Q.  How are solar systems formed?

A.  Space everywhere contains "Oort Soup", a mix of planetesimals and planets of every size. Over time, these may aggregate together. When an aggregation reaches the mass of a star, this forms the beginning of a solar system.

Q.  What's the lower limit of a star's mass?

A.  It's about 7.5% of the mass of our Sun.

Q.  What qualifies a body of this mass to be a star?

A.  That's the minimum mass for nuclear fusion to begin at its core.

Q.  What forms first in a solar system, the star or its planets?

A.  Surprisingly, the planets exist first, as components of the Oort Soup. When aggregation leads to a new star, this has to normalize the motion of other bodies in its vicinity.

Q.  How does it do this?

A.  Through the action of gravity, an effect called Equatorial Forcing.

Q.  Where is there more detail on this?

A.  See P1: The Cosmic Smog model for solar system formation, and the nature of 'Dark Matter'.