Currently Magnetic Cloud and Plasma Storm do nothing - they are bugged.
From memory however, Magnetic Cloud should disable strike cloud launch, and Plasma Storm disables ship's abilities. Maybe it was the other way around, but zooming out and clicking the planets Icon will allow you to see an ability in the control panel that will detail what its meant to do.
In addition, Asteroid fields reduce accuracy, and suns increase Anti Matter Regen [Though if I am correct, I believe different coloured suns will be getting different additional buffs/debuffs later on]. When a ship dies within a gas giant gravity well, it damages all other ships around it.
Stars have two purposes:
1. The anti matter regen buff, and other debuffs in Rebellion that either are in, or will likely be in the game later.
2. After having Researched "Long Range Jumps" or W/E the tech is, you can use a star's gravity well to jump to another star.