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.

They then turned to Greg Henry, a Tennessee State University astronomer who leads one of a handful of teams seeking to directly spot an extra-solar planet as it crosses, or transits, the face of its star.

(E.g., the planet Mercury will make such a crossing of our own sun on Monday, an event visible from much of Earth.)

On November 7, Henry used group of remotely operated telescopes in southern Arizona to observe a 1.7-percent dip in the star's brightness, indicating that a planet was likely crossing its disk. Like a solar eclipse on a much smaller scale, the passing shadow cast by the enormous planet - thought to be 60 percent larger than Jupiter -- caused the dimming.

"This planetary transit occurred at exactly the time predicted from Marcy's observations, confirming absolutely the presence of a companion," Henry said.

Henry was planning further observations on Sunday to catch the planet again as it passed between Earth and the star during its 3.523-day orbitational period.

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.

Astronomers have already calculated the planet's density and mass. The latter is just 63 percent that of Jupiter, despite its size, which indicates it also is a gas giant.

In an interview with space.com just a week ago, Marcy admitted to being "a little bit" worried no transit had yet been observed.

"With this one, everything hangs together," Marcy said following the announcement of the discovery. "This is what we have been waiting for."