Josh Hartnett has boarded Christopher Nolan's 'Oppenheimer'.

The star-studded drama has a new addition in the 'Pearl Harbor' star, Deadline reports.

It's not known at this time what role the 43-year-old actor will be playing.

Just last month, Florence Pugh, Rami Malek and Benny Safdie joined the cast.

The motion picture will be led by Cillian Murphy with Matt Damon, Robert Downey Jr. and Emily Blunt also set to star.

The movie has been written and directed by Nolan and will tell the story of J. Robert Oppenheimer, the scientist behind the Manhattan project that led to the invention of the atomic bomb.

The project is slated for release on July 21, 2023, and occupies a slot that the director has locked in for his movies in the past.

Florence will play Jean Tatlock, a member of the Communist Party of the United States who had an affair with Oppenheimer while Safdie will be the Hungarian physicist Edward Teller.

It is also thought that Rami will be playing another scientist but no specifics have been disclosed.

The movie is based on the award-winning book 'American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer' by Kai Bird and the late Martin J. Sherwin.

Nolan is producing alongside Emma Thomas and Charles Roven for Atlas Entertainment.

Universal acquired the rights to the movie after a bidding war between several major Hollywood studios, including Nolan's former collaborator Warner Bros.

Universal Filmed Entertainment Group Chairman Donna Langley said: "Christopher Nolan and Emma Thomas’ films have shattered the limits of what cinematic storytelling can achieve. We are thrilled to be working alongside them on this exceptional and extraordinary project and are grateful for their shared passion and commitment to the theatrical experience."

Oppenheimer's story has been adapted for the big screen previously in the 1989 drama 'Fat Man and Little Boy' – which starred Dwight Schultz as the scientist and Paul Newman as General Leslie Groves.