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Space · The Sun and its family

How the Solar System was born

It all began with a falling cloud

Only spacecraft can see it

How the Solar System was born
The disk of gas and dust around the newborn star: all the planets come from there.

The numbers

How long ago
4.6billion years
Width of the cloud
100astronomical units
When the Sun lit up
100million years later

How it happens, step by step

  1. The cloud falls

    A cold cloud of gas and dust gets squeezed, maybe by the blast of a star that exploded nearby, and starts falling toward its center.

  2. It spins faster

    The tighter it gets, the faster it spins, and as it tightens it heats up. The crowding matter has nowhere to go.

  3. It flattens out

    The spinning flattens the cloud into a thin disk, with a ball of gas at the center that gets hotter and denser.

  4. The grains stick

    In the disk, dust bumps together and sticks. The clumps grow, and the biggest pull in their neighbors with gravity.

  5. The center lights up

    When pressure and density at the center are high enough, fusion begins: the Sun is born, with the worlds already formed around it.

To get an idea

The planets grew like a snowball: grains that become pebbles, pebbles that become boulders.

Inside the frost line it was too hot for ice to last: that is why the worlds there are small and rocky.

Fun facts

These are not in the book: there was no room on paper.

  1. The Sun lit up about one hundred million years after the collapse began.

  2. Beyond the frost line water stays frozen, and there the planets were able to grow huge.

  3. Uranus and Neptune captured less gas than Jupiter and Saturn and formed around cores of ice.

  4. The rocky planets lost their first atmosphere: the one they have today comes from volcanoes and impacts.

  5. Between Mars and Jupiter the pieces never managed to join into a planet, stirred up by Jupiter's gravity.

  6. The oldest rocks ever dated say at least four and a half billion years: that is the lower limit.

Test yourself

The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.

Question 1 of 5★ is the one from your book

What did everything come from?

Question 2 of 5

Why did the cloud start spinning faster?

Question 3 of 5

How did the planets grow?

Question 4 of 5

Why are the inner planets rocky?

Question 5 of 5

Who first proposed this explanation?

Riddles

Before me there was nothing to look at: only a cold, wide, still cloud. Then someone pushed it, it fell in on itself and started to spin. What am I?

I began with one grain of dust sticking to another grain, and I kept going like that for millions of years until I became a world. What am I?

At my center a star lit up, and everything around stayed in orbit around it: rocks inside, gas giants outside. What am I?

Try it yourself

Activity 1

Sprinkle a handful of flour on a plate and blow on it while turning the plate: the dust settles into a flat ring, like the disk around the Sun.

Activity 2

Roll a small ball of snow or modeling clay over more powder: everything sticks and the ball grows. The planets grew like that, for millions of years.

The words

frost line
The distance beyond which water stays frozen and does not evaporate.
planetesimal
A small body made of dust and pebbles, a building block of future planets.

Sources: Wikidata (Q3535), Wikipedia (Formazione ed evoluzione del sistema solare).