How did the Earth end up filled with iron, oxygen and all the other ingredients for life? It all started when a cloud of dust gathered enough mass to collapse under gravity, as predicted by astronomer James Jeans.
In this extract from Rebel Star: Our Quest to Solve the Great Mysteries of the Sun (£16.99, Michael O’Mara Books), Colin Stuart explains the remarkable process that turned clouds of gas and dust into the Solar System as we know it.
How we got here
Today, there is widespread agreement among astronomers on the basic steps that formed the Sun. Around 4.6 billion years ago, there was a cloud of gas and dust in space around 65 light years (620 trillion kilometres) across. It began to contract until it reached the critical point at which James Jeans told us that irreversible collapse was inevitable.
As it continued to shrink and spin faster, the cloud fragmented into many parts. One of those fragments was about 3 light years across and would become the Sun and its family of orbiting worlds. It contained within it all the ingredients of the modern Solar System.
Not all were in their current form, but the atomic building blocks were there for Saturn’s rings and Mars’s moons; for every line of ink in every book ever written and every drop of blood spilled in every battle; for every bee that buzzes, flower that grows and bird that sings; Picasso’s paint and Churchill’s cigars; your hair, your eyes, your teeth.
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It took roughly 100,000 years – an astronomical heartbeat – for the cloud to collapse fully. In that time it became 2,000 times smaller. Gas molecules smashed into each other with increasing regularity, raising the temperature and pressure in equal measure.
A dense, hot sphere called a protosun emerged at the heart of the cloud. Like a human foetus, it was not yet fully formed. Becoming more compact increased its rotation speed to maintain angular momentum. Material falling towards the protostar found itself flung outwards into orbit around it, forming a protoplanetary disc that extended out to 1,000 times the modern Earth–Sun distance.
It would take another 50 million years for the young protosun to mature into the star that illuminates the solar system today. During this wild-child phase – perhaps akin to a human’s teenage years – it became what astronomers call a T-Tauri star.
Rotating at least twice as fast as it does now, the adolescent Sun belched vast amounts of high-energy radiation into space and its face was ridden with huge sunspots. Fierce winds roared away from its surface and it’s possible that twin jets of high- speed material erupted from its poles.
This is one way angular momentum is thought to be transferred from a protostar to the surrounding disc and it begins to slow its rotation rate.
Meanwhile, material in the disc has already started to clump. First, electrostatic forces draw miniscule dust grains together to form tiny pebbles. Then gravity takes over, colliding the pebbles together again and again, eventually fashioning them into huge boulders several hundred metres across.
They whack into each other in turn, creating mighty planetesimals up to 10 kilometres wide. The composition of these building blocks depends on where they form. Close to the protosun it is so hot that most materials simply evaporate away. The only elements that can survive the inferno are those with super-high boiling points, such as iron and nickel.
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Eventually, a couple of hundred heavy, rocky-metallic protoworlds roam the inner solar system. Further collisions see them turn into the four terrestrial planets, dotted all around by moons and asteroids – astronomical off-cuts.
Further from the heat, where temperatures plummet below freezing, molecules of water, methane and ammonia ice form and collide to construct cores of frozen material. Within a few million years there were icy objects four times the mass of the Earth marooned more than a billion kilometres from the protosun.
They collided, too, and attracted some of the gas remaining from the original nebula, becoming the four giant planets of the outer solar system. In 2019, a team of astronomers led by Ko Arimatsu from the National Astronomical Observatory of Japan discovered a kilometre-wide planetesimal still intact beyond the orbit of Neptune.
Any smaller icy blocks left over are now the comets that patrol the frigid outskirts of our neighbourhood. The young Sun’s strong winds cleared out most of the remaining gas and dust, leaving a nascent solar system behind. These gusts are so fierce that they can prevent other stars forming nearby, something astronomers spotted in the Orion Nebula for the first time in 2019.
Over time the new planets jockeyed and jostled for position, ejecting some protoplanets beyond the Sun’s gravitational clutches. Others slammed into each other. Just 100 million years after the Earth formed it was blindsided by a protoplanet the size of Mars. The resulting debris scattered into orbit eventually coalesced to form our moon.
Why did it happen?
It’s a fantastic story, but there’s one key plot point missing: what forced the collapse of the nebula in the first place?
We might just have found the answer thanks to recent analysis of asteroid material. These lumps of rock and metal are leftover chunks of planetesimals, predating the planets themselves. Sometimes they fall to Earth in the form of meteorites, providing an invaluable time capsule through which scientists can learn about the solar system’s past.
An asteroid to an astronomer is like a fossil to a palaeontologist – it contains history’s secrets. The oldest material inside these asteroids and meteorites is 4.6 billion years old, which is how astronomers are able to estimate how long ago the nebula that formed the solar system started clumping together.
They also contain evidence of short-lived radioactive isotopes – rare versions of familiar chemical elements such as aluminium and iron. Take iron-60. An ordinary atom of iron (iron-56) contains twenty-six protons and thirty neutrons in its nucleus, making a total of 56.
Iron-60 has twenty-six protons and thirty-four neutrons, making it heavy and unstable. In an attempt to stabilise the situation, one of its neutrons shape-shifts into a proton, turning the atom of iron-60 into one of nickel-60. Leave a lump of iron-60 alone for 2.6 million years and half of its atoms will have undergone this change.
After 21 million years, more than 99 per cent will have done so. As the solar system is nearly 5 billion years old, any iron-60 has long since vanished from asteroids. The stable nickel-60 it decayed into remains, however.
In 2017, a team of astronomers from the Carnegie Institution for Science in Washington, DC analysed a family of meteorites known as carbonaceous chondrites. They found enough nickel-60 to suggest significant quantities of iron-60 were present when the meteorites’ parent asteroids formed.
There aren’t many ways the universe can manufacture large amounts of iron-60. It’s known to be produced when massive stars die in a catastrophic explosion called a supernova.
So the Carnegie team’s findings hint at one or more supernovae detonating in close proximity to the Sun. That might have happened when the Sun had already formed, injecting an existing disc with iron-60.
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But if it happened before the Sun had formed then the shockwave(s) would have compressed the cloud, tipping it over the edge of the Jeans instability and setting it on a sure path to create our nearest star.
What this means is that we are all connected. The universe is one big recycling station: yesterday’s stars churned into tomorrow’s planets. It may also explain why this solar system ended up with life in it.
Only the heaviest stars are able to go supernova. For the very last part of their lives there’s a frenzy of element-building activity in their cores. Carbon becomes nitrogen, which morphs into oxygen. Eventually, silicon is crushed into iron, triggering the supernova as the star buckles under its own weight.
The explosion rockets these elements out into space, where they mix with surrounding material to form giant interstellar clouds – nebulae. The oxygen you are breathing and the iron in your red blood cells, which are currently ferrying that oxygen around your body, were made inside stars.
The solar nebula was peppered with shrapnel from stellar explosions, enriching it with those heavy elements. Eventually, some of it ended up as part of the Earth when material in the Sun’s protoplanetary disc came together under gravity.
Soon after our planet formed, some as-yet-unknown process turned a soup of those elements into the first life forms. Without supernovae we simply wouldn’t be here. No carbon, no oxygen, no iron.
As the American astronomer Carl Sagan once said, ‘We are all made of starstuff.’ It’s the circle of life played out on the grandest of stages. The death of one star leads to the birth of another.
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