They want the backyard stargazers to sign up to a programme called TransitSearch, to spur the discovery of planets that pass between us and their parent stars. Most of the 100 extrasolar planets discovered to date have been detected by the wobbles they cause in their stars’ orbits.

But you can only get so much information about a planet using this method. If it passes directly between Earth and its parent star, however, astronomers can study how the star’s light dims during the transit.

So far, astronomers know of only one star, HD 209458, with an orbiting planet that passes in front of it in this way. Follow-up observations yielded details about the planet’s mass and density, as well as its composition and atmosphere.

“We’ve been able to do amazing science from this one case, but we definitely need to find more transits,” says Debra Fischer, a planet hunter at the University of California, Berkeley. Other transits are likely to be found among the other 99 planets, she adds. “But professional astronomers don’t have enough telescope time to follow up on them all.”

Off-the-shelf kit

TransitSearch was set up by Tim Castellano of NASA Ames Research Center and Greg Laughlin of the University of California, Santa Cruz, after they heard that Finnish amateur astronomer Arto Oksanen had charted the passage of the planet across HD 209458 using off-the-shelf equipment.

Thousands of amateurs around the world have similar equipment, so Castellano and Laughlin wondered if they could tap into their expertise. After duplicating the feat with an 8-inch commercial telescope, they started recruiting in California this summer.

They are now searching for amateurs worldwide to monitor stars already known to have planets orbiting them, and have posted a list of target stars and predicted transit times at .

“This may not be as exciting as discovering new planets, but it can yield a greater scientific pay-off,” Castellano says.

Always midnight

One advantage of observers around the globe, notes Laughlin, is that “it’s always midnight somewhere”. When it is cloudy at one site, conditions may be better at another. And if two telescopes record the same event, the observations gain more credence.

Collaborations between professional and amateur astronomers are not new, but they are growing because of the falling cost of high-quality equipment, now within reach of many amateurs. Ron Bissinger, a Californian amateur, spotted HD 209458 transits with a 4-inch telescope costing only $2000.

But the team may get more information than it bargained for, cautions Brian Marsden, who as head of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics is regularly besieged with observations sent by amateurs.

“The trick is getting capable amateurs on board without wasting time on people whose contributions aren’t useful,” he says.