Russell Johnson, the actor best known for playing the Professor on Gilligan's Island, passed away this morning from kidney failure. He was 89.

"He died at home, peaceful, in his sleep at 5:21 am today," his wife Constance told ABC News. "[He was] a very brave guy who knew what he wanted, and he wanted to be at home."

Born in Northeastern Pennsylvania in 1924, Russell joined the U.S. Army Air Forces after high school and flew 44 combat missions during World War II before using the G.I. Bill to study acting. Following an early career dominated by Western and sci-fi movies -- among them, It Came From Outer Space, This Island Earth and The Stand at Apache River -- he settled into television with roles on such shows as Black Saddle, The Twilight Zone and Laramie.

However, it's his portrayal of high school science teacher Roy Hinkley, aka the Professor, on Gilligan's Island, for which he's best remembered. Johnson appeared in all 98 episodes of the comedy, which originally aired from 1964 to 1967 on CBS, and reprised the role in a pair of animated series and three made-for-TV movies.

"My 2 favorite people are now gone," Dawn Wells, who played Mary Ann, wrote in a Facebook post accompanied by a photo of herself with Russell and Gilligan's Island star Bob Denver. "The Professor past [sic] away this morning. My heart is broken. [...] Russell was a true gentleman, a good father, a great friend."

Wells and Tina Louise, who played Ginger, are the only surviving castaways: Jim Backus died in 1989, followed by Alan Hale Jr. in 1990, Natalie Schafer in 1991 and Denver in 2005.

Russell is survived by his wife Constance and a daughter Kim.