Astronomers have been looking for alien worlds for more than 15 years, and now you too can join the search.

The Planet Hunters project is the latest citizen-science campaign organized by the crew at Zooniverse. Hundreds of thousands of computer users are already helping Zooniverse classify galaxies through Galaxy Zoo, and analyze lunar craters through Moon Zoo. This new project aims to recruit users to check data gathered by NASA's Kepler mission, which is expected to detect hundreds of Earthlike planets in a region of the constellation Cygnus.

Kepler's science team detects planets by looking for the slight dimming in a star's light that's caused when a planetary disk passing over. By making precise measurements of that periodic dimming, astronomers can figure out how big the planet is, then follow up with other types of observations to confirm its existence and estimate its mass.

More than 500 planets have been detected beyond our solar system, and Kepler is just getting started.

"The Kepler mission will likely quadruple the number of planets that have been found in the last 15 years, and it's terrific that NASA is releasing this amazing data into the public domain," Yale astronomer Debra Fischer, a pioneer in the search for exoplanets, said in a news release.

The Planet Hunters project is not tied directly to the Kepler mission, but will serve to complement the studies being done by the Kepler team. The first big public release of Kepler data is scheduled to occur in February.

Astronomers are using computers to crunch the data from the Kepler probe and look for planet candidates. "But computers are only good at finding what they've been taught to look for, whereas the human brain has the uncanny ability to recognize patterns and immediately pick out what is strange or unique, far beyond what we can teach machines to do," Meg Schwamb, another Yale astronomer and Planet Hunters co-founder, said in today's news release.

Right now, the Planet Hunters program is compatible with the Firefox, Safari, Chrome and Opera Web browsers. "We aim to bring support for other browsers, including Internet Explorer, in early 2011," the organizers say.

When users log onto the Planet Hunters website, they'll be asked to answer a series of questions about a light curve from a distant star. The answers to those simple questions will help the Yale astronomers determine whether that particular star is displaying a pattern of dimming that could point to the existence of a planet.

"The great thing about this project is that it gives the public a front-row seat to participate in frontier scientific research," Schwamb said.

The online search process may have to be tweaked as time goes on, because it can be difficult to pick out the weak signal created by an Earth-scale planet as it crosses an alien sun. "Planet Hunters is an experiment — we're looking for the needle in the haystack," Fischer said.

But Galaxy Zoo has already proven that regular folks can make a real contribution to science. Several Galaxy Zoo users have been listed as co-authors on the more than 20 published scientific papers that have resulted from the project. The organizers of Planet Hunters are hoping to achieve a similar feat.

"When you join Planet Hunters, you're contributing to actual science — and you might just make a real discovery," said Yale astronomer Kevin Schawinski, who was involved in Planet Hunter's genesis as well as Galaxy Zoo's creation.

More about the planet search:

• How many alien Earths? More than expected
• How do you find life on an alien planet?
• Interactive: The search for other planets
• Interactive: The new solar system