ORPHAN PLANETS RIPPED AWAY FROM THEIR STELLAR PARENTS OR BORN IN ISOLATION MAY VASTLY OUTNUMBER STARS
Ever since astronomers first began discovering planets outside of the Solar System, dubbed ‘exoplanets’, in the mid-1990s, our picture of the Solar System as a ‘typical’ arrangement of planets and stars has been turned on its head. Among the 5,600 or so exoplanets that we have discovered over the course of the last three decades are scorching hot worlds bombarded by so much stellar radiation that they rain molten metal, planets that race around their stars so fast they can fit several years into a single day and planets squashed by the gravity of their stars so much they have flattened discs. While all of these discoveries have changed our perspective of the Solar System and its planets being ‘typical’ and may have even made our corner of the Milky Way look slightly mundane by comparison, they all share a similar theme: planets orbiting stars – albeit sometimes strangely, violently or chaotically.
In 2012, the universe let humanity know that even this ever-so-familiar concept can’t be taken for granted. That’s the year that astronomers operating the Canada-France Brown Dwarfs Survey discovered the first possible ‘rogue planet’, a cosmic orphan that drifts through the cosmos away from a parent star. Rogue planets go by a range of alternative names, including interstellar, nomad, orphan, starless, unbound or wandering planets, but the more official names for these worlds are free-floating planets (FFPs) or isolated planetary-mass objects (iPMOs).
The classification of iMPOs also includes so-called ‘failed stars’ called brown dwarfs. These are objects with masses up to around 12 times that of Jupiter – just outside the range of the largest planets – which form in isolation like stars but never quite gather enough mass to trigger the nuclear fusion of hydrogen to helium in their cores. Because they also wander the universe alone, it can be difficult to distinguish brown dwarfs from high-mass rogue planets.
The few hundred planetary rogues discovered thus far are merely the tip of the iceberg. Last year, a nine-year survey called Microlensing Observations in Astrophysics (MOA), conducted by scientists from NASA and Japan’s Osaka University, suggested that rogue planets in the Milky Way far outnumber planets that orbit stars, also outnumbering those stars by around 20 times. “Rogue planets could vastly outnumber the stars in the Milky Way. There are 1 to 300 billion stars in the Milky Way, so there could be trillions of free-floating planets,” NASA’s Jet Propulsion Laboratory postdoctoral fellow Samson A. Johnson tells All About Space. “The fact that we’re finding any at all means there’s a lot, but just exactly how many is a lot is hard to say right now.”
