Binary Star Systems
Versus Planetary Systems

Our Solar System may not be the norm for stars in the Universe. The observational evidence is that most stars are parts of multiple star systems, not single stars like our Sun.

Formation of Binary Star Systems

The most common occurrence of stars appears to be as parts of binary (two-star) systems. This suggests an alternative to the nebular hypothesis illustrated in the following figure.

Alternative to the nebular hypothesis that leads to binary star formation

Although planets might still form in such binary systems by a similar mechanism as discussed before, it is an open question whether they would have stable orbits that would keep them bound in the system without running into the stars. Another question, assuming such planets were on stable orbits, is whether they could have temperature ranges favorable for the formation of life.

If Jupiter Had Become a Star . . .

We note in this connection that if Jupiter had been about 100 times more massive than it is, it would have formed a star instead of a planet. Thus, maybe the Solar System very nearly became a binary star system instead of a single star with planets. We may speculate that in that case the Earth might not even exist, or even if it existed would be in an orbit giving surface conditions not favorable to the evolution of life.