Generating 2D voxel circles using a noise function

I'm creating a 2D space game where the map is made up of square tiles. I would like a noise function that I can use to generate circular planet, by circular planets I mean 'circles' made out of squares [basically like a circle in minecraft]. The planet's radius should be of all different sizes. The reason I want to use noise is that I want the user to be able to generate a map with a seed so they can generate the same planets again. [the planets should be randomly distributed, not uniformly spaced] How would I implement this using noise so that it procedurally generates?

Thanks! :)