There's no download (and no cost, as Planet War is free-to-play), so there's really nothing stopping you from checking it out. What sets the game apart from the legions of other browser games clamoring for your hard-earned free time? Contests, basically, and YooGames is running a lot of them during the buildup to the title's official release. According to the company's latest press release, "events include building the largest army of commanders, racing towards the best technology, and so much more."
Head to the official Planet War website and get your free account to see exactly how much more.