Robert Downey Jr. and his wife Susan welcomed baby Exton in February, and the little boy just got a visit from a very famous Friend... Jennifer Aniston and her beau Justin Theroux!

The A-list couple dropped by the Tropic Thunder actor's Malibu house Saturday, and they definitely weren't in any rush since they stayed from 1 - 6pm. We wonder if the little tot ignited a bit of baby fever for the Wanderlust stars?

Our photographer who spotted Aniston and her man heading to the Iron Man star's pad tells us:

    "Jen looked really happy as she and Theroux drove up to the house. When they left, they were holding hands and being affectionate with each other."

Maybe these lovebirds were getting all warm and fuzzy about the prospect of having their own bundle of joy?!