Hello folks, <br /><br />This is my very first post. I'm a long time reader of space.com and the forums here. I've finally had a little time to join. <br /><br />For my first post I wanted to pose a question:<br /><br />There are lots of different kinds of galaxies, but the spiral seems to be the most common structure. I'm guessing that most spiral galaxies contain a supermassive blackhole in the center, and that the rest of the mass of the spiral galaxy "orbits" this BH and creates the spirals we see. <br /><br />Planetary systems, moon systems and ring systems have something basic in common with galaxies: mass orbits a central large/dense object.<br /><br />Why are the spirals present on galactic scales and not on planetary ones? Or the inverse, why don't galaxies behave like planetary systems and have distinct rings of matter?<br /><br />Thanks!<br /><br />e.