Astronomers have been hunting for Planet Nine — the large, mysterious body thought to lurk at the edge of our solar system — ever since researchers at Caltech published evidence of its existence last year.

Ordinary people have now joined the search. And they've made some intriguing finds.

Participants in a citizen science campaign hosted by the crowdsourcing program Zooniverse, and the BBC pinpointed four previously unknown objects in the outer solar system that could be candidates for Planet Nine, according to researchers at the Australian National University.

Through the project, dubbed “Planet 9 Search,” space enthusiasts and astronomers alike were given access to thousands of images taken by ANU's SkyMapper telescope. Their task was to find anything that appears to move against the mostly motionless background of distant stars. This is how astronomers have looked for new solar system bodies for hundreds of years. (See the photos that helped pinpoint Pluto below.)

In just three days, about 21,000 volunteers sifted through more than 100,000 images and classified more than 5 million objects — work that would take an astronomy PhD student four years, ANU astronomer Brad Tucker wrote in the Conversation. They surveyed vast swaths of the southern sky and managed to rule out the possibility of an unknown Neptune-size object in about 90 percent of it.

The four objects identified by the campaign are considered interesting enough that professional astronomers are taking a closer look. Much as Pluto did, they appear as tiny moving dots of light in the SkyMapper images; researchers don't know their distance or dimensions. Although these objects could be Planet Nine, it's more likely that they are dwarf planets, asteroids or perhaps mere blips in the data. Scientists at ANU and elsewhere will conduct further observations to figure this out.

Mike Brown is the Caltech astronomer who published evidence of Planet Nine's existence, theorizing its presence based on perturbations of other outer solar system bodies. He tweeted out support for the initiative last week.

This isn't the only citizen science effort to find the enigmatic ninth planet. NASA and the University of California at Berkeley are running a similar project, called Backyard Worlds, which gives planet seekers access to archived images from NASA's Wide-field Infrared Survey Explorer (WISE) mission.

The two projects are complementary, not competitive: Unlike the Planet 9 Search, which hunted for distant bodies in the visible light spectrum, Backyard Worlds studies the infrared light useful for finding fainter objects.

Astronomers say they have evidence of a ninth planet in our solar system. (Joel Achenbach, Gillian Brockell/The Washington Post)

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