'The Big Bang Theory' casts Judd Hirsch as Leonard's dad -- but when will we meet Penny's mom?!

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'Taxi' Star Judd Hirsch Joins 'The Big Bang Theory' as Leonard's Dad!

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.

See photos of "The Big Bang Theory" cast:

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'The Big Bang Theory' casts Judd Hirsch as Leonard's dad -- but when will we meet Penny's mom?!
LOS ANGELES - APRIL 19: 'The Line Substitution Solution' -- Pictured: Trevor (Blake Anderson), Sheldon Cooper (Jim Parsons), Leonard Hofstadter (Johnny Galecki), Howard Wolowitz (Simon Helberg) and Rajesh Koothrappali (Kunal Nayyar). Sheldon hires Stuart to spend the day with Amy when he'd rather go to a movie screening. Also, Leonard's mother, Beverly (Christine Baranski), comes to town and Penny struggles to make a connection with her, on THE BIG BANG THEORY, Thursday, May 5 (8:00-8:31, ET/PT) on the CBS Television Network. (Photo by Michael Yarish/CBS via Getty Images)
LOS ANGELES - APRIL 19: 'The Line Substitution Solution' -- Pictured: Sheldon Cooper (Jim Parsons), Leonard Hofstadter (Johnny Galecki), Stuart (Kevin Sussman) and Howard Wolowitz (Simon Helberg). Sheldon hires Stuart to spend the day with Amy when he'd rather go to a movie screening. Also, Leonard's mother, Beverly (Christine Baranski), comes to town and Penny struggles to make a connection with her, on THE BIG BANG THEORY, Thursday, May 5 (8:00-8:31, ET/PT) on the CBS Television Network. (Photo by Michael Yarish/CBS via Getty Images)
LOS ANGELES - APRIL 19: 'The Line Substitution Solution' -- Pictured: Sheldon Cooper (Jim Parsons), Howard Wolowitz (Simon Helberg), Rajesh Koothrappali (Kunal Nayyar) and Leonard Hofstadter (Johnny Galecki). Sheldon hires Stuart to spend the day with Amy when he'd rather go to a movie screening. Also, Leonard's mother, Beverly (Christine Baranski), comes to town and Penny struggles to make a connection with her, on THE BIG BANG THEORY, Thursday, May 5 (8:00-8:31, ET/PT) on the CBS Television Network. (Photo by Michael Yarish/CBS via Getty Images)
HOLLYWOOD, CALIFORNIA - MARCH 16: (L-R) Moderator Alie Ward, executive producers Chuck Lorre, Steven Molaro, Bill Prady, actors Mayim Bialik, Jim Parsons, Kaley Cuoco, Johnny Galecki, Simon Helberg, and Kunal Nayyar attend The Paley Center For Media's 33rd Annual PALEYFEST Los Angeles ÃThe Big Bang Theory' at Dolby Theatre on March 16, 2016 in Hollywood, California. (Photo by Matt Winkelmeyer/Getty Images)
BEVERLY HILLS, CA - MARCH 09: (L-R) Actors Jim Parsons, Melissa Rauch, Kunal Nayyar, Kaley Cuoco, Simon Helberg, Mayim Bialik and Johnny Galecki perform onstage during the 24th and final 'A Night at Sardi's' to benefit the Alzheimer's Association at The Beverly Hilton Hotel on March 9, 2016 in Beverly Hills, California. (Photo by Jesse Grant/Getty Images)
MUST SEE TV: AN ALL-STAR TRIBUTE TO JAMES BURROWS -- Pictured: (l-r) Kaley Cuoco, Simon Helberg, Johnny Galecki, Jim Parsons -- (Photo by: Trae Patton/NBC/NBCU Photo Bank via Getty Images)
LOS ANGELES - FEBRUARY 2: 'The Positive Negative Reaction' -- Bernadette is concerned when Wolowitz freaks out over her pregnancy announcement, and Wolowitz is convinced he must make more money to provide for the baby, on THE BIG BANG THEORY, Thursday, Feb. 18 (8:00-8:31 PM, ET/PT) on the CBS Television Network. Pictured left to right: Kunal Nayyar, Simon Helberg, Johnny Galecki and Jim Parsons (Photo by Sonja Flemming/CBS via Getty Images)
LOS ANGELES - JANUARY 19: 'The Valentino Submergence' -- Sheldon and Amy host a live Valentine's Day episode of Fun with Flags, Leonard and Penny grapple with getting older, Howard and Bernadette find a surprise in their hot tub and Raj is torn between Emily and the cute girl he met at the Comic Bookstore, on THE BIG BANG THEORY, Thursday, Feb. 11 (8:00-8:31 PM, ET/PT), on the CBS Television Network. Pictured left to right: Johnny Galecki, Simon Helberg, Jim Parsons and Kunal Nayyar (Photo by Monty Brinton/CBS via Getty Images)
LOS ANGELES - FEBRUARY 8: 'The Celebration Experimentation' -- After more than nine years together, the gang finally celebrates Sheldon's birthday, surprising him with a special guest, on the 200th episode of THE BIG BANG THEORY, Thursday, Feb. 25 (8:00-8:31 PM, ET/PT) on the CBS Television Network. Pictured left to right: Jim Parsons, Kunal Nayyar, Sara Gilbert, Kevin Sussman, Adam West, Simon Helberg, Melissa Rauch, John Ross Bowie, Wil Wheaton, Johnny Galecki and Kaley Cuoco (Photo by Monty Brinton/CBS via Getty Images)
LOS ANGELES - FEBRUARY 2: 'The Positive Negative Reaction' -- Bernadette is concerned when Wolowitz freaks out over her pregnancy announcement, and Wolowitz is convinced he must make more money to provide for the baby, on THE BIG BANG THEORY, Thursday, Feb. 18 (8:00-8:31 PM, ET/PT) on the CBS Television Network. Pictured left to right: Simon Helberg, Kunal Nayyar, Jim Parsons and Johnny Galecki (Photo by Sonja Flemming/CBS via Getty Images)
LOS ANGELES - AUGUST 30: 'The Pulled Groin Extrapolation' -- Sheldon and Penny wonder how they'll get along when Leonard goes to a wedding as Amy's date. Meanwhile, Howard (Simon Helberg, left) and Bernadette (Melissa Rauch, right) spend the weekend with Mrs. Wolowitz, on THE BIG BANG THEORY, Thursday, Sept. 29 (8:00-8:31 PM, ET/PT) on the CBS Television Network. (Photo by Richard Cartwright/CBS via Getty Images)
LOS ANGELES - SEPTEMBER 23: 'The Workplace Proximity' -- Howard (Simon Helberg, left) finds himself sleeping on Raj'™s couch after a fight with Bernadette, on THE BIG BANG THEORY, Thursday, Oct. 17 (8:00 -“ 8:31 PM, ET/PT) on the CBS Television Network. Also pictured: Johnny Galecki (right) (Photo by Monty Brinton/CBS via Getty Images)
LOS ANGELES - APRIL 15: 'The Gorilla Dissolution' -- Howard (Simon Helberg, left) and Bernadette (Melissa Rauch, right) struggle to care for Mrs. Wolowitz, on THE BIG BANG THEORY, Thursday, May 8 (8:00-8:31 PM, ET/PT) on the CBS Television Network. (Photo by Sonja Flemming/CBS via Getty Images)
The cast and crew of €œ'The Big Bang Theory' accepts the award for favorite network TV comedy at the 40th annual People's Choice Awards at the Nokia Theatre L.A. Live on Wednesday, Jan. 8, 2014, in Los Angeles. (Photo by Chris Pizzello/Invision/AP)
LOS ANGELES, CA - SEPTEMBER 18: Kaley Cuoco is seen at 'Jimmy Kimmel Live' on September 18, 2014 in Los Angeles, California. (Photo by RB/Bauer-Griffin/GC Images)
LOS ANGELES, CA - SEPTEMBER 20: Kaley Cuoco is seen in Los Angeles on September 20, 2014 in Los Angeles, California. (Photo by Bauer-Griffin/GC Images)
LOS ANGELES, CA - SEPTEMBER 20: Kaley Cuoco is seen in Los Angeles on September 20, 2014 in Los Angeles, California. (Photo by Bauer-Griffin/GC Images)
LOS ANGELES, CA - AUGUST 25: Actress Kaley Cuoco (R) and husband Ryan Sweeting (L) attend HBO's 2014 Emmy after party at The Plaza at the Pacific Design Center on August 25, 2014 in Los Angeles, California. (Photo by Barry King/FilmMagic)
GOOD MORNING AMERICA - Rihanna, Steve Martin and Jim Parsons are all guests on 'Good Morning America,' 3/13/15, airing on the ABC Television Network. (Photo by Lou Rocco/ABC via Getty Images)
LOS ANGELES, CA - AUGUST 25: Actress Kaley Cuoco arrives to the 66th Annual Primetime Emmy Awards at Nokia Theatre L.A. Live on August 25, 2014 in Los Angeles, California. (Photo by C Flanigan/Getty Images)
BURBANK - JULY 23: Gallery Promo Coverage of CBS' THE BIG BANG THEORY, scheduled to air on the CBS Television Network. Pictured: Simon Helberg as Howard Wolowitz. (Photo by Monty Brinton/CBS via Getty Images)
Kaley Cuoco accepts the Favorite Comedic TV Actress award for The Big Bang Theory at the 40th annual People's Choice Awards at Nokia Theatre L.A. Live on Wednesday, Jan. 8, 2014, in Los Angeles. (Photo by Frank Micelotta/Invision/AP)
Mayim Bialik, winner of the award for favorite network TV comedy for "The Big Bang Theory," poses backstage at the People's Choice Awards at the Nokia Theatre on Wednesday Jan. 9, 2013, in Los Angeles. (Photo by Jordan Strauss/Invision/AP)
Craig Ferguson, right, and "The Big Bang Theory" panel on Day 2 of Comic-Con on Friday, July 25, 2014, in San Diego. (Photo by Richard Shotwell/Invision/AP)
Jim Parsons poses with the award for best actor in a comedy series for “The Big Bang Theory” in the press room at the Critics' Choice Television Awards at the Beverly Hilton Hotel on Thursday, June 19, 2014, in Beverly Hills, Calif. (Photo by Richard Shotwell/Invision/AP)
Jim Parsons poses backstage with the award for outstanding lead actor in a comedy series for his role on “The Big Bang Theory” at the 65th Primetime Emmy Awards at Nokia Theatre on Sunday Sept. 22, 2013, in Los Angeles. (Photo by Dan Steinberg/Invision/AP)
Kunal Nayyar and Johnny Galecki accept the award for best supporting actor in a comedy series on behalf of winner Simon Helberg for "The Big Bang Theory" at the Critics' Choice Television Awards in the Beverly Hilton Hotel on Monday, June 10, 2013, in Beverly Hills, Calif. (Photo by Frank Micelotta/Invision/AP)
CORRECTS BYLINE TO CHRIS PIZZELLO - Kaley Cuoco poses backstage with her award for best supporting actress in a comedy series for "The Big Bang Theory" at the Critics' Choice Television Awards in the Beverly Hilton Hotel on Monday, June 10, 2013, in Beverly Hills, Calif. (Photo by Chris Pizzello/Invision/AP)
In this Tuesday, May 7, 2013 photo, actor Simon Helberg poses for a portrait in Los Angeles. Helberg is a co-star of the CBS television show, "The Big Bang Theory." (Photo by Matt Sayles/Invision/AP)
From left, actors Johnny Galecki, Kunal Nayyar, Mayim Bialik, Jim Parsons, Melissa Rauch, Simon Helberg and Kaley Cuoco, from the cast of "The Big Bang Theory," pose backstage at the 21st Annual 'A Night at Sardi's' to benefit the Alzheimer's Association at the Beverly Hilton Hotel on Wednesday, March 20, 2013 in Beverly Hills, Calif. (Photo by John Shearer/Invision for Alzheimer's Association/AP Images)
IMAGE DISTRIBUTED FOR ALZHEIMER'S ASSOCIATION - From left, actors Kunal Nayyar, Melissa Rauch, Jim Parsons, Mayim Bialik and Simon Helberg, from the cast of "The Big Bang Theory," arrive at the 21st Annual 'A Night at Sardi's' to benefit the Alzheimer's Association at the Beverly Hilton Hotel on Wednesday, March 20, 2013 in Beverly Hills, Calif. (Photo by Jordan Strauss/Invision for Alzheimer's Association/AP Images)
Photo of Kunal Nayyar, left, and Simon Helberg courtesy of Samsung Galaxy, taken during the Paley Center for Media's PaleyFest, honoring The Big Bang Theory at the Saban Theatre, Wednesday March 13, 2013 in Los Angeles, California. (Photo by Kevin Parry/Invision/AP)
Photo of Jim Parsons courtesy of Samsung Galaxy, taken during the Paley Center for Media's PaleyFest, honoring The Big Bang Theory at the Saban Theatre, Wednesday March 13, 2013 in Los Angeles, California. (Photo by Kevin Parry/Invision/AP)
Photo of Johnny Galecki, left, and Kunal Nayyar courtesy of Samsung Galaxy, taken during the Paley Center for Media's PaleyFest, honoring The Big Bang Theory at the Saban Theatre, Wednesday March 13, 2013 in Los Angeles, California. (Photo by Kevin Parry/Invision/AP)
Photo of Mayim Bialik, left, and Jim Parsons courtesy of Samsung Galaxy, taken during the Paley Center for Media's PaleyFest, honoring The Big Bang Theory at the Saban Theatre, Wednesday March 13, 2013 in Los Angeles, California. (Photo by Kevin Parry/Invision/AP)
Photo of Kunal Nayyar courtesy of Samsung Galaxy, taken during the Paley Center for Media's PaleyFest, honoring The Big Bang Theory at the Saban Theatre, Wednesday March 13, 2013 in Los Angeles, California. (Photo by Kevin Parry/Invision/AP)
Photo of Simon Helberg courtesy of Samsung Galaxy, taken during the Paley Center for Media's PaleyFest, honoring The Big Bang Theory at the Saban Theatre, Wednesday March 13, 2013 in Los Angeles, California. (Photo by Kevin Parry/Invision/AP)
Kaley Cuoco poses on arrival at the Paley Center for Media's PaleyFest, honoring The Big Bang Theory at the Saban Theatre, Wednesday March 13, 2013 in Los Angeles, California. (Photo by Kevin Parry/Invision/AP)
Johnny Galecki arrives during the Paley Center for Media's PaleyFest, honoring The Big Bang Theory at the Saban Theatre, Wednesday March 13, 2013 in Los Angeles, California. (Photo by Kevin Parry/Invision/AP)
Mayim Bialik poses on arrival at the Paley Center for Media's PaleyFest, honoring The Big Bang Theory at the Saban Theatre, Wednesday March 13, 2013 in Los Angeles, California. (Photo by Kevin Parry/Invision/AP)
Melissa Rauch poses on arrival at the Paley Center for Media's PaleyFest, honoring The Big Bang Theory at the Saban Theatre, Wednesday March 13, 2013 in Los Angeles, California. (Photo by Kevin Parry/Invision/AP)
CORRECTS IDENTITY - Jim Parsons poses on arrival at the Paley Center for Media's PaleyFest, honoring The Big Bang Theory at the Saban Theatre, Wednesday March 13, 2013 in Los Angeles, California. (Photo by Kevin Parry/Invision/AP)
The cast of "The Big Bang Theory" pose backstage with the award for favorite network TV comedy at the People's Choice Awards at the Nokia Theatre on Wednesday Jan. 9, 2013, in Los Angeles. (Photo by Jordan Strauss/Invision/AP)
Kaley Cuoco, winner of the award for favorite network TV comedy for "The Big Bang Theory," poses backstage at the People's Choice Awards at the Nokia Theatre on Wednesday Jan. 9, 2013, in Los Angeles. (Photo by Jordan Strauss/Invision/AP)
BEVERLY HILLS, CA - DECEMBER 18: Actress Kaley Cuoco-Sweeting arrives at The PEOPLE Magazine Awards at The Beverly Hilton Hotel on December 18, 2014 in Beverly Hills, California. (Photo by Jon Kopaloff/FilmMagic)
NEW YORK - MAY 18: CBS announces their new fall schedule. (Jeffrey R. Staab/CBS via Getty Images) *** CBS presented its 2016-17 prime time schedule today (May 18) at Carnegie Hall, where many of the Networks new and returning series stars were in attendance. Pictured L-R: Melissa Rauch, Mayim Bialik, Simon Helberg, Kaley Cuoco, Jim Parsons, and Johnny Galecki from THE BIG BANG THEORY *** Melissa Rauch;Mayim Bialik;Simon Helberg;Kaley Cuoco;Jim Parsons;Johnny Galecki
LOS ANGELES - SEPTEMBER 9: 'The Hook-Up Reverberation' -- Raj's honesty about his past comes back to haunt him after his girlfriend, Emily, is cold to Penny. Also, the guys consider becoming part-owners in Stuart's comic book store, on THE BIG BANG THEORY, Monday, Oct. 6 (8:00-8:31, ET/PT), on the CBS Television Network. Pictured left to right: Laura Spencer, Simon Helberg, Melissa Rauch, Mayim Bialik, Jim Parsons, Johnny Galecki and Kaley Cuoco-Sweeting (Photo by Sonja Flemming/CBS via Getty Images)
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"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."

MORE: 'Big Bang' Baby Bombshell -- Can You Believe Bernadette Is Pregnant?!

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.

MORE: Big Bang: Sheldon Takes a Huge Step Forward in the 200th Episode!

"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!

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