Chapter 2
Our Solar System and Earth

Ever since the Big Bang, the Universe has been drifting and expanding. The birth and death of stars leave an aftermath of galaxies, planets, and even living organisms.

Watch the Earth transform from a violent, molten rock to a supporter of life. Discover how astronomers use collective learning to put our planet in its proper place. And learn about Earth's drifting surface that causes earthquakes, volcanic eruptions, and continental "surfing."

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Chapter at a Glance
43 Minutes
1 Threshold
3 Videos
2 Galleries

The Birth of the Sun

A New Day Begins

It was five billion years ago. A giant cloud of matter in our own galaxy, the Milky Way, condensed under its gravity, exploding in nuclear fusion.

This fusion released what we call sunshine. Very, very, very hot sunshine. And the newly formed star was our Sun. It drew in most of the surrounding matter, but some escaped. And some of this material clumped together, settling into a protoplanetary orbit.

Tasty morsels of gas and rock

Those chemically rich leftovers orbiting our young Sun were stewing with all the ingredients to form the planets in our Solar System.

The intense heat of the young Sun drove away most of the lighter hydrogen and helium elements — 99% of the leftovers — the furthest. These eventually condensed to form the gassy outer giants — Jupiter, Saturn, Uranus, and Neptune. The tiny bit of heavier elements that remained made up the rockier Mercury, Venus, Earth, and Mars.

Through a combination of gentle collisions and gravity these atoms and molecules began attracting other like-sized material. Over millions of years, they gradually shaped themselves into solid planetesimals, and later protoplanets with their own unique orbits.

Astronomers call all this smashing and joining together accretion. After 10 to 100 million years of this banging, eight spherical, stable planets remained. Our Solar System spun into place.


The lifecycle of our Sun

See how our Sun and stars will evolve

Age of star (billions)

Our Solar System

How Did the Planets Form?

The cosmic creation of our Solar System

New elements, combined with the just-right Goldilocks Conditions came together and formed our Solar System.

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Join John Green and Crash Course Big History as they say goodbye to Pluto to see the formation of the eight planets and Sun in our Solar System.

The Rock We Call Home

What Did the Young Earth Look Like

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Though Earth was neatly orbiting the Sun as a rocky mass four and a half billion years ago, no organism could survive there. Radiation from the recent supernova kept the planet extremely hot, its surface molten, and oxygen was non-existent. Plus, incredibly massive meteorites and asteroids frequently slammed onto the surface — creating even more heat.

The Earth got so hot, it began melting. Heavier material sank to the bottom, lighter stuff rose to the top. Some elements evaporated. This transformation created the Earth's layered core and mantle, crust, and atmosphere.

Even today the Earth undergoes constant change. Shifting, sliding, and colliding tectonic plates "surf" atop its semi-molten mantle. This relentless drifting speeds along at the rate of fingernail growth, yet causes mountains to rise, volcanoes to erupt, and earthquakes to strike.

Finding Earth

Letting the Sun take center stage

It took billions of years for the Earth to form and settle into orbit around the Sun. But how do we know that? What makes it so? These questions burned and plagued astronomers for millennia.

To study the movements of heavens back then, you would look up into the sky. You would see the Sun and stars revolve around the very spot where you were standing, the Earth — just as Ptolemy did some 1,900 years ago. This geocentric view, backed by the very powerful religions at the time, endured for more than 1,400 years until it was toppled by Copernicus and confirmed by Galileo. Through their observational evidence, and by using the newly invented telescope, they produced data and logic supporting a Sun-centered, heliocentric model of the Solar System.

Through these revolutionary findings, geocentrism began to crumble. In the later 1600s, Newton developed his three basic laws of motion and the theory of universal gravity by combining physics, mathematics, and astronomy. These ideas laid the foundation for our current understanding of the Earth and the cosmos, and helped astronomer Edwin Hubble construct the modern-day Big Bang theory.

The universal view

The geocentric view of the cosmos held by Aristotle and Ptolemy persisted for more than 1,400 years.


Astronomers See the Light

The Speed of Light

Light travels fast. In one second it races around the Earth seven times. Then in a blink of an eye, light reaches the Moon.

Going out to the stars, Astronomers know that by studying Cepheid variables, the fluctuation in brightness of certain stars, we can calculate the star's distance from Earth. The longer the period of fluctuation, the brighter the star. So even though a star might appear extremely dim, if it had a long period it must actually be extremely large. The star appeared dim only because it was extremely far away. By calculating how bright it appeared from Earth and comparing this to its intrinsic brightness, Astronomers could estimate how much of the star's light had been lost while reaching Earth, and how far away the star actually was.

Touching the edge of the Universe

In the scale of the Universe, light would take eight minutes to reach the Sun. And four years to reach Proxima Centauri, the next nearest star. But could light ever cross the entire Universe? Or might it still have a long way to go? Nobody knows for sure.

The Biosphere

Out With the Bad, in With the Good

Different elements joining, colliding, breaking apart, and joining again is a very ferocious stage in the life of any planet. Even after the Earth formed, when the atmosphere began to stabilize, it was under siege. Early microbes, in their struggle for life, clashed with and consumed hydrogen gas. Hundreds of millions of years passed. These microbes evolved into prokaryotes and adapted further, finding energy in sunlight. Then, in a process called photosynthesis, they flooded the atmosphere with oxygen.

The rise of oxygen formed a protective layer around the Earth and also helped cool the Earth, eventually encasing the planet with ice in a series of "Snowball Earths" 2.4 to 2.2 billion years ago. Some life forms survived, some proliferated, pushing oxygen levels higher. This enabled a greater diversity of life.

Naming the biosphere

Combining "bio," meaning life, and "sphere," referencing the Earth's rounded surface, English-Austrian Geologist Eduard Suess coins the term that expressed the portion of the Earth that supports life.

Suess invented the word because he felt it was important to try to understand life as a whole rather than singling out particular organisms. He believed "biosphere" combines an understanding of the distinct layers that make up the Earth, its atmosphere, and an awareness of all life on our planet and relationships surrounding us.

Meet the young Earth

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As knowledge of life on Earth evolves, thinking about it as a biosphere helps explain the entire intertwined network of life. Here's an early look at how the Earth warmed, cooled, and built its biosphere over time.


Goldilocks Conditions

Not too hot... Not too cold... Where in our Solar System are the conditions just right to support life?

  • Sun

    Even the coolest sunspots on the surface of the Sun are 5,500 °C. So, yes, way too hot.

  • Mercury

    With no real atmosphere to retain heat, the temperature is a freezing -180 °C at night to a scorching oven of 430 °C during the day.

  • Venus

    Because of a dense atmosphere (over 96% carbon dioxide) it's a runaway greenhouse effect. At 480 °C, actually makes it the hottest planet in the Solar System.

  • Earth

    Our planet contains just the right amount of energy and water to support a diverse variety of life.

  • Mars

    Even though Mars reaches a temperate 20 °C at noon at the equator in summer, it's usually a frozen, arid world. The poles are way too cold to support humans — around -153 °C.

  • Jupiter

    This is a "gas giant" — nothing more than a giant ball of hydrogen, helium, and other gases with little solid surface, with an average temperature of -148 °C.

  • Saturn

    Saturn is too cold and gassy. Life-supporting planets usually posses a heavy-metal core surrounded by a rocky mantle.

  • Uranus

    The surface of Uranus is mostly composed of ices: methane, water, and ammonia. This -216 °C hydrogen and helium atmosphere isn't hospitable.

  • Neptune

    The only energy is lightning, ultraviolet light, and charged particles. Although it's the kind of environment in which scientists believe life began, it's not viable today.

  • Pluto

    Not only does liquid freeze solid on this dwarf planet, but even gases, like methane, will harden when Pluto is at its most distant, 5.9 billion kilometers from the Sun.

Tectonic Plates

The Massive Supercontinent Breaks

The Earth's tectonic plates

Even as the Universe drifts, the Earth's surface is in continual motion — moving a little more than two centimeters per year, floating on a semi-molten bed of lava.

Along the edges where the continental and oceanic crust plates meet, all sorts of crazy things happen. These massive plates scrape past each other sideways. They dive under each other. And in places, they get snagged, causing tremendous pressures to build. When this tension suddenly releases things happen much, much faster than two centimeters per year.

But how do we know that the Earth's surface is moving? Some of the early scholars studying the first world maps began to notice some very odd things — for instance, that West Africa seems to fit nicely into Brazil.

In the early 20th century, a German meteorologist named Alfred Wegener began assembling evidence suggesting that the continents were once connected. He found very similar geological strata in West Africa and in Brazil. And during World War I, he wrote a book arguing that at one time all the continents on Earth had been united in a single supercontinent that he called Pangaea.

Why we're all Lava Surfers

Why we're all Lava Surfers

Journey with our Big Historian team on assignment in Iceland, a land of fire and ice, as they walk upon the spot the North American and Eurasian plates collide.

Proving Continental Drift

A Case for Pangaea


While other scientists put forth the theory that the Earth's landmasses had once been connected by land bridges that had since sunk into the ocean, and had always been located where they are today, a few renegade scientists postulated that the Earth once contained one huge supercontinent. In 1858, Austrian geologist Eduard Suess postulated a supercontinent called Gondwanaland, and American astronomer William Henry Pickering suggested in 1907 that the continents broke up when the Moon was separated from the Earth.

These theories found near-hostile scorn in the scientific community. So did a theory of a meteorologist named Alfred Wegener. He regarded the Earth as fundamentally dynamic. He believed the great continent, eventually named Pangaea, had broken apart due to continental drifting.

Together, decades apart, they proved it

Alfred Wegener

Alfred Wegener (1880 — 1930)

Alfred Wegener was not the first to present continental drifting, but he was the first to put together extensive evidence from several different scientific approaches. Submitting fossil evidence of tropical life on Arctic islands to matching geographical features and formations on separate continents, he argued against transcontinental land bridge claims. He also disputed the theory that mountains formed like wrinkles on the skin of a drying apple, proposing instead that they were created by continents drifting.

But he was unable to explain what force could be immense enough to cause continents to plow through the Earth's crust. Wegener would eventually perish during a ski journey on the Greenland ice cap conducting his scientific research.

Harry Hess

Harry Hess (1906 — 1969)

During World War II, Harry Hess was placed in command of an attack transport ship in the Pacific Ocean. His ship was using a new sonar technology that emitted underwater sound waves to detect enemy submarines. But, driven by his own scientific curiosity even during wartime, he kept the sonar turned on to read the topography of the ocean bottom.

Using his own data along with newer research from the Atlantic, Hess postulated that the ocean floors were growing through the process he called seafloor spreading. Further research along the Mid-Atlantic Rift in the 1960s confirmed Hess's theory — it was discovered that rocks closest to the rift are newer than those farther away. The Earth's crust was now shown to be growing and spreading apart along the rift.

Quiz: Threshold 4

Earth & Solar System

  • Ingredients
  • Goldilocks Conditions
  • New Complexity
  • Which of the following is not a necessary ingredient in planetary formation?

  • Which process best describes how new planets are planets?

  • Why are planets considered to be more complex than stars?


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