This Island Earth

A team of top nuclear scientists is assembled by the mysterious Exeter at a high-tech laboratory in Georgia. His mission, kept secret even from the researchers, is to develop new sources of atomic energy, which are needed to defend his home planet, Metaluna, from its interplanetary rival, Zagon. When two of the recruits begin to suspect Exeter's true identity and motive, they attempt to escape. But before the runaways can reach safety, the aliens intervene and carry them off in a flying saucer, plane and all, in a last-ditch effort to save Metaluna.
Remember the scene where the flying saucer lands on the lawn at the nation's capital and the Army surrounds it and a robot named Gort comes out to destroy the Earth? If you do, you're thinking of another movie: Day The Earth Stood Still

Full Credits:
Director: Joseph M. Newman
Star: Jeff Morrow, Faith Domergue, Rex Reason (where there's a Rex, there's a Reason), Russell Johnson, Lance Fuller, Robert Nichols, Douglas Spencer, Karl L. Lindt
Producer: William Alland
Writer: Franklin Coen
Director of Photography: Clifford Stine
Editor: Virgil Vogel
Composer: Herman Stein
The movie was based on a novel by Raymond F. Jones, recently reprinted by Forrest J. Ackerman.

The best book for looking up movie titles and descriptions is the Movie / Video Guide by Leonard Maltin. Here is his review:
"Suspenseful, intelligent science fiction about scientists lured to mysterious project, only to find they've been recruited - or more appropriately, shanghaied - by aliens to help them defend their invasion-torn planet. Existentialism of Raymond F. Jones' novel has been muted, but it's still thoughtful and exciting, with excellent visuals." He's talking about "This Island Earth," not Invaders From Mars

This Island Earth is available on video or DVD in the original version (1955). It was also seen as the movie they're watching in Mystery Science Theater 3000: The Movie, reviewed by Mike Nelson and his robot pals...