This is a drawing by my daughter which inspired one of the elements in my story/world:

The perspective is a little bit mixed here (certainly I don't have a 15 meter-high volcano in this world), but for the sake of this question let's assume that such planet is between 1 and 2 kilometres in diameter or 500-1000 meters in radius. Planets like this are sold by various agencies in the world of my story.

Given the fact that the whole story is set in exactly the same universe, as ours (only a little bit into the future), with exactly the same physics and other laws, the question is, if such planet:

• could exists naturally (what are the limits here) or
• these companies are selling artificial creations.

What is the smallest possible planet that can exists naturally in our universe? Must I assume that such small planets cannot exist naturally?