This story is a continuation of my previous ones, Putting More Truth in the First Great Lesson, and Telling the Story of the Sun and Solar System.

Note: There are good illustrations for the events I describe below in the book, Planet Earth: A 4.5-Billion-Year Story by Anthea Lacchia, published by DK in 2026.

Our Earth is a rocky planet. Here is how astronomers think it formed. In the solar disk, the rocky particles, metals, and gases rotating around the Sun bumped into each other and stuck together. They formed large clumps, and then bigger clumps until they were chunks called planetesimals, which were about a kilometer across. The process of building planetesimals is called accretion. To accrete means to grow by adding material to the outside. The force that makes accretion happen in space is gravity.

Accretion is very different from the way living things grow. We grow from the inside. Planets grow from the outside. If you have ever rolled a snowball around in a shallow layer of snow to make a snowman, you have made a bigger ball using accretion.

When the planetesimals grew to about a kilometer in diameter, their gravity was strong enough to attract even more material. Larger masses exert a stronger gravitational pull than smaller ones. The planetesimals came together and formed a few hundred protoplanets that were about the size of the Moon or smaller. Then these collided with one another, and finally enough material stuck together to form planets.

To be considered a planet, a celestial body must be large enough that its gravity can pull it into a sphere. It also must orbit the Sun (or another star), and its gravity must be strong enough to gather up any other large objects left in its orbit. (The dwarf planets – Pluto, Ceres, Erdis, Makemake, and Haumea – are considered to be protoplanets by some astronomers.)

There were many collisions in the early Solar System. As planets form, the force of the collisions heats them up so much that they are molten. The heaver elements like metals sink to the center of the sphere. The lighter rocky ones move to the outer part. It is hard to think of rocks as floating, but that is what they did. They floated on the metal core.  (See demonstration below for layering liquids by density.) (Show a diagram of the Earth’s interior. https://www.usgs.gov/media/images/earth-cross-section)

In the outer core of the Earth, molten iron and nickel move around and generate a magnetic field. This gives us magnetic north and south poles, but it does something much more important. Remember how the early Sun spewed out a solar wind? It still does, and this wind is hot and made of charged particles traveling at very high speed. If the Earth didn’t have a magnetic field, the solar wind would sweep away our atmosphere. Life would not be possible on the Earth’s surface.

(Show a diagram of the Earth and the solar wind. See https://en.wikipedia.org/wiki/Habitable_zone#/media/File:Magnetosphere_rendition.jpg.)

Soon after the Earth formed (if you can call 50 million years soon), it was struck by another planet about the size of Mars. Astronomers have named that planet Theia. In Greek mythology, Theia was the mother of the goddess of the Moon. Theia’s collision dealt the Earth a giant blow; it remelted the outer layers of both planets and flung a huge volume of debris out into space. Some of that debris fell back to Earth, but some was flung farther away. It came together and formed our Moon. The collision caused the Earth’s axis to tilt, which is what gives us our seasons.

NASA has a dramatic simulation of the Moon’s formation that was done using a supercomputer. See https://science.nasa.gov/moon/formation/.

Our Moon is unusual. Compared to the other moons in the Solar System, it has the largest diameter compared to its planet. It is just over ¼ of Earth’s diameter. Other moons are a much smaller fraction of their planets’ size. The largest moon in the Solar System is Ganymede, and it would take 27 Ganymedes to equal the diameter of the planet it orbits, Jupiter.  

When it first formed, the Moon was closer to the Earth. We have evidence that the Moon is slowly moving away from the Earth. When the Apollo astronauts visited the Moon over 50 years ago, they left a mirror on its surface. Astronomers on Earth can aim a laser beam at it and measure the time it takes for light to reach the Moon and be reflected back to Earth. They found that the Moon is moving away at 3.8 cm (1.5 in.) per year. If you are 8 years old, the Moon has moved about a foot (30 cm) farther away since you were born.

The Moon must have looked much larger when it first formed. Astronomers estimate that 3.9 billion years ago, the Moon looked nearly three times as large as it does today. The Earth was rotating faster, so its days were shorter. Gradually, the Moon moved farther away, and the Earth’s rotation slowed, but this didn’t happen quickly. The Earth and Moon became locked together in their orbits so that the same side of the Moon always faces the Earth.

When the Earth first formed and got its Moon, astronomers estimate the day length was about 6 hours. At the beginning of the Cambrian Period, nearly 4 billion years later, a day was about 21 hours long. Scientists studied the microscopic layers in seashell fossils and layering in sedimentary rocks to produce estimates of Cambrian day length.

Our Earth has many characteristics that make it a place where life can exist. It is the right distance from the Sun to have liquid water. All life requires liquid water.

Where did Earth’s water come from? Most of it was locked up in the rocks that formed the Earth. The early Earth had many volcanos, and they brought water vapor from the mantle to the atmosphere. Some of our water came from comets and asteroids. The Earth had oceans very early, before the first signs of life, which have been found in rocks about 3.8 billion years old.

The oceans formed when the crust of our planet cooled and solidified, which blocked most of the heat from the inner layers. After that, the atmosphere cooled enough to allow liquid water to form. It is hard to imagine, but there were thousands of years of rain. Water gradually filled the low areas of Earth’s crust and formed oceans. The first oceans didn’t look like today’s. They were green because they had a lot of dissolved iron, and there was very little oxygen in the atmosphere to rust it. Early oceans were hot and acidic but not salty, quite unlike today’s oceans.

Can you imagine standing on the shore of that first ocean? If your time machine could take you there, you would have to wear a spacesuit and an oxygen tank. Above the hot, green oceans, you would see an orange sky. The atmosphere was thick, so thick that the oceans were hotter than our current temperature of boiling water.

(This may be a hard concept for children to understand, but even today, there is a difference between the temperature of boiling water at sea level and on a high mountain. Atmospheric pressure decreases with higher altitude, and fewer molecules of air above the mountain means that water boils at a lower temperature. My personal experience – a “3-minute” boiled egg takes about 9 minutes at an altitude of 8000 feet. A pressure cooker is another example. The increased pressure inside it allows liquids to be heated to a temperature above the usual boiling point.)

With the oceans present, life could emerge, and it did, even though there was little or no oxygen in the atmosphere. We will look at the history of life on Earth in our future studies.     

(Note: older stories described how the first raindrops would have evaporated like water hitting a hot skillet. If the surface of the Earth was still that hot, the atmosphere would probably have been too hot for rain to form.)

References and resources:

Natural History Museum of London’s webpage on Moon formation. https://www.nhm.ac.uk/discover/how-did-the-moon-form.html  

Lunar and Science Exploration. https://www.lpi.usra.edu/exploration/training/illustrations/earthMoon/  

NOAA Ocean Exploration. https://oceanexplorer.noaa.gov/explainers/intro/

The Simple English Wikipedia article on the Moon. https://simple.wikipedia.org/wiki/Moon

Demo for planet differentiation: You can demonstrate the layering of liquids with different densities. The American Chemical Society has this activity: https://www.acs.org/education/activities/three-layer-float.html. You can find many other examples online. Use a tall glass, beaker, or test tube. Add dark corn syrup to make a layer in the bottom. Slowly add water and then salad oil. Children should be able to see the layers clearly.