'The Big Bang Theory' Casts Judd Hirsch as Leonard's Dad -- But When Will We Meet Penny's Mom?!

The Big Bang Theory family tree is growing even bigger!

During Wednesday's PaleyFest panel in Los Angeles, it was announced that Taxi star Judd Hirsch has been tapped to play Leonard's dad for an upcoming episode this season.

Johnny Galecki admitted that he actually had a hand in casting Hirsch as his on-screen father after meeting at NBC's James Burrows special.

“I met this fellow a month or two ago and he’s a hero of mine and I knew that he was a hero of Chuck [Lorre] and Bill [Prady’s], so I just accosted him and cornered him and practically made him commit on the spot to play my father,” Galecki said, adding that he was inspired by Hirsch’s Taxi character, Alex Rieger, while filming The Big Bang Theory pilot. “I’m really excited about it. It’s going to be a lot of fun.”

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Over the past nine seasons, very few details have been spilled about Leonard's old man, however, we do know that he is an anthropologist who withheld affection from his son during Leonard's childhood. He and Leonard's mom, Beverly -- played by The Good Wife's Christine Baranski -- got divorced after he cheated on her with a waitress from the Princeton cafeteria.

Although we're pumped to finally meet Leonard's dad, we're still wondering when the CBS comedy is going to tackle the rest of that long list of Big Bang's M.I.A. family members.

"We're open to meeting any of these people," executive producer Steve Molaro dished exclusively to Entertainment Tonight ahead of the panel.

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"There's so many family members that we haven’t met yet," Molaro continued. "Penny's Mom, Leonard's dad, Penny's brother -- which is a great one that I'm interested in -- Sheldon's brother, and Howard's dad is floating around out there."

So, now that we can officially add Dr. Hofstadter’s picture to the family tree, who’s next? "There's no list," the showrunner explained. "It's which stories are we excited about and how do we make that happen?"

Fingers crossed we won't have to wait another nine years before we meet Penny's other parent.

The Big Bang Theory returns Thursday, March 31 at 8 p.m. ET on CBS.

Which Big Bang family member are you dying to meet? Who do you think should play Penny's mom? Share your thoughts with @LeanneAguilera on Twitter!