Artist impression of the Early Earth as a lava planet during the Hadean eon
Artist's impression of Archean Earth, showing an orange atmospheric haze, leading to the alternative description of the planet in that stage of its development as a "pale orange dot"
Early Earth, also known as Proto-Earth, is loosely defined as Earth in the first billion years — or gigayear (109 y or Ga) — of its geological history, from its initial formation in the young Solar System at about 4.55 billion years ago (Gya), to the end of the Eoarchean era at approximately 3.5 Gya. On the geologic time scale, this comprises all of the Hadean eon and approximately one-third of the Archean eon, starting with the formation of the Earth about 4.6 Gya, and ended at the start of the Paleoarchean era 3.6 Gya.
This period of Earth's history involved the planet's formation from the solar nebula via a process known as accretion, and transition of the Earth's atmosphere from a hydrogen/helium-predominant primary atmosphere collected from the protoplanetary disk to a reductant secondary atmosphere rich in nitrogen, methane and CO2. This time period included intense impact events as the young Proto-Earth, a protoplanet of about 0.63 Earth masses, began clearing the neighborhood, including the early Moon-forming collision with Theia — a Mars-sized co-orbital planet likely perturbed from the L4 Lagrange point — around 0.032 Ga after formation of the Solar System, which resulted in a series of magma oceans and episodes of core formation. After formation of the core, meteorites or comets from the outer Solar System might have delivered water and other volatile compounds to the Earth's mantle, crust and ancient atmosphere in an intense "late veneer" bombardment. As the Earth's planetary surface eventually cooled and formed a stable but evolving crust during the end-Hadean, most of the water vapor condensed out of the atmosphere and precipitated into a superocean that covered nearly all of the Earth's surface, transforming the initially lava planet Earth of the Hadean into an ocean planet at the early Archean, where the earliest known life forms appeared soon afterwards.
Although little crustal material from this period survives, the oldest dated rock is a zircon mineral of 4.404 ± 0.008 Gya enclosed in a metamorphosed sandstone conglomerate in the Jack Hills of the Narryer Gneiss terrane of Western Australia. The earliest supracrustal terrains (such as the Isua greenstone belt) date from the latter half of this period, about 3.8 Gya, around the same time as peak Late Heavy Bombardment.
