Think of it this way: each object has a classification, a name, and perhaps a nickname or generic name.
So, the Sun is a particular star (our star). The word "sun" can also be applied generically, just like "star" to mean any star ... just not our star, whose name is the Sun (a proper noun). Just as there are many stars in the Milky Way, you could say there are many suns. But if you were to say the Milky Way contains many Suns, you would be implying "many stars like our Sun: perhaps as to spectral class, harboring a system of planets, etc."
Similarly, any star can be part of a solar system, but ours is called the Solar System (its name, a proper noun).
Gliese 581 (again, a proper noun) is a particular star, or sun, but it's not Sol, the Sun, our star. It apparently has its own solar system, but it's not the Solar System, just one among many solar systems.
Solar is not a proper noun. It means related to the Sun, but could be applied to any other star within the context of its own neighborhood. You could say that Gliese 581 is known to harbor a planet in its solar neighborhood, or solar system, but not its Solar System (that one belongs to us, or more properly to the Sun). It would be less confusing to refer to the neighborhood of Gliese 581as a planetary system (though that might tend to leave aside asteroids, comets, and the like).