A possible free-floating exoplanet, drifting through interstellar space without a star of its own to orbit, has been discovered a hundred light years from Earth. If confirmed, it will be the first of its kind to be proven to exist - and there may be billions more like it out there.

An artist's impression of the rogue planet CFBDSIR2149, glowing with its own internal heat from its formation millions of years ago. Image: ESO/L Calcada/P Delorme/Nick Risinger (skysurvey.org)/R Salto/VVV Consortium
 
The object, named CFBDSIR2149, was discovered by the Canada-France-Hawaii Telescope on Mauna Kea and is part of the AB Doradus moving group, which is a cluster of around three dozen stars that formed together and are now loosely moving through space in unison. Although rogue planet candidates had been discovered previously, it has been impossible to accurately determine their age, which is crucial in determining their mass. As such, it is uncertain whether they can be classed as planets or as brown dwarfs, which are small, failed stars with masses between 13 and 80 times that of Jupiter, rendering them unable to reach temperatures hot enough in their cores to ignite the nuclear fusion of hydrogen atoms.

Planets and brown dwarfs begin life hot and gradually cool down with age at a rate depending upon their mass, their mass and age dictating their luminosity.

"A bright object can be either young and low mass, or old and very massive, and all the age/mass combinations in between," says Philippe Delorme of the Institut de Planetologie et d'astrophysique de Grenoble, France, who led the discovery team. Fortunately we know that the age of the stars in the AB Doradus group is between 20-200 million years old, which allows Delorme's team to place constraints on the properties of CFBDSIR2149. Knowing its age, distance and absolute luminosity Delorme's team have calculated the mass of the rogue object as between four and seven times that of Jupiter. This is much too small to be a brown dwarf and hence the only definition that makes sense is that of a gas giant planet.

The next big question is, how did it get out there? Some models of star-formation propose that planetary-mass objects can condense directly out of the pre-natal star-forming gas cloud like regular stars can. Alternatively, CFBDSIR2149 may have been expelled from a planetary system around one of the stars of the AB Doradus group following interactions with and gravitational perturbations by its sibling planets.

Although CFBDSIR2149 is the only known free-floating planet in the AB Doradus group, and the closest candidate rogue planet to Earth, our models for the origin of free-floating planets indicates that hundreds of billions could exist in the Galaxy and that many more may be awaiting discovery in the moving group and other areas of the nearby Universe.

"There could be many more free-floating planets that we have not identified yet," says Delorme. "Perhaps some of the known low-mass brown dwarfs are free-floating planets, but until we have an age we cannot say for sure what their mass is. We are currently checking if some of the 'weird' brown dwarfs we already know of could belong to moving groups."

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