A new survey has turned up more than two dozen failed stars, including one lightweight only about six times the mass of Jupiter.
The Canadian-led discovery could shed light on these strange objects, known as brown dwarfs, which straddle the boundary between stars and planets.
Brown dwarfs fail to accumulate enough material, and hence mass, to begin the nuclear fusion process that makes stars like our Sun shine.
Astronomers using the Subaru Telescope in Hawaii and the Very Large Telescope in Chile made optical and infrared observations of two clusters of young stars while searching for brown dwarfs. Their study has been accepted for publication in the Astrophysical Journal.
One paper focuses on the star cluster NGC-1333, which is 1,000 light years away in the constellation Perseus. Astronomers found that up to a third of stars in the cluster were actually brown dwarfs, raising questions about the kind of environmental conditions needed to encourage brown dwarf development.
New evidence on how they form
They also discovered one of the least massive brown dwarfs ever detected. Despite being the size of a large planet, it doesn't orbit a host star, raising questions about how it formed.
Stars form from the collapse of gas and dust clouds, while planets form out of protoplanetary disks of material left over from star formation.
Scientists believe most brown dwarfs form like stars. However, the small brown dwarfs may form like planets around a host star and are later ejected into interstellar space.
"The findings suggest [planet-sized] objects not much bigger than Jupiter could form the same way stars do," lead researcher Professor Ray Jayawardhana of the University of Toronto said.
"What's more, one cluster contains a surprising surplus of them, harbouring half as many of these astronomical oddballs as normal stars. In other words, nature appears to have more than one trick up its sleeve for producing planetary mass objects."
The second paper examines the star cluster Rho Ophiuchi, 460 light years away in the constellation Ophiuchus. Scientists discovered six new brown dwarfs, making up a fifth of the known stellar population in the cluster.
Star vs. planet blurs
Fred Watson from the Australian Astronomical Observatory said the papers indicate brown dwarfs may be more common than previously thought.
"The work sheds new light on the sort of conditions in which brown dwarfs form," he said. "It also pushes down the lower limit for brown dwarf star mass, previously thought to be at least 13 times Jupiter's mass. That figure was set because deuterium burning, which powers brown dwarfs, drops off below this point.
"All this further blurs the boundary between what we call stars and what we define as planets," he added.