I noticed that little bit hidden underneath the bottom-most object too, but I wasn't sure if it should be counted. I double-checked the full-color version, and sure enough, it wasn't removed and was given the same color as the object it's next to, so perhaps it was indeed intended to be a satellite around that particular object.
The real question that remains in my mind, though, is whether they're meant to be moons or planets. It's not too important for the sake of your argument, as the connection of "eight objects around the Earth" still holds up, I'm just extremely curious what Oda wants to convey with this model.
Does the One Piece world believe in geocentrism, with the Earth as the center of its solar system, implying that this is an inaccurate model?
Is the One Piece world actually geocentric somehow? If so, does the sun revolve around the Earth too, or is the Earth itself heliocentric while also being so large that the other planets behave geocentrically?
Obviously, they aren't all so close as to be seen in the night sky, at least not in the same way that what we call "the moon" is, so that really calls into question their size. I wonder if they can be seen among the stars though, similar to the closest planets to our Earth in our solar system?
If they are indeed planets, how big would this world need to be in order for them to orbit it as if they were moons instead of orbiting the sun and still be so far as to not be seen at the scale of the moon?
Do these planets support life? Is that how the Space Pirates came to this world's moon; because they were close enough to do so rather than needing to travel lightyears to reach it?
And most importantly, if Monet is in fact an astronomer, do they have any special properties that would be unusually helpful for celestial navigation?