First Photo Taken of Planet Outside the Solar System

The first direct photographic evidence of a planet circling another star outside Earth’s solar system has been found by a team headquartered at UC Berkeley.

The planet is a bloated gas giant half again as big as Jupiter, but with only 63% of Jupiter’s mass, astronomer Geoffrey Marcy and his colleagues revealed Saturday.

Marcy, his collaborator Paul Butler of the Carnegie Institution of Washington and other astronomers have previously produced a variety of indirect evidence hinting at the existence of at least 28 other planets circling distant stars. But Saturday’s finding marks the first time a planet itself has actually been photographed.

Planets orbiting distant stars are normally too dim to be seen from Earth, even with the finest telescopes. In this case, however, astronomer Greg Henry of Tennessee State University was able to photograph the gas giant passing in front of the star HD 209458, producing an eclipse-like image.

The planet is scheduled to pass in front of its star again today and Henry hopes to capture further images of the extraordinary find.

“This is the first independent confirmation of a planet,” Marcy said. “And it also gives us the first-ever measure of the size of one of these planets.”

Although most cosmologists are convinced that planetary systems are common throughout the universe, proving it has been difficult. The best evidence to date has been provided by observing “wobbles” in distant stars.

Although we generally think of planets as orbiting suns, a planet and a sun actually orbit a common center of mass. Because the sun is so much bigger than the planet, its orbital motion is much smaller. Nonetheless, researchers have now observed many stars whose motion suggests that they are being orbited by a large planet.

Researchers have also seen stars surrounded by large discs of dust--the primordial material from which planets are formed. In some cases, they have even observed voids in the dust that they believe have been swept clean by newly forming planets. But until now, they have never been able to see a planet itself.

Marcy and Butler first detected a wobble in HD 209458 on Nov. 5. The star is 153 light-years from Earth in the constellation Pegasus, and is about the same age, color and size as our own sun.

Based on the star’s wobble, Marcy and Butler concluded that the planet orbits its star once every 3.523 days. They notified Henry of their discovery and told him the best chance of observing the planet was last Sunday.

Observing a planet is a hit-and-miss proposition because it can only be seen if its orbital plane brings it between the star and a telescope on Earth. With previously discovered wobbling stars, that has not happened.

In this case, however, the astronomers were lucky and the planet was captured on film. Henry observed a 1.7% dip in the star’s brightness as the planet passed in front of it.

“This planetary transit occurred at exactly the time predicted from Marcy’s observation, confirming absolutely the presence of a companion,” Henry said. “We’ve essentially seen the shadow of the planet and used it to measure the planet’s size.”

All of the data and observations “hang together” to indicate the existence of the planet, Marcy said. “This is what we have been waiting for.”

Although the planet’s mass is only 63% of Jupiter’s mass, its radius is 60% bigger than that of Jupiter. That conforms well to theories that predict a gas giant should be bloated when its orbit is close to its sun, Marcy said. The short orbital period indicates that the planet is very close to HD 209458, and that closeness suggests that the planet is also very hot and unlikely to have any life on it.

The team hopes to be able to learn a lot more about the planet. Because the planet passes in front of the star, some of the star’s light will pass through its atmosphere. Telescopes on Earth thus should be able to learn a great deal about its composition.