Formation of the Solar System: Birth of the Sun and Planets

The formation of the Solar System began about 4.568 billion years ago, when part of a cold cloud of gas and dust in the Milky Way collapsed under its own gravity. At the centre the Sun ignited, while the surrounding spinning disc slowly built the planets, moons, asteroids and comets. Every later chapter of Earth’s history, from the first rocks to the first life, depends on what happened in those first few tens of millions of years.

Background: a cloud in the galaxy

Stars are born in giant molecular clouds, cold and dense regions of interstellar gas made mostly of hydrogen and helium, with small amounts of heavier elements and dust. Those heavier elements, such as carbon, oxygen, silicon and iron, had been forged inside earlier generations of stars and scattered by stellar winds and supernova explosions.

The Sun probably formed in a cluster with hundreds or thousands of other stars, which later drifted apart. Some meteorites contain traces of short-lived radioactive isotopes, notably aluminium-26, which decays with a half-life of about 717,000 years. Their presence suggests that fresh material from nearby massive stars was mixed into the cloud, and some researchers think a supernova shock wave helped trigger its collapse. This remains debated.

What happened: from cloud to disc

Once a dense core of the cloud began to collapse, it spun faster as it shrank, in the same way a figure skater speeds up by pulling in their arms. The material flattened into a rotating disc, known as the solar nebula or protoplanetary disc, with most of the mass falling to the centre.

That central concentration became the proto-Sun. As it contracted, it heated up until, after a few tens of millions of years, nuclear fusion of hydrogen into helium became stable in its core. Today the Sun contains about 99.8 percent of the mass of the entire Solar System.

The first solids

In the hot inner parts of the disc, the first solid grains condensed from the gas. The oldest known are calcium-aluminium-rich inclusions (CAIs), pale, millimetre- to centimetre-sized fragments found in primitive meteorites. Slightly younger are chondrules, small once-molten droplets of rock that formed over roughly the next three million years.

Uranium-lead dating of CAIs gives an age of about 4,567 to 4,568 million years. Scientists use this moment as “time zero” for the Solar System. The Allende meteorite, which fell in Chihuahua, Mexico, in February 1969 and scattered tonnes of fragments, has been one of the most important sources of these ancient grains.

Building planets

Dust grains stuck together to form pebbles, and pebbles grew into planetesimals, bodies roughly a few kilometres to hundreds of kilometres across. How grains jumped from centimetre to kilometre sizes is still an active research question. One leading idea, called the streaming instability, suggests that pebbles clumped together in the gas and collapsed directly into planetesimals.

Planetesimals then collided and merged into planetary embryos, some as large as the Moon or Mars. The Soviet scientist Viktor Safronov worked out much of this theory of gradual growth in the 1960s, and his ideas remain the foundation of modern models.

Rocky planets and giant planets

Temperature across the disc shaped the kind of planets that formed. Close to the Sun, only metals and rocky silicate minerals could stay solid. Farther out, beyond a boundary known as the frost line or snow line, water and other volatile substances froze into ice, providing much more solid material.

In the outer disc, solid cores grew large enough, perhaps around 10 Earth masses, to pull in vast envelopes of hydrogen and helium. This produced the gas giants Jupiter and Saturn. Studies of isotopes in iron meteorites suggest that Jupiter’s core grew to perhaps 20 Earth masses within roughly the first million years and kept growing over the next few million, quickly enough to separate the inner and outer parts of the disc. Uranus and Neptune, the ice giants, captured less gas.

The gas disc dispersed within roughly 3 to 10 million years, blown away by the young Sun’s radiation and wind. In the inner Solar System, the remaining embryos kept colliding for tens of millions of years, forming Mercury, Venus, Earth and Mars. One of the last giant collisions is thought to have created Earth’s Moon.

A restless young Solar System

The planets did not necessarily form where they are today. Computer models suggest that the giant planets migrated. In the “Nice model”, published in 2005 by Rodney Gomes, Harold Levison, Kleomenis Tsiganis and Alessandro Morbidelli, the orbits of Jupiter and Saturn became unstable, scattering Uranus and Neptune outward and flinging vast numbers of icy and rocky bodies around the Solar System.

Another model, the “Grand Tack”, proposes that Jupiter drifted inward towards the Sun and then back outward, which could explain why Mars is so small and why the asteroid belt contains a mix of rocky and icy material. Both models remain debated, and the timing of any giant-planet instability is uncertain.

The leftovers of planet formation survive today. The asteroid belt between Mars and Jupiter holds rocky debris that never became a planet. Beyond Neptune lie the Kuiper Belt and, much farther out, the hypothesised Oort Cloud, reservoirs of icy bodies that occasionally become comets.

How scientists know

The theory rests on several independent lines of evidence:

• Meteorites. Primitive meteorites called chondrites are almost unchanged since the Solar System’s birth and can be dated precisely with radioactive decay.
• Orbits. The planets orbit in nearly the same plane and in the same direction as the Sun rotates, just as expected for bodies born from one spinning disc.
• Other stars. Telescopes now see protoplanetary discs around young stars. In 2014 the ALMA radio telescope array in Chile imaged the disc around the young star HL Tauri, revealing rings and gaps that may be carved by forming planets.
• Space missions. Samples returned from comets and asteroids, such as those collected by the Stardust, Hayabusa2 and OSIRIS-REx missions, show the chemistry of material left over from the disc.

Key thinkers

The basic idea is old. In 1755 the German philosopher Immanuel Kant proposed that the Solar System formed from a rotating cloud. In 1796 the French mathematician Pierre-Simon Laplace independently set out a similar “nebular hypothesis”.

The theory fell out of favour in the late 19th and early 20th centuries, partly because it could not explain why the Sun spins so slowly while the planets hold most of the Solar System’s angular momentum. Rival ideas, such as planets pulled out of the Sun by a passing star, were popular for a time. From the mid-20th century, work by Safronov and others, combined with meteorite dating and later observations of discs around other stars, restored and transformed the nebular model. The Sun’s slow spin is now explained by magnetic interactions between the young star and its disc.

By the numbers

Timeline

• c. 4,568 million years ago: Collapse of a dense core in a molecular cloud; the first solids (CAIs) condense in the disc.
• c. 4,568–4,565 million years ago: Chondrules form; planetesimals begin to grow.
• Within the first few million years: Jupiter’s core forms and begins capturing gas.
• c. 4,565–4,558 million years ago: The gas disc disperses, ending the growth of the giant planets.
• c. 4,540 million years ago: Earth largely assembled, according to the commonly cited age of the Earth.
• c. 4,510–4,450 million years ago: A giant impact forms Earth’s Moon.
• AD 1755: Immanuel Kant publishes his nebular theory.
• AD 1796: Pierre-Simon Laplace sets out his nebular hypothesis.

Why it matters

The birth of the Solar System set the conditions for everything that followed on Earth: the planet’s distance from the Sun, its rocky make-up and the water and carbon it inherited. It led directly to the formation of the Earth and the giant impact that formed the Moon, and its leftover debris shaped the Late Heavy Bombardment. Studying it also helps astronomers understand the thousands of planetary systems now known around other stars.