Sallie Baliunas, a Harvard-Smithsonian astronomer and adjunct professor at TSU, said the search looks not for the shadow of the planet per se, but the slight dimming it would cause to the stars light as it crosses its disk. Since the planets in question are thought to be Jupiter-sized, a transit would lead to an estimated 1 percent change in the candidate stars brightness.
"The change would be easy to see," Baliunas said. "We could confirm in a positive way that there are transits."
Although indirect evidence leaves many astronomers all but certain the planets indeed exist, others eagerly await the observation of a transit as final proof.
"A transit will mark the first definitive confirmation of the discovery of planets, which up to now depends precariously on the Doppler measurements," said Geoff Marcy, an astronomer who has led the search for extra-solar planets with his colleague Paul Butler.
The Tennessee team has used a remotely operated constellation of seven telescopes in Patagonia, Arizona, to observe nine candidate stars. So far, the team has been without luck.
"Its beginning to get interesting," said Greg Henry, the TSU astronomer heading the program. "Eventually, if the planets are there, we have to see transits."
Such an observation would -- literally -- open those new worlds to further discovery, since it would allow astronomers to calculate the planets diameter. Armed with information about the planet's size and mass, astronomers can calculate its density.
"The density tells us if the planet is gaseous, like Jupiter, or solid like the Earth," Marcy said. "If solid, the chances are stunningly apparent that liquid water could puddle on their surfaces, perhaps into lakes and oceans. This water could then serve as the solvent for biochemical reactions."
Baliunas said she expects to see a transit within the next two years.
"Its going to be shooting fish in a barrel," Baliunas said.
Although transits are fairly rare -- Mercury, for example, passes between the Earth and sun only once every eight years or so -- they are predictable.
"If we observe 15 or 20 stars that supposedly have a close planet, but none shows a transit, I will be terrified out of my tenure," joked Marcy, who admits to being "a little bit" worried that none have yet to be observed.
If Earth-based efforts do not pay off, the search will soon receive a space-borne boost. In 2005, NASA plans to launch the Space Interferometry Mission. The flying observatory will allow astronomers to probe nearby stars for planets. The Terrestrial Planet Finder, another space-borne observatory, would follow in 2011. It would allow astronomers to pinpoint and study extra-solar planets with unprecedented accuracy.
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