The Earth is thought to be about 4.54 billion years old. Along with other planets, the Earth was born in the early days of the Solar System, which first started forming about 4.6 billion years ago.
How did the Earth form?
The Solar System formed about 4.6 billion years ago from material in a massive, rotating cloud of gas and dust called the solar nebula. Gravity caused this cloud to collapse in on itself, spin, and flatten into a disk shape. Most of the material in that cloud was pulled toward the center, forming the protostar that would eventually become our Sun. The rest of the material began to come together into clumps called planetesimals. These in turn gradually came together with other planetesimals, forming larger bodies called protoplanets. Earth started as one of these protoplanets, likely about 4.5 billion years ago.
The Earth’s history
As the proto-Earth grew, heavier elements within it began to sink toward the center, forming the core, and lighter elements rose to the surface. This process, called differentiation, likely took place over tens of millions of years.
During these early stages a Mars-sized protoplanet, often referred to as Theia, collided with the young Earth, ejecting material from both protoplanets into space. Some of this material fell back to Earth, but some of the material eventually coalesced in orbit around Earth to form the Moon.
The Earth continued to experience impacts throughout its early life, though none as dramatic as the collision with Theia. During a period called the Late Heavy Bombardment, which likely happened between 4.1 and 3.8 billion years ago, there was an increased rate of asteroid and comet impacts in the inner Solar System. The Late Heavy Bombardment had major geological consequences, including causing Earth’s crust to melt and differentiate and shaping the early atmosphere and oceans. Although geological activity has erased the craters from this time on Earth, they are preserved on the Moon. These are some of the craters you can see from Earth.
By about 4.3 billion years ago, the Earth's surface had cooled enough for water vapor in the atmosphere to condense on the surface, leading to the formation of oceans. Volcanic activity, which was more widespread at the time, released gasses that shaped the early atmosphere. Life emerged around 3.5 to 4 billion years ago in the form of simple, single-celled organisms.
The Earth has probably been as we know it today — with recognizable continents, oceans, a hospitable climate, and diverse life — for the past few hundred million years. But it continues to evolve through its own gradual tectonic and volcanic activity, and through the more rapid effects of climate change.
How do scientists determine the age of the Earth?
Scientists have been able to piece together our planet’s timeline thanks to techniques including radiometric dating of rocks and minerals, examining layers of sedimentary rock, and studying the Earth's magnetic field.
The most precise method is radiometric dating, which measures the decay of radioactive isotopes in rocks. Because geologists know how long these isotopes take to decay, they can determine a rock’s age by looking at the ratio of parent (pre-decay) and daughter (post-decay) isotopes in a sample.
One challenge with dating the Earth via rocks is that most of the original rocks that formed on our planet at the earliest stages of its creation have likely been recycled into the mantle since then. Because of this, geologists also learn about the history of the Solar System by studying rocks from beyond Earth, including meteorites that were formed billions of years before falling to Earth, meteorites of Earth material that have been found on the Moon, and asteroids that have coasted through space undisturbed for billions of years without undergoing any major composition-altering change. The asteroid Bennu, for example, is thought to have formed in the first 10 million years of the Solar System’s history. By studying the samples returned to Earth by the OSIRIS-REx mission, scientists can learn a lot about the early Solar System.