For decades Pluto was the undisputed heavyweight champion in the far reaches of the outer solar system. Now astronomers know that the beloved world is just one of many known dwarf planets, most of which orbit the sun out beyond Neptune.

The discoveries that led to Pluto's demotion from planet to dwarf planet arrived in a rapid burst that peaked about a decade ago. Between 2002 and 2007 astronomer Mike Brown of the California Institute of Technology and his colleagues discovered several major objects, including the dwarf planets Eris, Makemake and Haumea (although another group also claims credit for Haumea). Since that flurry of activity, the discovery of large objects in the outer solar system has stalled, even though Brown's group left broad swaths of the sky unsearched.

The reason? Most of the big, bright objects have already been found, according to a new study. Megan Schwamb, a former graduate student of Brown's, now at the Academia Sinica in Taiwan, conducted a large-scale survey of the outer solar system, then extrapolated from the search to estimate the total numbers of objects. “It says that there are about 12,” Schwamb says, adding that nine are already known. “That's really telling us that we are pretty complete on the inventory of bright dwarf planets.” Schwamb and her colleagues published their findings in January in the Astronomical Journal.

Even though astronomers have not scanned the entire sky, they appear to have covered the areas laden with bright objects. It is possible, however, that a dwarf planet has escaped notice, says Darin Ragozzine of the Florida Institute of Technology. The starry plane of the Milky Way could obscure a dwarf planet, he notes, but it is unlikely that several await discovery.

“We had this golden age of finding these dwarf planets,” Schwamb observes. “That era is over.” But there may be similar objects farther out that are just too faint to spot today. “They're lurking in the shadows waiting for someone to detect them,” she adds.