The solar system formed from an immense
rotating cloud of gas and dust called
the solar nebula.
The sun's nuclear reaction began at
the dense center of the nebula.
The planets formed by accumulating
material within the swirling currents
of the cloud.
Planets near the sun evolved as relatively
small spheres of rocky material.
In the outer portions of the solar
nebula, debris and gases accumulated
to form massive gaseous planets.