Astronomers announced yesterday that, for the first time, they had seen the shadow of a planet outside Earth's solar system pass over its star, providing the most direct evidence yet of the existence of a faraway world.

"The significance is quite huge. This is the first time a planet has been detected in this way. It is sort of a direct detection. We actually see the effect of the planet on the star," said Paul Butler of the Carnegie Institution of Washington, who helped make the discovery. "You see it in shadow."

Astronomers have identified 28 planets outside our solar system since the first one was found in 1995. But the existence of all of them has been inferred from the very faint wobbles of their stars, presumably caused by the gravitational tugs of the planets as they orbit the stars. Butler and Geoffrey Marcy of the University of California at Berkeley pioneered this technique and used it to identify 20 of the putative planets.

In the new work, Butler, Marcy and Steve Vogt of the University of California at Santa Cruz detected a wobble in a star called HD 209458 on Nov. 5. The star is about 153 light-years (859,000 billion miles) from Earth in the constellation Pegasus. It is about the same age and size as Earth's sun.

The astronomers notified Greg Henry of Tennessee State University in Nashville. Henry, who operates several telescopes at the Fairborn Observatory in the Patagonia Mountains in southern Arizona, used an automated telescope to watch the star on Nov. 7, when Marcy and Butler predicted the planet would cross between the star and Earth. The astronomers hoped the planet would travel around its star in an orbit that made it pass over the star from Earth's perspective.

When Henry monitored the star, he saw a 1.7 percent drop in the star's brightness exactly when Marcy, Butler and Vogt predicted it would pass, indicating that the planet had indeed cast a shadow over the star.

"This is wonderful," Butler said in a telephone interview. "We can now learn so many things. We can derive all the orbital elements. This removes the ambiguity."

Based on the dimming, the astronomers estimated that the planet is a giant ball of gas similar to Jupiter but has a mass only about 63 percent that of Jupiter and a radius that is 60 percent bigger than Jupiter's.

The planet orbits its star every 3.523 days, which means it is so close that the planet's surface would be extremely hot, making it unlikely to be hospitable to any forms of life as we know it, Butler said.

In addition to confirming the validity of looking for wobbling stars to find planets, the discovery also provides astronomers with a new, more accurate way to determine the features of newly found planets.

"The future prospects here are absolutely delightful. When a planet crosses in front of the star, one of the effects is some of the light will leak out from around the planet," he said. "The spectrum of the planet will be impressed on the starlight. We can start learning the physics and chemistry of the atmosphere. We can begin studying it as a physical object."

Today, Henry plans to watch the planet pass its star again.

CAPTION: Based on the dimming of the star as the planet passed it, astronomers concluded that the planet is a giant ball of gas, bigger than Jupiter.