Desktop App for Planner?

Not currently,

There is a uservoice for it here

https://planner.uservoice.com/forums/330525-microsoft-planner-feedback-forum/suggestions/34471954-desktop-application

Would recommend you vote on it to push it up the agenda. Voting for the uservoice will also notify you if it has been picked up by Microsoft. I would also keep an eye out on this tech community for Planner AMA's where you can also raise this with the product team. There is a big appretite to push Planner forward so hopefully, we'll see something in the future

Hope that answers your question

Best, Chris