They are calling it the 'Forbidden Planet' because it isn't something that's supposed to be, it defies expectations. Perhaps a more apt name would be the 'Impossible Planet' but let's not get into the nomenclature.
What makes this planet's existence impossible is that it could be found in a Neptunian desert, a region with intense radiation that nothing should be able to survive. But this one planet does and scientists aren't yet sure why.
(Image credit: University of Warwick)