PASADENA, Calif. -- For the first time, astronomers have spotted the silhouette of a planet as it circles another star, apparently confirming that other worlds exist beyond our own solar system.

Until now, astronomers had only inferred the presence of roughly two dozen planets orbiting other stars. This weekend's announcement marks the first, direct evidence that extra-solar planets in fact exist.

"This is the first independent confirmation of a planet discovered through changes in a star's radial velocity and demonstrates that our indirect evidence for planets really is due to planets," said Geoff Marcy, a University of California, Berkeley astronomer.

Marcy and colleague Paul Butler, of the Carnegie Institution of Washington, have inferred the presence of 18 extra-solar planets by measuring wobbling changes in sun-like stars' velocities, which cause changes in their light. The theory, now confirmed, is that the changes are caused by the tug of orbiting planets' gravitational pull.

Marcy, Butler and fellow astronomer Steve Vogt, of the University of California, Santa Cruz, first noticed a telltale wobble in the star HD 209458 on November 5. Deducing the regularity of the wobble was due to the presence of a nearby planet, they estimated its orbit and approximate mass.

The star is 153 light-years from Earth (859,000 billion miles, or 1.4 million billion kilometers) in the constellation of Pegasus. About the same age, color and size of our own sun, HD 209458 is near 51 Pegasi, the star around which the first extra-solar planet was discovered in 1995.