Too lightweight to be stars but bigger than most planets, a handful of hot, young, freefloating objects have the raw materials to make their own mini-planetary systems, astronomers have reported.
Like some young stars, these planetary-mass objects, or "planemos," have disks of cosmic dust and gas circling them -- the ingredients for planet-making.
But planemos do not orbit stars, as normal planets do, said Ray Jayawardhana of the University of Toronto. He and other researchers presented their findings Monday at a meeting of the American Astronomical Society in Calgary, Canada.
The researchers detected four newborn planemos, just a few million years old, in a star-forming region about 450 light-years from Earth.
All four of these objects had dust disks around them, the astronomers reported.
Scientists also found a disk-skirted planemo interacting with a brown dwarf -- a failed star -- even closer to Earth, just 170 light-years away.
Such a planet-sized object might have been expected to be pulled into orbit around the brown dwarf, but instead the two revolve around each other.
These objects, several times the mass of the giant planet Jupiter but 1% the mass of our sun, fill the gap between the least massive stars and the most massive planets, Jayawardhana said.
When young, planemos are still warmed by the heat of formation and are more like stars, he said. But as they age, they shrink and cool.
If a satellite formed very close to a young planemo, it might be temporarily warm enough for liquid water to exist, a requirement for earthly life.
But Jayawardhana said that in the long run, life would have dim prospects: "Any kind of planet that forms around them is committed to an eternal freeze."