Solar 2 is a simple sandbox with not a lot to keep you compelled. Gameplay revolves around smacking into things to grow (asteroid and black hole stages) or get things in your orbit so you can press some keys to grow (planet and star stages). Missions and challenges play around with these goals, mostly by telling you to avoid contact with most things and not grow.
Some people say that the game is relaxing, and it can be if you don't care about doing anything. If you want to see anything new in the game, you will have to fly through 2D space and hope that the strange physics doesn't impede your progress. I can accept being attacked by spaceships and having to avoid obstacles, but my biggest issue is that pulling an object into orbit has a bad habit of rubber-banding the object into you, reducing your mass. So the method of growing your planet or star can randomly decide to remove your progress just because the programming has clear-cut orbit paths with the first object being yanked to the closest orbit.
Even the black hole stage is underwhelming, HOW could they mess up something that should be total fun to let loose and eat everything! Once you reach black hole stage, other black holes begin to appear, although they never spawned in the previous stages of the game. These other black holes can take objects from you, and if you hit a larger black hole than your own, you are forced to restart the state with no progress. This restart is made more terrible as black holes have a nasty issue of failing to pick up many items. Apparently entire stars and systems can fly around a black hole and not actually be swallowed.
I remember liking this game originally, but I only played for about an hour. The game has definitely become dated and feels especially mediocre now. Controls are weak, physics are annoying, and the gameplay is a repetitious grind full of elements that impede your progress. I believe I got the game for cheap in a bundle, but I wouldn't really recommend it for even $2. It's a time consumer, but similar games could be found for free. I'd recommend just playing a game you know you like, as this won't last long for most players.