Counting Rogue Planets

A new study suggests rogue planets-- planets not associated with any star-- may be more common than main sequence stars in the galaxy, and may outnumber orbiting planets by fifty percent. The study emphasized Jupiter-mass worlds, but the objects no doubt range from smaller planets to brown dwarfs-- failed stars.

If the number of rogue planets is in fact so huge, scientists say, current theories of planetary formation will need to be expanded. The accretion model alone, in which planets build up slowly, couldn't produce the numbers implied by so many rogue worlds. There may be a parallel to the history of geology. Early modern scientists argued geologic processes slowly shaped Earth's surface, but now we see catastrophic events as also playing a role.