Astronomers at NASA and the University of California at Santa Cruz have launched a Web-based project that has amateur astronomers lining up to have a chance to discover extra-solar planets that 'transit' or pass in front of their parent stars.

Of the more than 100 known extra-solar planets discovered so far, only one (called HD 209458 b) is known to pass in front of its star, as seen from Earth. The small dimming of a star during such a transit will allow amateur astronomers to perform valuable measurements that can aid scientists by determining the planet's size, and potentially reveal the planet's atmospheric composition and the presence of rings or moons in orbit around it.

"We welcome the assistance of a large number of dedicated and experienced amateur astronomers around the world to add to our understanding of the nature of extra-solar planets," said Dr. Tim Castellano, an astronomer based at NASA Ames Research Center and co-investigator of the Web project.

During the nights of Oct. 5 and Oct. 30, backyard sky-watchers will get their chance. On those dates, a planet twice as big as Jupiter, orbiting the star HD 68988, has an 8 percent chance of passing in front of its star, giving amateur astronomers the chance to confirm the existence of a Jupiter-sized planet outside our solar system. The star, located near the Big Dipper in the northern sky, is too faint to see without a telescope.

The basic search technique used by the amateurs scanning the autumn skies this month will involve taking a night-long series of electronic images of the star HD 68898 and surrounding stars. The astronomers will use these images and specialized software to look for small changes in brightness characterizing a planet's transit, a technique called 'transit photometry.'

Candidate stars such as HD 68898 for the amateur astronomers to observe, are chosen using the "wobble method," by which the first extra-solar planet was detected in 1995. With this method, professional astronomers use large telescopes to watch for the minute 'wobble' (the Doppler shift) of a star caused by the tug of an unseen planet orbiting it.

"The participation of dozens of astronomers means that more planet-bearing stars can be scrutinized during the intervals when possible transits are predicted," said the site's co-creator, Dr. Greg Laughlin, an assistant professor of astronomy at the University of California, Santa Cruz. Having observers in various locales around the world will provide 24-hour availability and reduce the dependence on local weather, he said. "The signature of a planetary transit is very subtle, so multiple simultaneous observations provide a vitally needed redundancy."

The two researchers work as a team, with Laughlin serving as the theorist, calculating transit times and probabilities based on the radial velocity data from the California Planet Search team. Castellano is the observer, who demonstrated that the necessary measurements can be made with only a small backyard telescope. He will provide guidance on observing and data analysis to the amateur astronomers.

Requirements for astronomers who want to sign up for an observing shift are a computer-controlled telescope, a charge coupled device (CCD) camera and personal computer, and software to record and analyze the small changes in stellar brightness that will be produced by a planet. Previous experience measuring the brightness of variable stars or success in observing the known transit of HD 209458 with a CCD camera also are highly recommended, the researchers say.

Interested participants should visit: www.transitsearch.org for exact transit time predictions and further details.

The development of the Website was partially supported through the NASA Ames Research Center's Director's Discretionary Fund, via a grant of two-year startup funds.

In December 2001, NASA selected the Kepler Mission, a project based at NASA Ames, as one of the next NASA Discovery missions. The Kepler Mission, scheduled for launch in 2006, will use a spaceborne telescope to search for Earth-like planes around stars beyond our solar system.