Autosport has a piece about it on their website. Although heavily trailed this still comes as a big surprise to me, though not as big as if he’d returned to Williams. My prediction is that he’ll have a tough season and be bested by Petrov.
Rubens’s seat is safer though not a given. Williams still have him, Bottas, Sutill, Maldonado, and maybe even Hulkenberg to choose from to fill their two race seats.