Pluto, please step aside. The beleaguered world has for the past several years been at the center of the debate about what defines a planet. Now a new crop of astronomical objects has arrived to further cloud the matter.

The Pluto issue was formally resolved in 2006, when the International Astronomical Union (IAU) relegated Pluto and its diminutive ilk to dwarf planet status. (The conversation has not stopped, however; among a number of recent Pluto-centric books are titles like The Case for Pluto and How I Killed Pluto and Why It Had It Coming.)

Outside the solar system the issue is less clear—what distinguishes a star from a brown dwarf, which is a sort of failed star not massive enough to burn hydrogen in fusion reactions? What distinguishes a brown dwarf from a planet? And now, most puzzling of all, what to call an object that seems too small to be anything other than a planet but that floats freely through interstellar space rather than orbits a star? Such objects are numerous in the galaxy, and possibly more numerous than ordinary stars, a new study concludes.

Two astronomical collaborations report in the May 19 Nature that they have located a population of 10 celestial objects, each with about the mass of Jupiter, with no detectable host star. (Scientific American is part of Nature Publishing Group.) By extrapolation, the study's authors, from the Microlensing Observations in Astrophysics (MOA) collaboration and the Optical Gravitational Lensing Experiment (OGLE) collaboration, calculate that there should be almost twice as many such objects in the Milky Way as there are stars. Some of the newfound objects may simply orbit a star at a distance so great that their host star is not apparent, but the researchers estimate that most of them are indeed free-floating.

"This is a pretty remarkable claim," says astronomer Mark McCaughrean of the European Space Agency. And it will no doubt undergo close scrutiny. But if the interpretation proves correct, a future NASA mission could turn up such free-floating bodies in droves. A planned spaceborne observatory called the Wide-Field Infrared Survey Telescope (WFIRST) should be able to discover 1,000 such objects, says study co-author Takahiro Sumi of Osaka University in Japan, a member the MOA group.

Other researchers have expressed cautious enthusiasm about the research. "This is a very exciting discovery, but still it has to be confirmed by other teams, other techniques, more statistics and so on," says Joachim Wambsganss of the University of Heidelberg in Germany, who wrote a commentary accompanying the research in Nature. "It's plausible" that these planetlike objects are indeed adrift in the galaxy rather than simply loosely bound to a star, Wambsganss says. "I wouldn't say it's proved. I wouldn't bet my house on it, let me put it this way, but it's pretty convincing."

Microlenses for Macro Objects
MOA and OGLE look for so-called microlensing events in the night sky. Microlensing occurs when a chance alignment brings an otherwise unseen celestial object between Earth and a distant background star. The celestial object's mass bends light rays from the background star, focusing the light like a magnifying lens toward Earth and making the background star appear temporarily brighter. Those events are rare, so microlensing searches monitor large numbers of stars for long periods of time; the new discovery came out of a MOA survey of 50 million stars in 2006 and 2007.

The duration of a microlensing event indicates the mass of the celestial object that acts as the lens. The MOA group found 10 microlensing events that lasted less than two days, which points to a lens of at most a few times Jupiter's mass. The independent OGLE group broadly confirmed that finding. But none of the objects were accompanied by a stronger lensing signal indicating a more massive host star nearby. So either the objects are drifting alone through the galaxy or they orbit their stars at a great remove. But there seem to be far more of these objects than can be explained by wide-orbit planets, and also far more than can be explained by very lightweight stars or brown dwarfs.

The researchers speculate that these newfound objects may have formed in relatively ordinary planetary systems before losing a game of gravitational tug-of-war with sibling planets and being flung clear of their birthplaces. "This implies that star formation must be producing many planets that are ejected away from their star," says Pavel Kroupa of the University of Bonn in Germany. Those ejections, Kroupa says, could even be caused by nearby stars perturbing a planetary system. "Clearly any theory of star formation will need to account for this population of free-floating planetary-mass objects," he adds.

Planet or Dwarf?
Free-floating objects in the planetary-mass range have been found before, but they were located in star-forming regions of the galaxy and had a bit more heft than the newfound objects. That led some astronomers to think of them as being more akin to lightweight brown dwarfs than to planets—that is, they probably formed on their own rather than in orbit around a larger parent body. But the newfound objects seem to be solidly in the planetary-mass range and also seem to dot the galaxy, not confining themselves to regions where stars and failed stars such as brown dwarfs are being born.

As to what to call these newfound objects, Wambsganss favors brevity. "I think the most intuitive name is 'free-floating planets,' but if we decide to adopt that name then we have to give up one of our definitions of a planet," he says. "A free-floating planet is a contradiction, because a planet is by definition bound" in an orbit around a star.

That contradiction will no doubt fuel controversy—McCaughrean calls "free-floating planets," a term that appears once in the new study, "a red rag to a bull." Even the more conservative "free-floating planetary-mass objects" can be misleading, McCaughrean says. "To me, that's somewhat still equivalent to calling a Chihuahua a 'cat-massed object,'" he says.

Sumi predicts that his and his colleagues' new discovery will spur discussion at the IAU, much like the 2006 debate that led to Pluto's demotion. "I think that people have not thought seriously about this yet because there is little evidence of these objects so far," he says. So what is Sumi's take? If a celestial object formed just like any other planet, which he and his colleagues suspect is the case for their newfound objects, then he says "it is natural to be called a planet."