What is the atmosphere and what is it made of?
How do weathering and erosion affect the Earth?
Formation of Earth and Our Solar System
We can construct the formation history of our solar system by looking at regions where other
stars are forming now. Star formation begins when a giant cloud of gas and dust collapses
under its own gravity. As the cloud contracts, it begins to spin faster and settles into a disk-
shaped structure. We see these disc-shaped objects (called proplyds) in the Orion Nebula
(Figure12.13), where the new stars are forming today. Most of the dusty disk material
drains toward the center where the density gradually increases until the enormous central
pressure triggers nuclear fusion reactions and the star is born.
Figure 12.5: Orion Nebula ( 9 )However, a relatively small fraction of the disk material is left behind in the form of ice-
coated dust grains. The icy mantles of the grains begin sticking together and eventually grow
to meter-sized rocky boulders called planetesimals. The planetesimals collide and accrete
into larger bodies that are tens of kilometers in diameter called protoplanets. Once the
protoplanets clear a gap in the disk, they become bonafide planets and their orbits begin to
stabilize (Figure12.14).
The process of planet formation is messy. Not all of the planetesimals are accreted into
planets. Millions of planetesimals remain as the leftover debris and are now the asteroids
and ice-coated comets in our solar system. In the first hundred million years after the
formation of the Sun, collisions between the leftover planetesimals and the planets were