Ames Researchers Confirm New Technique, Extra-Solar Planet

Using a Vulcan photometer at the Lick Observatory, the team of researchers from NASA's Ames Research Center in Silicon Valley, CA, have demonstrated that the "transit photometry" method is a viable way to detect extra-solar planets. Transit photometry works by measuring the change in a star's brightness as an orbiting planet transits its face.

"We are extremely pleased about this proof of our technology," said Mr. William Borucki, principal investigator at Ames. "This will give the scientific community a valuable additional tool to use in astrobiology research."

The planet, orbiting star HD209458, was discovered by Dr. David Charbonneau and colleagues at the High Altitude Observatory in Boulder, CO in September and announced in early November. The data showed that the diameter of the planet was about 1.3 times that of Jupiter, and that it had an orbital period of 3.52 days.

Three different research groups at Lick attempted to make observations of the transit on November 14. However, heavy clouds precluded success until November 21, researchers said. The November 21 observation window was marginal because the planet's transit across the face of its parent star occurred during a full moon and just before the star set, resulting in a bright sky and a rapidly changing atmospheric transparency. The Ames team's Vulcan photometer succeeded in observing the planet because it monitors a huge slice of sky and measures thousands of nearby stars. Such large amounts of data allowed the photometer equipment to correct for the poor conditions.

Ordinarily, the photometer monitors approximately 6,000 stars each night. Discoveries of possible planets using photometry -- which reveal a planet's size, transit times, orbital period and the identity of its parent star -- are then forwarded to other research groups who use the "Doppler velocity" method to obtain the mass of the star-companions. The Doppler velocity method monitors changes in the color of a star as it moves towards or away from the Earth.

The two planetary observation techniques are complementary, researchers say. Used together, scientists can make inferences about an extra-solar planet's density and composition, providing a more complete picture. Combining the data from the Doppler velocity and photometry observations, for example, "it is clear that the planet's atmosphere is inflated by its closeness to the central star and that the planet has a density of approximately 40% the density of water, much less dense than Saturn," Borucki noted.

Plans for future research using the photometry technique include extending the current ground-based observations to a space-based platform in 2004, the researchers said. Unlike the current ground-based methods, which must deal with atmospheric "noise" and other problems, observations from a small telescope in space will allow Earth-sized planets to be discovered and will determine if the planets are in the habitable zone, Borucki said. Ground-based observation techniques are primarily designed to locate and observe large, Jupiter-sized gas giants orbiting other stars and are not able to observe planets as small as the Earth.

The Ames Vulcan photometer was designed and built at NASA Ames Research Center, with funding from the Astrobiology program. It was installed only a few days before it was called on to monitor HD209458 to confirm the existence of the planet.