Solar system formation. Artwork of asteroids (planetesimals) in the early solar system about 4.7 billion years ago. The planetesimals lie in an interstellar cloud (nebula) of gas and dust that surrounds the Sun. The solar system was formed when this nebula contracted due to gravitational forces; this process was possibly triggered by shock waves from a nearby exploding star (super- nova). The planets were formed by a process called accretion in which dust particles collided and to form planetesimals. These in turn attracted dust, gas and other planetesimals by gravitational attraction to form larger bodies.