An international team of astronomers using ESO’s Very Large Telescope and the Canada-France-Hawaii Telescope has detected a rogue planet, an interstellar object wandering through space without a parent star, at a distance of about 100 light-years.

Rogue planets, also known as interstellar, nomad, free-floating or orphan planets, are planetary-mass objects that roam through space without any ties to a star. Possible examples of such objects have been found before, but without knowing their ages, it was not possible for astronomers to know whether they were really planets or brown dwarfs – ‘failed’ stars that lack the bulk to trigger the reactions that make stars shine.

The newly detected object, labelled CFBDSIRJ214947.2-040308.9 (CFBDSIR2149 for short), is about 4-7 times more massive than Jupiter and probably belongs to a nearby stream of young stars known as the AB Doradus Moving Group.

The AB Doradus Moving Group is the closest such group to the Solar system. Its stars drift through space together and are thought to have formed at the same time. The link between the new object and the moving group is the vital clue that allows astronomers to find the age of CFBDSIR2149. This is the first isolated planetary mass object ever identified in a moving group, and the association with this group makes it the most interesting free-floating planet candidate identified so far.

“Looking for planets around their stars is akin to studying a firefly sitting 1 cm away from a distant, powerful car headlight,” said Dr Philippe Delorme of the Institut de planétologie et d’astrophysique de Grenoble in France, lead author of a paper reporting the discovery in the journal Astronomy & Astrophysics (ESO’s version).

“This nearby free-floating object offered the opportunity to study the firefly in detail without the dazzling lights of the car messing everything up.”

Rouge planets like CFBDSIR2149 are thought to form either as normal planets that have been booted out of their home systems, or as lone objects like the smallest stars or brown dwarfs. In either case these objects are intriguing – either as planets without stars, or as the tiniest possible objects in a range spanning from the most massive stars to the smallest brown dwarfs.

“These objects are important, as they can either help us understand more about how planets may be ejected from planetary systems, or how very light objects can arise from the star formation process,” Dr Delorme said. “If this little object is a planet that has been ejected from its native system, it conjures up the striking image of orphaned worlds, drifting in the emptiness of space.”

These worlds could be common – perhaps as numerous as normal stars. If CFBDSIR2149 is not associated with the AB Doradus Moving Group it is trickier to be sure of its nature and properties, and it may instead be characterized as a small brown dwarf. Both scenarios represent important questions about how planets and stars form and behave.

“Further work should confirm CFBDSIR2149 as a free-floating planet,” Dr Delorme said. “This object could be used as a benchmark for understanding the physics of any similar exoplanets that are discovered by future special high-contrast imaging systems, including the SPHERE instrument that will be installed on the VLT.”

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Bibliographic information: P. Delorme et al. 2012. CFBDSIR2149-0403: a 4–7 Jupiter-mass free-floating planet in the young moving group AB Doradus? Astronomy & Astrophysics, vol. 548, A26; doi: 10.1051/0004-6361/201219984