So far, from what we have seen in 'The Expanse' series and the comments above, what I have concluded is that the Protomolecule concept is utterly stupid as it is now.
Why would some species will send their technology/embryo/biomolecule to other planets and solar systems without any real clear purpose, except growing…

I haven't read the books, so I would like to guess and hypothesize that the guy with that fast yacht may be the cause of the arrival of alien protomolecule in our solar system.

Only yelper can click it. For that special treatment, you know!
B&C.