The Earth was formed approximately 4.54 billion years ago. The process began with a large cloud of gas and dust called the solar nebula. This nebula was likely the remnant of a supernova explosion. Under the influence of gravity, this cloud began to collapse inward. As it collapsed, the cloud began to spin faster due to the conservation of angular momentum, similar to how an ice skater spins faster when they pull in their arms.
As the nebula flattened into a rotating disk, the Sun formed at the center. In the disk, tiny dust particles began to collide and stick together due to electrostatic forces. These collisions led to the formation of larger and larger objects. Over time, these growing bodies, called planetesimals, ranged from a few kilometers to hundreds of kilometers in size. In the region where Earth would eventually form, about 150 million kilometers from the Sun, these planetesimals continued to collide and merge, gradually building up what would become our planet.
As the early Earth grew through collisions and accretion, it began to heat up significantly. This heating came from three main sources: radioactive decay of elements within the Earth, gravitational compression as the planet grew larger, and the kinetic energy from countless impacts. This intense heat caused the early Earth to melt partially or completely. In this molten state, a process called differentiation occurred. Denser materials, primarily iron and nickel, sank toward the center to form Earth's core, while lighter silicate materials floated upward to form the mantle and crust. This layered structure, with a dense metallic core surrounded by a rocky mantle and thin crust, remains a fundamental feature of our planet today.
One of the most dramatic events in Earth's formation was a catastrophic collision that occurred about 4.5 billion years ago. A Mars-sized planetary body, which scientists call Theia, struck the young Earth at an oblique angle. This giant impact was so powerful that it ejected a massive amount of material from both Earth's mantle and the impactor into space. Much of this debris was captured by Earth's gravity and began orbiting our planet. Over time, these orbiting fragments collided and merged, eventually coalescing to form our Moon. This giant impact theory explains many observations about the Moon, including why the Moon's composition is similar to Earth's mantle, and why the Moon has a small iron core relative to its size.
After the giant impact and the formation of the Moon, Earth continued to evolve. Initially, our planet was extremely hot, with a molten surface and virtually no atmosphere. As Earth cooled, a solid crust began to form, creating the first landmasses. Intense volcanic activity was common during this period. These volcanoes released gases such as water vapor, carbon dioxide, and nitrogen, which gradually formed Earth's early atmosphere. As the planet continued to cool, water vapor in the atmosphere condensed to form rain, which fell for thousands or millions of years, eventually creating the oceans. This early Earth, with its oceans, atmosphere, and continents, set the stage for the later development of life, which would begin to appear about 3.5 to 4 billion years ago.