One cluster contains a surprising number of them - half as many as normal stars.
"Our findings suggest once again that objects not much bigger than Jupiter could form the same way as stars do," says professor Ray Jayawardhana of the University of Toronto.
"In other words, nature appears to have more than one trick up its sleeve for producing planetary mass objects."
Brown dwarfs are somewhere in between between stars and planets. They glow brightly when young, from the heat of formation, but gradually cool down to end up with planet-like atmospheres.
Most are believed to have formed in isolation, like stars, from contracting gas clouds. However, some of the smaller free-floaters may have formed like planets around a star and later been ejected.
Astronomers took extremely deep images of the NGC 1333 and rho Ophiuchi star clusters with the Subaru Telescope in Hawaii at both optical and infrared wavelengths. Once they identified candidate brown dwarfs from the very red colors, they confirmed them with spectra taken at Subaru and the Very Large Telescope (VLT) in Chile.
The six-Jupiter-mass brown dwarf found in the NGC 1333 cluster is one of the least massive free-floating objects known.
"Its mass is comparable to those of giant planets, yet it doesn't circle a star. How it formed is a mystery," says Aleks Scholz of the Dublin Institute of Advanced Studies in Ireland.
Several other newly identified brown dwarfs in both NGC 1333 and rho Ophiuchi clusters have masses below 20 times that of Jupiter.
"Brown dwarfs seem to be more common in NGC 1333 than in other young star clusters. That difference may be hinting at how different environmental conditions affect their formation," says the University of Toronto’s Koraljka Muzic.