Summer of Science: how the asteroid belt was formed | The young witness


How was the asteroid belt formed? I heard there must be another planet out there, but there doesn’t seem to be enough matter for a planet.


That’s a great question, Jessie. To answer it, we must board a time-traveling spacecraft and return to the birth of the solar system itself.

Let’s go back to about 4.6 billion years ago, just before the existence of the solar system. Imagine we’re floating in our spacecraft in the Orion Arm of the Milky Way, which is our galactic address – but the Sun and the planets aren’t there yet. Instead, when we look out the window of the spaceship, we see the dark void of space.

But suspended in this void is a gigantic cloud of gas and dust. They are cosmic debris, left by the explosion of a star.

And another star is about to be born before our eyes.

As we watch, the cloud spins, slowly at first and then faster and faster. It’s like an ice skater, who spins slowly when his arms are up, and faster when he retracts his arms. (Try this at home: instead of ice skates, use a spinning desk chair.)

Because we’re in a time-traveling spaceship, we can speed up time a bit and watch what happens.

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Over the years, the swirling cloud of gas and dust begins to flatten into a disc, like a Frisbee. It also heats up to a searing temperature.

Soon the center of the cloud becomes so hot that a star ignites – our newborn Sun.

The Sun is made up of much of the cloud’s gas and dust, but there’s still a lot of it, still spinning rapidly around it.

All those bits of gas and dust start colliding and sticking together. At first, these stuck-together pieces are tiny. Then they become the size of pebbles, then boulders, then boulders, then small quasi-planets – then whole planets.

It’s quite spectacular to watch from our spaceship. We would see these rocky quasi-planets shatter and crash into each other in a series of violent collisions. From a million tiny particles, the planets we know and love today form, from Mercury right next to the Sun to Neptune in the cold regions of the solar system.

But the story is not over.

Even after the birth of the planets, we can see that there are still pieces of rock left – which did not end up on the planets. We call these asteroids. They are made up of rocks and metals like iron and nickel. Some pieces are small, just a few meters in diameter, while others are over 900 kilometers (roughly the distance between Melbourne and Sydney, if you’ve ever made that trip).

They are still orbiting our newborn Sun. Some asteroids are scattered throughout the solar system, but most of them are clustered in a ring between the planets Mars and Jupiter. This is because Jupiter is gigantic and exerts a strong gravitational pull on all other objects in the solar system. It acts as a shepherd and guides all the asteroids into place, in what we now call the asteroid belt.

Astronomers once thought the asteroid belt was a planet broken into small pieces. But then we realized there wasn’t a lot in the asteroid belt.

It’s true that there are big objects out there, like Ceres – which has a third of the mass of the entire asteroid belt! Astronomers call it a dwarf planet.

But if you could glue all the asteroids together, including Ceres, you’d end up with a planet even smaller than the Moon (which is already only a quarter the size of Earth).

So astronomers no longer think the asteroid belt is a broken planet. Instead, they think it formed the way we saw through the window of our time-traveling spacecraft.


Now, let’s step back in time and back to the present day – it’s time to disembark our spaceship.

Our journey through the asteroid belt must not be over. From here on Earth, here and now, you can look at the night sky through binoculars or a telescope and spot big asteroids like Pallas, Vesta and Juno.

If you don’t know where to look, go online and check out the Melbourne Planetarium Celestial notes. The sky is constantly changing, so each month they’ll tell you cool things to spot, from asteroids to planets to constellations. You can also see comets and meteors, which are left over from the birth of the solar system, just like asteroids.

Happy asteroid watching, Jessie, and keep asking questions!

  • This article is published in partnership with Cosmos magazine. Cosmos is produced by the Royal Institution of Australia.

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