Early theories
People have guessed about the Earth's birth for centuries. A few important ideas:
- Nebular hypothesis (Immanuel Kant, then Pierre Laplace, 1796): a slowly rotating cloud of gas (a nebula) around the young Sun cooled and shrank. Rings of matter left behind became planets.
- Planetesimal idea (Chamberlain and Moulton, 1900): a passing star pulled material out of the Sun; small bodies called planetesimals joined to make planets.
- Tidal / binary star ideas (Jeans and Jeffrey): a companion or passing star played a role.
- Revised nebular hypothesis (Otto Schmidt, Carl Weizsäcker, about 1950): the Sun was surrounded by a nebula of hydrogen, helium and dust; friction and collisions made a flat disc and planets grew from it by accretion (sticking together).
The revised nebular idea is close to what scientists accept today for the solar system.
Modern theory: the Big Bang
The Big Bang theory (also called the expanding universe hypothesis) explains the whole universe, not just the Earth.
- At the start, all matter was packed in one tiny, extremely hot and dense point (a singularity).
- About 13.7 billion years ago it expanded violently. In the first moments, simple atoms began to form.
- Within about 300,000 years it cooled enough for atoms to form and the universe became transparent.
- It is still expanding. In 1920 Edwin Hubble showed that galaxies are moving away from each other.
Think of dots drawn on a balloon: when you blow it up, every dot moves away from every other dot. The space between them grows.
Some scientists once supported a steady state idea (the universe always looked the same), but growing evidence supports expansion.
Stars and planets
Stars: the early universe had uneven gas. Where gas was thicker, gravity pulled it together into clumps. These became galaxies (huge groups of stars). Inside a galaxy, smaller clumps became hot enough to shine: these are stars. Galaxies began forming around 5–6 billion years before our Sun (the Sun is about 5 billion years old).
Planets: formed in three stages: (1) a star forms inside the gas cloud; (2) the leftover disc of gas and dust breaks into small lumps that stick together into planetesimals; (3) planetesimals collide and join to make a few big planets.
| Inner (terrestrial) planets | Outer (Jovian / gas giant) planets |
|---|---|
| Mercury, Venus, Earth, Mars | Jupiter, Saturn, Uranus, Neptune |
| Small, rocky, dense | Huge, mostly gas, less dense |
| Close to the Sun: too hot for gases to stay | Far away: cold, so gases condensed |
| Weak gravity; solar wind blew gas away | Strong gravity held thick gas |
The Moon: scientists think a body about the size of Mars (or larger) hit the young Earth. The splashed-out material came together to form the Moon, about 4.44 billion years ago. This is the giant impact idea.
Evolution of the Earth: lithosphere, atmosphere, hydrosphere
Lithosphere (the solid Earth)
The young Earth was hot and partly molten, mostly because of collisions and radioactive heat. Heavy materials like iron sank to the centre; lighter materials rose to the surface. This sorting is called differentiation. It made layers: crust, mantle, outer core and inner core. As the surface cooled, a solid crust formed.
Atmosphere (air) in three stages
- The first air (hydrogen and helium) was blown away by the solar wind.
- Gases escaped from the hot interior through volcanoes: water vapour, nitrogen, carbon dioxide, methane, ammonia. This is degassing.
- Much later, living things doing photosynthesis added oxygen. By about 2.5–3 billion years ago oxygen began to fill the air.
Hydrosphere (water)
As the Earth cooled, water vapour condensed into clouds. Rain fell for a very long time, collecting in low places. Carbon dioxide dissolved in rainwater, cooling the planet further, so more rain fell. The oceans formed within about 500 million years of the Earth's birth, so they are about 4 billion years old.
Try it at home: hold a cold steel plate over steam from a kettle (with an adult). Watch vapour turn to drops. This is how the first rain formed.
Origin of life
Scientists think life began through chemistry. In the early oceans, simple molecules joined into complex ones (like amino acids), then into molecules that could copy themselves. These became the first simple cells.
- Life began about 3.8 billion years ago in water.
- Fossils of blue-green algae (in layered rocks called stromatolites) are about 3 billion years old or more.
- These algae added oxygen, which later allowed complex life.
- Plants and animals spread onto land in the last 500 million years; humans came very recently.
| Time ago | Event |
|---|---|
| 4.6 billion years | Earth forms |
| ~4 billion years | Crust hardens, oceans form |
| ~3.8 billion years | First simple life in the sea |
| ~2.5 billion years | Oxygen builds up in the air |
| ~0.5 billion years | Life spreads on land |
If the Earth's history were one day, humans would appear only in the last few seconds.
Key formulas and definitions
- Age of the universe ≈ 13.7 billion years; age of the Earth ≈ 4.6 billion years
- Nebular hypothesis: Kant, Laplace (1796); revised by Otto Schmidt and Carl Weizsäcker
- Planetesimal idea: Chamberlain and Moulton (1900)
- Big Bang = expanding universe; Edwin Hubble (1920) showed galaxies move apart
- Planet formation: star forms → dust makes planetesimals → planetesimals join into planets
- Inner planets = rocky (terrestrial); outer planets = gas giants (Jovian)
- Differentiation: heavy iron sinks to core, light rock rises to crust
- Atmosphere: primordial gas lost → degassing → photosynthesis adds oxygen
- Moon: giant impact, about 4.44 billion years ago
Worked examples
1. Why are the inner planets rocky while the outer planets are gassy?
Near the Sun it was too hot for light gases to condense, and the solar wind blew them away. Far away it was cold, so gases condensed, and the big outer planets had enough gravity to hold them.
2. Explain the balloon example of the Big Bang.
Draw dots on a balloon and blow it up. Every dot moves away from every other dot because the rubber (space) stretches. Galaxies move apart in the same way as the universe expands.
3. List the three stages of the Earth's atmosphere.
1) First hydrogen–helium air lost to solar wind; 2) degassing from the interior and volcanoes; 3) oxygen added by photosynthesis of living things.
4. How did the oceans form?
The cooling Earth's water vapour condensed into clouds. Continuous rain collected in low areas. This happened within about 500 million years of the Earth's birth.
5. If the Earth is 4.6 billion years old and life began 3.8 billion years ago, for how long was there no life?
4.6 − 3.8 = 0.8 billion years, that is about 800 million years.
Common mistakes
- Saying the Big Bang was an explosion in space. It was the expansion of space itself from one point.
- Thinking the Earth formed with oxygen-rich air. Oxygen came much later from living things.
- Mixing up the ages: the universe is about 13.7 billion years old; the Earth is about 4.6 billion years old.
- Thinking all planets are made the same way. Inner ones are rocky, outer ones are giant balls of gas.