Ripley the cat is an alien.
She's a flerken, or a cat from outer space. She stowed away to try to go home. Leon Kovi set up the mission to Ceres as a distraction to let Walt go home.
Leon will tell everyone that Walt actually left on the one-man mission to Ceres, then will cause the (actually empty) ship to be lost. Everyone will be sad, then forget about the viral sensation. Walt will go home and resume his life with a little financial help and assistance from Leon.