Earth did not begin as a small version of the familiar blue planet. Its ingredients passed through several radically different states: microscopic grains orbiting a young star, kilometer-scale bodies colliding in a disk, a growing world repeatedly melted by impacts, and finally a layered planet with a crust, atmosphere, and surface water. No single moment separates all of these stages. “Earth’s formation” names a sequence that began with the Solar System and continued until the young planet acquired most of its mass and internal structure.
A Star and Its Disk Came First
About 4.57 billion years ago, part of a cold molecular cloud collapsed under gravity. Most of its material collected at the center and became the young Sun. Conservation of angular momentum flattened the remainder into a rotating protoplanetary disk of gas and dust. The disk was not chemically uniform. Close to the Sun, temperatures were high enough that water and many other volatile compounds remained gaseous. Metals and heat-resistant silicate minerals could condense there, providing the principal solids from which the rocky planets formed.
Earth’s location therefore influenced its starting composition. The material around its orbit was rich in rock-forming elements such as oxygen, silicon, magnesium, and iron. Yet Earth is not an exact match for any meteorite group now known. Its final chemistry probably records the mixing of multiple reservoirs, the loss of some volatile elements during heating, and the arrival of additional material as the planet grew.
Dust Became Worlds Through Collisions
At first, the solid ingredients were smaller than pebbles. Electrostatic attraction helped grains stick, but simple sticking becomes inefficient as aggregates grow. Concentrations of drifting particles may have collapsed gravitationally into planetesimals, bodies large enough for gravity to dominate their encounters. Collisions and gravitational capture then built Moon- to Mars-sized planetary embryos within the inner disk.
The word “accretion” can sound gentle, but the late stages were violent. Embryos perturbed one another’s orbits and collided at speeds capable of melting or vaporizing rock. Some encounters added mass; others stripped mantles, scattered fragments, or merged two bodies only after debris fell back together. Earth emerged from this changing population rather than traveling along a predetermined assembly line.
The Growing Planet Sorted Itself
Three energy sources heated the young Earth: impacts converted motion into heat, compression raised temperatures in the interior, and radioactive isotopes released energy as they decayed. Large regions melted. In molten or partly molten rock, dense iron-rich metal descended while less-dense silicate remained above it. This differentiation produced the basic division between metallic core and rocky mantle.
Core formation did not necessarily occur once, after accretion had finished. Metal could separate within early planetesimals, and fresh metal delivered by later impacts could sink through Earth’s mantle. Chemical clues in mantle rocks indicate that growth and differentiation were intertwined. Elements that readily dissolve in metal were drawn toward the core, while others stayed predominantly in silicate. The resulting pattern is one of the records used to reconstruct Earth’s hidden beginning.
A Giant Impact Remade the Planet
Near the end of terrestrial planet formation, a protoplanet commonly called Theia struck the growing Earth. The exact geometry, speed, and composition remain active research questions, but the leading family of models links this collision to the Moon’s origin. Material placed into orbit assembled into the Moon, while the impact profoundly heated and mixed Earth.
This event is often treated as the last major chapter of Earth’s construction, although smaller additions continued afterward. It may also have affected Earth’s rotation, chemical inventory, and early atmosphere. The close isotopic resemblance between terrestrial and lunar rocks imposes demanding tests on every impact model: a successful explanation must reproduce both the Moon’s mass and the compositions measured in samples.
Cooling Made a Recognizable Planet Possible
After giant impacts, Earth could sustain a deep magma ocean. As heat escaped to space, minerals crystallized and a crust began to form. Gases released from molten rock helped build an atmosphere containing abundant water vapor, carbon dioxide, nitrogen-bearing compounds, and other species. The earliest atmosphere was not a breathable predecessor of today’s air, and impacts and escape to space could remove part of it.
Cooling did not wait until the Hadean Eon was almost over. Zircon crystals as old as about 4.4 billion years preserve chemical evidence consistent with differentiated crust and interaction with liquid water. They do not reveal a calm modern surface, but they show that at least some regions cooled surprisingly early after Earth formed. Oceans may have condensed as atmospheric water vapor cooled, while water and other volatiles also arrived with rocky material during accretion.
Earth thus got started through a chain of linked transformations. Solar-nebula chemistry supplied the ingredients; gravity and collisions accumulated them; melting separated metal from rock; a giant impact created the Earth–Moon system; and cooling allowed crust and surface water to persist. The planet continued changing afterward, but its identity as a differentiated rocky world was established during this extraordinary opening interval.