You guessed it! Potters make them.
So if aliens came to this planet in something that potters make, then the obvious conclusion would be that, yes, potters are aliens.
Oh sure, you scoff, but think about this for a second. Potters arrived here at least 20,000 years ago (as evidenced by pottery shards discovered in China). They found a planet experiencing an ice age whose crust was 59% silica (the main component of clay). They started turning the earth's crust into mugs, bowls, plates, ewers, vases, dildos, amphora, butter dishes, gravy boats, etc., etc., and with all of their pottery kilns going, they ended the ice age, freeing up more of that silica-rich crust and many ewers worth of fresh drinking water.
They also ate all of the megafauna because pottery-making is hungry work.
But the alien potters' scheming doesn't end there. Remember, they're used to working with wet clay; they're slippery buggers.
The computer revolution was brought about with the invention of the microprocessor, and what is the main material in microprocessors? Yup. Silicon. Computers are just a type of thinking pottery.
And what do you need computers for?
Exactly. Space ships!
My hypothesis is that potters crash-landed their flying saucer on this planet, and for tens of thousands of years they've been working at building another one so they could continue on their merry way through the cosmos converting planets into pottery.