Some 1,150 light-years distant, the possible planets are gargantuan by local standards, dwarfing Jupiter many times over. Researchers say they might not be planets at all, but rather brown dwarfs, compact star-like objects that never amassed enough material to fuel the nuclear burning that heats a real star.

While more than 50 extrasolar planets have previously been identified, these are the first that don't orbit a star, instead nestling in the gravitational matrix of a star cluster. They are also the first for which scientists have gathered information about temperature and composition by analyzing the light spectrum reflected off the objects' surfaces.

Current theories hold that it takes tens of millions of years of cosmic engineering -- when gas and dust swirl and gather -- to create a large round collection of material that qualifies as a planet. Our own solar system, as an example, is billions of years old.

But the region of the cosmos where the 18 planet-like objects were found is thought to be no more than 5 million years old.

"The formation of young, free-floating, planetary-mass objects like these are difficult to explain by our current models of how planets form," said lead author Maria Rosa Zapatero Osorio, a Spanish researcher currently working at the California Institute of Technology.

Others agree: "There is no consensus yet on how to form free-floating planet-mass objects," says David Charbonneau of the Harvard-Smithsonian Center for Astrophysics. "Tackling this question over the next few years will be a lot of fun."

Charbonneau would know. Last year, he and colleagues were among the first to use direct observations of light to identify extrasolar planets. Before that, astronomers could only presume their presence by noticing how a suspected orbiting planet created a slight wobble in a central star.