Science › Cosmic timeline
The Sun and Earth form
1755· Cosmic event
ProvenTested many times; part of working physics.
About 4.6 billion years ago, roughly 9 billion years after the start, a collapsing cloud of enriched gas formed the Sun, Earth and planets.
See it on the Science map →How it works
A cloud of gas and dust in the Milky Way collapsed, most of it falling to the center and igniting as the Sun. The rest flattened into a spinning disk. Dust grains stuck into pebbles, then boulders, then kilometer-sized bodies, which collided into planets. Earth assembled over tens of millions of years. A Mars-sized body then struck it, and the debris formed the Moon. Life appeared by at least 3.5 billion years ago.
Why it matters
It is the local chapter of cosmic history: where general processes, gravity, stellar chemistry and cooling, produced one particular home with rock, water and the ingredients for life.
How we know
Radioactive dating of the oldest meteorite grains gives 4.567 billion years. Kant (1755) and Laplace (1796) proposed the collapsing cloud; telescopes now image planet-forming disks, sharply since ALMA's 2014 picture of HL Tauri. Australian zircon crystals show Earth had crust 4.4 billion years ago.
Still a mystery
How exactly Earth got its water, the details of the Moon-forming impact, and how common systems like ours are all remain open.
Connections (2)
▸Stars forge the heavier elements supplied the elements for The Sun and Earth formProven
The relationship
By 4.6 billion years ago, about nine billion years of stellar generations had raised the Milky Way's heavy elements to roughly 1.4% of gas by mass. The cloud that formed the Sun inherited this mix: enough silicon, iron and oxygen for rocky planets, plus carbon and water ice. Short-lived radioactive atoms show one or more stars enriched the cloud just before it collapsed.
How the link was found
In 1976–77 Typhoon Lee, Dimitri Papanastassiou and Gerald Wasserburg found traces of aluminum-26's decay in the Allende meteorite (fell in Mexico, 1969), pointing to fresh stellar input.
Why it's proven
The Sun's spectrum and the most primitive meteorites share the same element ratios. Aluminum-26, with a half-life of about 700,000 years, implies an injection within a few million years of collapse.
Open questions and mysteries
Did the aluminum-26 come from a nearby supernova or the winds of a massive star, and did that event trigger the collapse?
▸The Sun and Earth form explains why You are made of stardustProven
The relationship
Earth's ingredients were sorted by temperature in the young disk. Near the hot Sun, only rock and metal could condense, so the inner planets are rocky. Earth's iron sank to form its core; lighter rock formed the crust. Water and carbon likely arrived partly from colder material farther out, carried in by asteroids. Life then assembled star-made carbon, nitrogen and oxygen with Big Bang hydrogen.
How the link was found
The temperature sequence of condensing minerals was worked out by Lawrence Grossman in 1972. Comparing hydrogen isotopes in meteorites, comets and oceans since the 1980s traced Earth's water.
Why it's proven
Earth's water has a heavy-to-light hydrogen ratio matching carbon-rich meteorites rather than most comets. Samples from asteroids Ryugu (returned 2020) and Bennu (2023) contain water-bearing minerals and organic molecules.
Open questions and mysteries
How star-made chemistry became the first life is unknown. Bennu's samples (2025) hold 14 of the 20 protein amino acids and all five DNA and RNA bases, but no lab has made life from such parts.