History #8: Solar Systems

This is a continuation of an ongoing series on big history. It follows the structure of the course developed by historian David Christian. This post will explore how our solar system and planet came to be.

Past posts can be found here: The Big Bang, Star Formation

The story so far

(adjusted scale: if all of time was condensed into 13 years)

About 13 years ago, unknown conditions set off the Big Bang. This produced space, time, energy, and matter (mainly hydrogen and helium). The universe was born.

About 36 days later, cooling and tiny variations in the density of matter allowed gravity to bring together hydrogen and helium into high pressure clumps. Stars were born.

High temperatures within those stars, along with supernova explosions, allowed for the synthesis every other element on the periodic table. Chemistry was born.

Our Solar Systems

We now had a universe that consisted of 98% hydrogen and helium, and 2% every other element (thanks to stars). These new elements could mix together in elaborate new ways and form even more complicated structures.

When stars explode, they fertilize the universe with these new elements. Gravity once again takes over and forms new dense clouds of matter. From these clouds, new stars form. Evidence suggests that our sun was one of these second generation stars.

About 4.56 billion years ago, our sun formed. We calculate this based on the age of objects we find that were around at that time – meteorites. When the sun formed, its gravitational pull consumed 99.99% of all the matter available in our solar system. The left over .01% formed everything else, including us.

Rings of left over matter orbited the newly formed sun and electrostatic forces began to clump the matter together in increasingly complex ways. As the clumps of matter got larger, they would collide with and absorb other clumps of matter. Eventually, clumps of matter got so big that they bulldozed through everything else in their orbital path.

Remember that the universe is mainly made up of hydrogen and helium. So it would follow that most planets would be made up of hydrogen and helium, and this is true for Jupiter and Saturn. The heat from the sun blasted away lighter elements until they cooled and formed gas giants in the outer rings of the solar system. The heavier elements, left behind in the inner rings of the solar system, formed solid planets.

The Earth

About 4.5 billion years ago, our earth formed. Heat from collisions and radioactive material meant that the earth was a large molten ball at the time. As it cooled, heavy elements (iron, nickel, etc) sank to the center of the planet and lighter elements (oxygen, hydrogen, etc) floated to the surface. This process formed the earths crust and early atmosphere.

As the earth cooled even more, the water vapor in the atmosphere (some brought to earth from comet collisions) began to rain down, and roughly 200 million after the original formation of the earth, we got oceans. This set the stage for our next stop, the origin of life on earth.


As stars die, they seed the universe with new elements that clump together in dense pockets of space. This leads to the formation of new astronomical bodies that are much more chemically rich than stars. Solar systems and planets were born.




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