Chapter 12: How the Sun and Earth Were Born

Night 137: A Call Home on a Rest Day

Mission day 137 was a rest day. On the station, rest days come about once a week, and they are not completely free. In the morning, Noor still did her exercise, because bones and muscles do not take a day off from weakening in weightlessness. She also cleaned her sleeping cabin and wiped the surfaces near the kitchen area, where small drops of food and water like to hide. But after lunch, the schedule from the ground was empty, and the afternoon belonged to her.

Rest days on the station have their own quiet habits. Tomás played music from his tablet in the main module, not loudly, and hummed along when he thought nobody was listening. Ade read a long book on his screen and floated upside down relative to everyone else, because he said it helped him think. Nobody minded. In weightlessness there is no real up or down, only the direction that the crew has agreed to call the floor, so that labels and switches are easy to find.

Noor spent part of the afternoon at the large window with a camera. The station was passing over a wide ocean. From about 400 kilometres up, the water was a deep, dark blue, and the clouds lay on it in long white lines and curls. Some clouds were thin and soft, like spilled milk. Others stood up in tall towers with flat tops. At the edge of the planet, a thin band of pale blue air separated the Earth from the black of space. Noor took picture after picture. Scientists use such photographs, but she also took them simply because she could not stop.

In the late afternoon, there was a video call with home. Sami was there, on his day off, sitting at their mother's kitchen table. Their mother held the screen too close to her face, as she always did, and asked whether Noor was eating enough. Sami asked about the view. Noor turned the camera to the window so that they could see the ocean and the clouds sliding past. For a moment, nobody said anything.

Then their mother asked a question that sounded simple. "Was the Earth always there? Was it made at the same time as everything else?" Sami laughed and said that Noor would need a whole night to answer that. He was right. After the call, Noor floated by the window for a long time. The planet below her looked so finished and so calm. Yet it had a birth, and that birth was not gentle at all.

So tonight's story is about how the Sun and the Earth were born. On the previous night, we saw how stars make elements and scatter them into space. Tonight, some of those scattered elements come together again. They gather into a new star, our Sun, and into the planets that go around it, including the blue world under Noor's window.

A Cloud That Fell Inward

About 4.6 billion years ago, the place where our solar system is now was part of a large cloud of gas and dust in the Milky Way. By then, the universe was already more than nine billion years old. Many generations of stars had lived and died, and their elements had been mixed into the clouds between the stars. The cloud that would become our solar system was mostly hydrogen and helium, but it also contained small amounts of carbon, oxygen, silicon, iron and many other elements made by earlier stars.

Clouds like this can stay quiet for a long time. Then something makes a part of the cloud start to fall inwards under its own gravity. What started the collapse in our case is not known for certain. One idea is that a nearby exploding star pushed on the cloud with its blast and squeezed part of it. Some elements found in old rocks fit with this idea, but other explanations are possible, and scientists are still discussing it. What is clear is that a part of the cloud began to collapse.

As the gas fell inwards, it began to spin faster. You may have seen a skater pull in her arms and spin faster on the ice. The same thing happens to a spinning cloud as it gets smaller. The spinning cloud also flattened into a disc, the way a lump of dough flattens when a baker spins it in the air. Most of the material fell into the centre of the disc. There, the pressure and heat grew and grew, until the centre became hot enough for fusion to begin. A new star was born. That star was the Sun.

The Sun took almost all of the material. Today, more than 99 percent of the mass of the whole solar system is in the Sun. Everything else, all the planets, moons, asteroids and comets together, is only a small remainder. If the solar system were a bakery's whole day of baking, the Sun would be nearly all the bread, and the planets would be a few crumbs left on the table. But as we know, crumbs can matter a great deal, because one of them is our home.

Today, astronomers can see this process happening in other places. With large telescopes, they find young stars surrounded by flat discs of gas and dust. In some of these discs, there are dark rings and gaps, which may be places where young planets are clearing a path as they grow. Looking at these discs is a little like looking at old family photographs of other people's children to understand what you yourself looked like as a baby. We cannot go back and watch our own birth, but we can watch the births of others.

Reading the Oldest Stones

How do we know when the solar system was born? Nobody was there with a calendar. The answer comes from stones, and in particular from stones that fall from the sky. Some meteorites are pieces of material that formed in the disc around the young Sun and never became part of a planet. They have stayed much as they were since the beginning of the solar system, like letters kept in a drawer for billions of years.

Some elements in rocks are radioactive. This means that their atoms slowly change into other atoms at a steady rate that does not depend on heat or cold. Scientists can measure how much of the original atoms are left and how much of the new atoms have formed. From this, they can work out how long ago the rock became solid. It is a natural clock, a little like an hourglass whose sand runs at the same speed whatever happens around it.

In the middle of the twentieth century, scientists used clocks of this kind on meteorites and found an age of about 4.5 billion years. Later measurements became more and more exact. Today, the oldest solid pieces found in meteorites are about 4.6 billion years old. They are the oldest solid material we can hold in our hands. Many museums keep pieces of such meteorites, and some allow visitors to touch them.

On the Earth itself, it is harder to find rocks from the very beginning. The young Earth was hot, and its surface has been melted, broken, worn away and recycled many times since. So the oldest rocks on Earth are younger than the oldest meteorites. But tiny, very hard mineral grains have survived from a time not long after the Earth formed, and they tell us that the Earth already had a solid crust quite early. Different kinds of evidence, from meteorites, from Earth rocks and from rocks brought back from the Moon, all point to the same age for the solar system.

Planets From the Disc

While the Sun was forming in the centre, the rest of the disc was busy too. Near the young Sun, it was hot. Only materials that can stand great heat, such as rock and metal, could become solid there. Farther out, it was cold enough for water and other substances to freeze into ice. This difference helps to explain why the planets near the Sun, Mercury, Venus, Earth and Mars, are small and rocky, while the planets farther out, Jupiter, Saturn, Uranus and Neptune, are much larger and contain a lot of gas and ice.

In the disc, tiny grains of dust stuck together, a little like dust gathering under a bed. The clumps grew into pebbles, the pebbles into rocks, and the rocks into bodies many kilometres across. These bodies collided and joined, again and again. In the end, a small number of large bodies cleared their paths and became the planets. The whole process of building the rocky planets probably took tens of millions of years, which is short compared with the age of the solar system.

The giant planets had a race to win. Jupiter and Saturn are made mostly of hydrogen and helium gas, and they could only gather that gas while it was still in the disc. Some of the gas fell into the young Sun, and the rest was slowly driven away by the young Sun's strong light and the particles it blew outwards. Within a few million years, most of the gas in the disc was gone. So the giant planets must have grown quickly, first building solid cores and then pulling in huge amounts of gas before it disappeared. The rocky planets near the Sun did not need gas, and they could take their time.

The young Earth did not have a calm life. It was hit again and again by smaller bodies, and its surface was hot and often melted. Heavy metals such as iron sank towards the centre and formed the Earth's core. Lighter rock floated above it. Gases escaped from the hot rock, and slowly an early atmosphere formed. Over time, the surface cooled, and water collected into the first oceans.

Not every piece of the disc became part of a planet. Many rocky leftovers still go around the Sun in a wide belt between Mars and Jupiter, and these are called asteroids. Farther out, beyond Neptune, there are many icy leftovers, and some of them, when their paths bring them close to the Sun, become comets with long glowing tails. These leftovers are precious to scientists, because they have changed less than the planets. Space probes have visited several asteroids and comets, and some have even brought small samples back to Earth, so that the oldest material of the solar system can be studied in laboratories.

Where the Earth's water came from is still being discussed. Some of it may have been in the rock from which the Earth was built. Some of it may have been brought later by icy bodies from farther out in the solar system, such as certain asteroids or comets. Scientists compare the kinds of water found in comets, asteroids and the Earth's oceans to find out which sources fit best, but the full answer is not yet known. So the next time you drink a glass of water, you are drinking something whose exact origin is still a small mystery.

The Moon and the Sun's Family History

The Moon also had a dramatic birth. The leading idea today is that, early in the Earth's life, another body roughly the size of Mars crashed into the young Earth. The collision threw a huge amount of hot material into space around the Earth. Some of that material gathered together and became the Moon. This idea explains several facts, for example why the Moon has a small iron core and why its rocks are similar in many ways to the rocks of the Earth. But the details are still being studied, and scientists continue to test other versions of the idea.

The young Moon was much closer to the Earth than it is today. Since its birth, it has been slowly moving away. Scientists can measure this very exactly by bouncing laser light off reflectors that were left on the Moon's surface and timing how long the light takes to come back. Today, the Moon moves away from the Earth by about four centimetres a year, roughly the speed at which your fingernails grow. Over billions of years, those small steps have added up to a great distance.

Now we can answer the question that Noor's mother asked. No, the Earth was not made at the same time as everything else. The universe is about 13.8 billion years old. The Sun and the Earth are about 4.6 billion years old. That means the universe had already existed for about two-thirds of its present age before our solar system began. Many stars had come and gone before the Sun was born.

In fact, the Sun carries the proof of this inside it. When astronomers study the spectrum of sunlight, they find not only hydrogen and helium but also small amounts of heavier elements, such as carbon, oxygen and iron. The very first stars could not have contained those elements, because the first minutes of the universe made almost none of them. So the Sun must have formed from material that had already passed through earlier stars. The Sun is not a first child of the universe. It is a grandchild, or perhaps a great-great-grandchild, of many earlier generations.

This is one of the quiet lessons of tonight's story. The Earth is not ancient in the way the universe is ancient. It arrived late, built from what earlier stars left behind. Everything under Noor's window, the oceans, the clouds, the rocks and the living things, depends on a long history of stars that worked and ended long before there was anyone to see them.

On this young, changing Earth, life eventually began. How and when exactly that happened is still one of the great open questions of science. The oldest widely accepted signs of life that scientists have found are about three and a half billion years old, and some possible signs may be older, although the earliest signs are small and difficult to read, and their exact age is still discussed. What seems clear is that life appeared on the Earth fairly early in its history, once there were oceans and a cooler surface. From those first simple living things, a very long story led to the clouds of plankton in the oceans, the forests on the land and, much later, to the people looking down from the station's window.

After the Call

When the station passed into the Earth's shadow, the ocean below disappeared into darkness, and the stars appeared above the thin line of air. Noor put the camera away and wrote to Sami and their mother together.

"The Earth is younger than the universe," she wrote. "About 4.6 billion years old, while the universe is about 13.8 billion. The Sun and the Earth were made from a spinning cloud full of dust from older stars. The Moon was probably made when something big hit the young Earth. So, Mum, the Earth was not always there. It arrived late, but it arrived well."

After sending the message, Noor stayed by the window a little longer. The next time the station came out of the Earth's shadow, a thin line of orange light spread along the edge of the planet, and the ocean slowly turned blue again. It was one more sunrise in a day full of them. She had stopped counting them, but she had not stopped watching. Then she went to her cabin and climbed into her sleeping bag.

Sami answered in the middle of his shift. "Mum says that sounds like you. Arrived late, but arrived well." Noor laughed quietly in her sleeping bag, so as not to wake Ade in the next cabin.

If you look at the ground under your feet tomorrow, on your way to work or on your way home, you might remember that it was once dust in a spinning disc. It took a long time and many collisions to become a solid place where people can walk, build and rest. Like many things worth having, it was not made in a hurry.

Common Misunderstandings

Many people think that the Sun and the Earth are as old as the universe, as if everything appeared at once at the beginning. It is easy to imagine it that way, because the Sun and the Earth feel like the permanent background of our lives. But the universe is about 13.8 billion years old, and the Sun and the Earth are about 4.6 billion years old. For more than nine billion years, the universe existed without our solar system. Stars were born and died, and galaxies grew, long before the Sun began to shine.

Many people also think that the Sun is one of the first stars. In fact, the very first stars were made almost only of hydrogen and helium, and most of them probably lived short lives and ended long ago. The Sun contains heavier elements that only earlier stars could have made. That is why astronomers say that the Sun belongs to a later generation. It is a young member of a very old family.

Finally, some people think that the Earth has always looked the way it looks today, with blue oceans, white clouds and green land. But the young Earth was very different. For a long time, it was hot, often hit by other bodies and without the oceans and air we know. Its continents have moved, its air has changed and its life has changed many times. The calm blue world that Noor photographs is the result of billions of years of change. On the next night, we will look at a change of a different kind, much bigger and still unexplained: the way the whole universe has begun to grow faster.