As far as I know, circular collision polygons are not possible. I can't give full solution to this, but I have one idea that maybe could work.
You could make it as regular flat floor platformer, but make it look like your on a planet. Just make the actual ground on a layout invisible and make a non-solid sprite image of a planet under players feet. Make planet.x = player.x. Then rotate the planet and backround according to player movements (e.g. set planet angle relative to player.x and pin background to the planet). Any collectible objects and enemies on the planet must be pinned to the planet (position and rotation).
This only works if you have single planet in each layout. If you want jumping from planet to planet or something like that, then it's more complicated.