The PET or P.E.T science weapon location in the Outer Worlds Peril On Gorgon. This unique weapon is found in two steps first you must collect 4 magazine issues.
The magazine is called the Exhilarating Adventures of Archibald Excelsior and the location of all 4 is shown in the video guide.
The second stage is to answer 4 question to enter the secret hideout. Once done the Science weapon is yours.