1. Most of the stars you view from the earth at night are actually binary stars. That is, two stars circle each other creating an imagined point of gravitation, or a smaller star circles around the “main star”. Sometimes these main stars draw material from the smaller one, as they come closer to each other. There is a mass limit that a planet can hold without fusing nuclear reaction. If Jupiter was larger it might have fused as a brown dwarf, a kind of semi-star, many moons ago. It is actually more common in other solar systems, evidenced by their relative lack of planets. This is because most of the material which is locked in the gravitational field of the main star gathers up in one place, which in the end lights up as a star and forms a binary system. There can also be more than two stars in one system, yet binary systems are more common.