Even after the major planets in our Solar System had already formed, planetesimals -- thousands or millions of them -- still roamed around the Solar System. Many of them ultimately collided with the major planets.

http://www.psrd.hawaii.edu/Jan01/lunarCataclysm.html

This last rush of relatively large collisions is known as the Late Heavy Bombardment or the Lunar Cataclysm. It ended around 3.9 billion years ago.

This 3.9 billion years number is going to be important later on in the course.

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