'The Big Bang Theory' Casts Sean Astin, Kal Penn for Final Season

The Big Bang Theory is adding two big names to the cast for the second half of its final season.

The CBS comedy announced Wednesday that actors Kal Penn (Designated Survivor) and Sean Astin (Stranger Things) will be guest starring in an upcoming episode of the final season.

Penn and Astin will play Dr. Campbell and Dr. Pemberton, respectively, a team of physicists who accidentally confirm Sheldon and Amy's super-asymmetry theory. The air date for the episode has not been announced yet, according to The Hollywood Reporter.

The outlet writes the new casting helps hint at the final season storyline that newlywed couple Sheldon (Jim Parsons) and Amy (Mayim Bialik) have made a scientific discovery that will dramatically change their careers and lives.

For the past few episodes the couple has been seen working diligently on their super-asymmetry theory after altering their discovery after initially being proven wrong.

The casting news comes a day before Big Bang Theory kicks off the second half of their twelfth and final season, and as it remain the No. 1 comedy on television among total viewers.

Penn will next reprise his Designated Survivor role on the new season of the series, which was canceled by ABC back in May, but revived by Netflix later in 2018. The actor is also known for roles on House, How I Met Your Mother and the Harold and Kumar franchise.

Astin starred in season two of Netflix hit series Stranger Things and also starred in the iconic, Lord of the Rings trilogy.

The series also recently announced the casting of Grace and Frankie actress Lindsey Kraft in the role of Marissa, the new wife of Penny's (Kaley Cuoco) ex-husband. She will make her debut for a recurring role during the Jan. 10 episode of the comedy series.

The episode's official synopsis reads: "Zack and his wife want a baby, but when Zack is infertile, they ask Leonard for help."

Big Bang will return with a new episode on Thursday titled, "The Paintball Scattering."

The episode description reads: "Penny and Leonard (Johnny Galecki) organize a paintball game that results in mayhem when Sheldon (Jim Parsons) is jealous of Amy (Mayim Bialik). Also, Koothrappali (Kunal Nayyar) catches Anu (Rati Gupta) with her ex-boyfriend and Stuart (Kevin Sussman) doesn't want to move in with Denise (Lauren Lapkus)."

The Big Bang Theory airs Thursdays at 8 p.m. ET on CBS. The show will wrap up its run in May.