The Carina Nebula in the southern skies

A pillar of dust and gas forms in the Carina Nebula, a large region of star formation in the southern skies © Yury Dmitrienko/Shutterstock

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How our solar system was born

Discover how a giant interstellar cloud known as the solar nebula gave birth to our solar system and everything in it. 

The solar system as we know it began life as a vast, swirling cloud of gas and dust, twisting through the universe without direction or form.

About 4.6 billion years ago, this gigantic cloud was transformed into our Sun. The processes that followed gave rise to the solar system, complete with eight planets, 181 moons, and countless asteroids.

Researcher Tim Gregory explains how it burst into being.

Gravitational collapse

Before it was moulded into a neat set of planets, every scrap of matter in the solar system was part of a gigantic nebula - a floating interstellar cloud. This giant cloud was made up of dust, hydrogen, and other gases.

It began to collapse in on itself after becoming gravitationally unstable. This was possibly because of a nearby supernova - an exploding star - sending shock waves rippling through space.

Gravity then caused dust and gas to be continually tugged to the centre of the cloud, making its core very hot and dense.

The remains of a supernova, captured by NASA

NASA's Hubble Space Telescope captured the expanding remains of a massive supernova, the Veil Nebula © NASA/ESA/Hubble Heritage Team

Gregory says, 'It became a snowball effect. As more matter got pulled in, the centre got denser, increasing the gravity and pulling even more dust inwards.

'About 99.9% of the material fell into the middle of the cloud and became the Sun. Once the centre became hot and dense enough it triggered nuclear fusion. Then visible light flooded the solar system for the first time.

'The 0.1% of matter that remained orbited around the Sun, causing this randomly shaped gas cloud to form a flat disc shape. This flat disc, called the protoplanetary disc, was where the planets formed.'

The formation of planets                                                

As this rotating disc span around the Sun, it began to cool and form different types of solid material.

Gregory says, 'Near to the Sun, the temperature was very high, so minerals and metals formed. And on the edge of the disc, far away from the heat of the Sun, less volatile solids like ice and ammonia formed.

'As the disc continued to cool down, these whirling solids stuck together to form big clusters of mass. Gradually they got larger and larger, sweeping up all the leftover dust, until they grew into the planets we recognise today.'

Part of a chondrite found in the Sahara desert

Part of a chondrite found in the Sahara desert, showing the formation of chondrules. Chondrites are the most common type of meteorite to fall to Earth © Tim Gregory

The hot, rocky material near the centre of the solar system was sculpted into terrestrial planets with metal cores: Mercury, Venus, Earth and Mars.

And on the cool edges, the gas and ice giants were born: Saturn, Jupiter, Neptune, and Uranus.

The asteroid belt

Rocks that escaped the pull of planets were left as asteroids, scattered through the solar system without a permanent home.

Many of these rocks orbit the Sun in an area between Mars and Jupiter known as the asteroid belt. They can be very large - the biggest, Ceres, has a diameter of nearly 600 miles.

Gregory says, 'The asteroids are rocky debris left over from the era of planet formation, 4.5 billion years ago.

'They are very valuable to us as scientists, because they contain material that Earth and the other planets were originally made from, frozen in time. The study of these rocks can tell us a lot about what conditions were like in the disc, when planets were still forming.'

The first solids

Many of the asteroids in the solar system melted early on in their history to form an iron core and rocky mantle. During melting the heavier material, metal, sinks to the centre while the lighter rock floats up to form a crust.  

The bodies that didn’t melt are a type of meteorite known as chondrites - sedimentary rocks that formed in the early solar nebula.

Because they didn't melt, they’re pristine samples of the original solids that formed in the cooling protoplanetary disc. For scientists they’re some of the most valuable leftover materials we have.

They are also the most common type of meteorite that falls to Earth.

Gregory, whose research focuses on these rocks, says, 'Chondrites contain the first solids that formed in the solar system. By analysing them we can figure out how old the solar system is.

'We can unpick the 4.5 billion year journey from the solar nebula, to the protoplanetary disc, to the solar system we see today. 

'Earth formed from this nebula, so our journey to understand it is also a journey of self-discovery. It lets us understand our own home in space.'