A VERY interesting paper came out recently that proposed that the theoretical Planet Nine may in fact be a primordial black hole lurking in our solar system. These types of black holes were formed early in the Universe and are thought to be quite numerous and have masses on the scale of our planet.
The paper postulates that the Planet Nine black hole would be about five Earth masses and provides an actual to scale diagram (Figure 1 in the paper) of the size of such a body:
What if Planet 9 is a Primordial Black Hole? Jakub Scholtz1 and James Unwin2
Sky Guy in VA