Previous research suggested there may be as many as 200 billion free-floating planets in the Milky Way galaxy, with most astronomers assuming such "rogue planets," which don't orbit a star, must have been ejected from existing planetary systems.
Not necessarily, the Swedish researchers said; they might also be born free.
Along with colleagues in Finland, scientists at Sweden's Chalmers University of Technology and Stockholm University used several telescopes to observe the Rosette Nebula, a huge cloud of gas and dust 4,600 light-years from Earth, a Chalmers release reported Tuesday.
A number of tiny dark clouds drew their attention, they said.
"The Rosette Nebula is home to more than a hundred of these tiny clouds -- we call them globulettes," project leader Gosta Gahm of Stockholm University said.
"They are very small, each with diameter less than 50 times the distance between the sun and Neptune.
The researchers made measurements to determine the bodies' mass and density.
"We found that the globulettes are very dense and compact, and many of them have very dense cores," Chalmers astronomer Carina Persson said. "That tells us that many of them will collapse under their own weight and form free-floating planets."
In the history of the Milky Way countless millions of nebulae like the Rosette have bloomed and faded away, the researchers said, and in all of these many globulettes would have formed.
"If these tiny, round clouds form planets ... they must be shot out like bullets into the depths of the Milky Way," Gahm said.
"There are so many of them that they could be a significant source of the free-floating planets that have been discovered in recent years."