The following information is from the Adventure Timecomic book series. It may not be part of the same canon or continuity as the animated series.
Adventure Time: Marceline Gone Adrift Issue 2 is the Second issue of the 2015 mini-series.
Marceline finds herself deep in space while Princess Bubblegum realizes her mistake and starts planning how to bring her friend back. The threat to Ooo might be down for now, but one unlikelybun is going to suddenly find himself with too much power to handle!