c. 4.54 billion years ago · The early Solar System
The formation of Earth
Rocky bodies coalescing in the gas-and-dust disc around the young Sun formed Earth around 4.54 billion years ago; the Moon emerged shortly after, from a giant collision.
The Solar System emerged about 4.57 billion years ago, when an interstellar gas cloud collapsed under its own gravity. The Sun ignited at its centre; in the surrounding disc, dust grains collided and stuck together, growing into planetesimals and eventually into rocky planetary embryos. Within a few tens of millions of years, Earth had taken something close to its present mass.
Earth's age is measured from lead-isotope dating of the oldest meteorites (Allende, Murchison) and of Apollo lunar samples: about 4.54 billion years. The early Earth was nothing like today — its surface was largely molten, and its atmosphere held hydrogen, water vapour, carbon dioxide and methane, but no free oxygen.
Not long after, the young planet collided with a Mars-sized body called Theia. This "giant impact" hypothesis proposes that material thrown into orbit from Theia and from Earth's mantle coalesced into the Moon — accounting both for the chemical similarity between Earth and Moon and for Earth's tilted spin axis.
Sources
- How Earth and the Moon Formed — NASA
- Earth — Age and origin — Encyclopaedia Britannica
- The age of the Earth in the twentieth century — Scottish Journal of Geology