Early solar system. Illustration of the early solar system during its formation. The Sun (upper left) formed from a collapsing nebula some 4.6 billion years ago. The remaining dust and gas formed a rotating disc surrounding the Sun. Slow gravitational attraction (accretion) of small particles formed rocky asteroids (planetesimals). They in turn clumped together to form Earth-like rocky planets, or remained scattered like the present-day asteroid belt.