hundred million years, it changed the composition of the atmosphere. Our modern atmosphere is made up of 78 percent nitrogen and 21 percent oxygen, among other gases. Life on Earth evolved to thrive within this atmosphere.
