Passing star yanks tongues of material from the proto-sun. Problem is that an expanding gas cloud should overcome the gravitational attraction of the Sun. Recent Theory - Revives nebular hypothesis. Sun condenses under the force of rotation and gravity. Compression causes temperature to exceed one million C. Thermonuclear reaction occurs which synthesizes the various elements.
Material is blown out into space.
Hence, heavy planets are near Sun and lighter ones farther out. Mantle - 2900 km thick. Core - 3400 km thick. No recent hypothesis. Thought that planetesimals form by accretion of condensed clumps of silicon.