How the Solar System Was Born: The Stones Older Than Earth

At 1:05 in the morning, on 8 February 1969, a fireball lit up the sky over northern Mexico.

It burst apart, and thousands of black stones rained down around the village of Pueblito de Allende. Over the years, collectors and scientists gathered more than two tonnes of them.

Inside the stones were small white specks. They turned out to be older than the Earth itself. Older than every planet. They are the closest thing we have to a birth certificate for the solar system.

The Spinning Cloud

Look at the solar system from far above, and something stands out. All eight planets go around the Sun in the same direction. Almost all of them in the same flat plane. Even the Sun spins the same way.

That is not what you would expect if the planets had been caught one by one. It looks as if everything was made together, out of one thing, turning in one direction.

In 1755, a young philosopher in Königsberg, Immanuel Kant, suggested what that thing was. A huge cloud of gas and dust, pulling itself together under its own gravity.

As it shrank, it spun faster, like a skater pulling in her arms, and flattened into a disc. The middle became the Sun. The disc became the planets. Forty years later, Pierre-Simon Laplace worked out a version of his own.

It was a beautiful idea. But for two hundred years, nobody could test it. The cloud was long gone.

The Oldest Rocks

Which brings us back to Allende. The stones fell just months before Apollo eleven, and laboratories all over the world were ready and waiting to study rocks from space.

The white specks are called calcium aluminium-rich inclusions. They are minerals that only form at enormous temperatures, so they probably crystallised in the hottest part of the cloud, close to the young Sun.

Uranium inside them slowly turns into lead, at a known rate, like a clock. Measured carefully, that clock says the specks formed 4,567 million years ago. For planetary scientists, that is time zero.

Around them, the stone is packed with tiny glassy beads called chondrules: drops of molten rock that cooled in minutes, while floating in space. Something kept flash-heating the dust of the young disc. Shock waves, lightning, collisions? Nobody is quite sure.

And in the 1970s, a team at Caltech found traces of something else inside them: aluminium-26, a radioactive kind of aluminium that disappears within a few million years.

So it must have been made just before the specks formed, inside a star. Perhaps a nearby star exploded, seeded the cloud, and even helped push it into collapse. Scientists still debate that part.

Other Stars

Then telescopes got good enough to catch other stars doing it. In the early 1990s, the Hubble Space Telescope looked into the Orion Nebula, and found young stars wrapped in dark discs of dust. Exactly what Kant had imagined.

They were nicknamed proplyds, short for protoplanetary discs. Some of them are being boiled away by the glare of giant stars nearby. Not every young solar system gets to finish.

In 2014, the ALMA telescope array in Chile took the sharpest picture yet, of a star called HL Tauri, about 450 light-years away and less than a million years old.

Its disc is striped with bright rings and dark gaps. The gaps are thought to be lanes, swept clean by young planets as they grow.

These features are almost certainly the result of young planet-like bodies that are being formed in the disk.

And in 2018, astronomers photographed a planet still in the making, around a star called PDS 70. A few years later, they found a disc around one of its planets, which may be building moons of its own.

But how do you get from dust to a planet? Grains of dust stick together, into fluffy clumps, then pebbles. But pebbles are a problem. They should drift into the star, or smash each other apart, long before they grow.

One answer is that the pebbles bunch together in the gas, until a whole cloud of them collapses under its own gravity, all at once, into a body tens of kilometres across.

On New Year's Day, 2019, the New Horizons spacecraft flew past an object 6.5 billion km away, called Arrokoth. It is two lumps, stuck together, that touched at about walking pace, and have barely changed since the solar system was young.

This is a decisive result in favor of one theory: local cloud collapse.

Jupiter First

Further out from the Sun, beyond the frost line, it was cold enough for water to freeze. With ice to build from, the cores of giant planets could grow fast, and pull in great envelopes of gas.

Jupiter seems to have been first. In 2017, a team studying iron meteorites found two families of them that had been kept apart for millions of years. They concluded Jupiter's core had reached about 20 Earth masses within the first million years, and its pull kept the inner and outer solar system apart.

Some scientists think Jupiter then wandered, drifting in towards the Sun and back out again, scattering asteroids and stealing material that might have made Mars much bigger. If that idea is right, it is one reason Mars is so small.

Within a few million years, the young Sun blew the leftover gas away. The giants stopped growing. Closer in, rocky worlds kept colliding for tens of millions of years. One of those collisions, as we saw in our last film, made the Moon.

The Leftovers

We are still collecting the leftovers. In 2023, the OSIRIS-REx mission dropped a capsule into the Utah desert, carrying dust from the asteroid Bennu, a time capsule from the beginning.

The Sun ended up with almost all of the cloud, about 99.8%. Everything else, the planets, the moons, the oceans, and you, was made from what was left over.

The science in this story

Key facts

• The Allende meteorite fell on 8 February 1969 over Chihuahua, Mexico; more than two tonnes were recovered. source ↗
• Calcium-aluminium-rich inclusions in meteorites like Allende are about 4,567 million years old, the oldest solids of the solar system. source ↗
• In 2014 ALMA imaged rings and gaps in the disc around the young star HL Tauri, about 450 light-years away. source ↗
• New Horizons flew past Arrokoth on 1 January 2019; its two lobes merged gently, supporting local cloud collapse. source ↗
• Iron meteorites suggest Jupiter's core reached about 20 Earth masses within the first million years. source ↗

Sources

• Kant (1755), Allgemeine Naturgeschichte und Theorie des Himmels; Laplace (1796), Exposition du système du monde
• Connelly et al. (2012), Science 338: the absolute chronology of the oldest solids
• Lee, Papanastassiou & Wasserburg (1976–77): aluminium-26 in Allende
• O'Dell et al. (1993), ApJ: proplyds in the Orion Nebula
• ALMA Partnership (2014/15): HL Tauri
• ESO eso2111 (2021): the moon-forming disc of PDS 70c
• McKinnon, Stern et al. (2020), Science 367: Arrokoth
• Kruijer et al. (2017), PNAS 114: the age of Jupiter
• Walsh et al. (2011), Nature 475: the Grand Tack (hypothesis)

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