The earth is the largest terrestrial planet in our solar system, and the third planet from the sun. It is also known as Planet Earth, the World, Gaia, and Terra.

With millions of species, Earth is the only planet where life definitely exists. The planet was formed 4.5 billion years ago, and within a billion years, life appeared. Earth’s biosphere has changed the planet in many ways, including forming the ozone layer which blocks harmful radiation from space.

Around 70% of the Earth’s surface is covered with salt water oceans. The remainder is covered by continents and islands. Underneath the surface, there are several layers of the earth. Tectonic plates move across the surface, causing entire continents to move. Under the thick mantle layer, there is a liquid core deep in the earth which generates a magnetic field. Inside that, a solid iron inner core exists.

The Earth orbits the sun every 365.26 days. The Earth also spends like a top, providing us with day and night. The axis of rotation is tilted 23.4°, which gives us the changing seasons. Ocean tides are caused by the movement of the moon.

The Earth was created from the solar nebula – a mass of gas and dust left over from the sun – over 4 billion years ago. Over time, the outer layer of the planet cooled and hardened. Shortly after, the moon was formed. It is believed that a meteorite the size of Mars impacted the earth, ejecting enough material to create a moon.

The primordial atmosphere was created by volcanic activity and condensing water vapor. Water, combined with ice from impacting comets, collected to create the oceans. How life formed is still a mystery, although it is believed that around 4 billion years ago, a self-replicating molecule developed. Half a billion years later, life existed on Earth.

As creatures changed over time, some developed the ability to perform photosynthesis. This allowed them to harvest the energy of the sun. These lifeforms then emitted oxygen, which over time formed the ozone layer. As harmful UV radiation was absorbed by the ozone layer, life began to colonize the Earth’s surface.

Over hundreds of millions of years, continents formed and separated, only to form and separate again. Sometimes they would form to create a supercontinent. The most recent of these was Pangaea, which broke apart 180 million years ago.

Around 535 million years ago, an event known as the Cambrian explosion occurred. Suddenly, life was everywhere. Multicellular lifeforms began to colonize the earth.

Since the Cambrian explosion, there have been five mass extinctions. In each of these, a majority of the life forms on Earth were wiped out; however, some remained. The most recent mass extinction was 65 million years ago. The cause is unknown, although it is believed a meteorite collision was the primary reason. Large reptiles and (non-avian) dinosaurs were wiped out, while small mammals were spared.

Since the last extinction, mammals spread across the globe. Birds laid claim to the skies. Several million years ago, ape-like creatures took their first tottering steps on the African savanna. This freed up their hands, allowing tool use and communication like never before. The increased nutrition allowed for a bigger brain, which in turn allowed for greater tool use. Over time, this led to agriculture and civilization. No other creature has had such a profound influence on earth in such a short period of time.

Earth is prone to ice ages, which started 40 million years ago and intensified during the Pleistocene era 3 million years ago. The repeated cycles of glaciation and thawing repeat every 40,000 to 100,000 years. The last Ice Age on earth ended 10,000 years ago.

Earth cutaway from core to exosphere. Not to scale.
Photograph of Earth taken from Apollo 17. Courtesy Wikipedia.