Object: Gas giant planet
Name: CFBDSIR2149
Location: The AB Doradus moving group of stars
Just drifting: artist's impression of CFBDSIR2149
(Credit: ESO/L. Calçada/P. Delorme/N. Risinger
(skysurvey.org)/R. Saito/VVV Consortium)
The giant planet doesn't even remember home. Once it was the largest in a system orbiting a young star, nestled in a star cluster that drifted peacefully through the galaxy. But a chance encounter with a massive foe ripped the cluster apart. The planet was separated from its star and sent wandering through space with a small band of stellar companions.
If this tale proves true, the loose gathering of stars known as the AB Doradus moving group is harbouring a newly discovered orphaned world. Known as CFBDSIR2149, the object may be the closest known rogue planet, offering the best glimpse yet of one of these bizarre worlds.
The object turned up during a recent survey of young
brown dwarfs – "failed" stars that never grow big enough to ignite – done with the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii. The warm brown dwarfs all glowed in the infrared. But the scientists noticed that one object, CFBDSIR2149, was unusually bright in a section of the near-infrared spectrum called the K band.
Such brightness suggests lower gravity, says Philippe Delorme of the Institute of Planetology and Astrophysics in Grenoble, France. Colliding hydrogen molecules absorb more K band light shining from the interior. But if an object has less gravity, it will have lower atmospheric pressure, which means fewer collisions and thus more K band light getting through.
Alone in a crowd
This hints that the object is less than 13 times the mass of Jupiter – too small to be a brown dwarf. But to be certain, Delorme and colleagues needed some idea of its age. "If you see a bright object, you cannot be sure if it's small and very young, so it still has energy from its formation, or if it's [old and] massive," says Delorme.
That's where being in a stellar caravan comes in handy. Follow-up observations of the object's motion through space show that it seems to be travelling with the AB Doradus group, a collection of about 30 stars of the same age and composition a mere 65 light years from Earth. These stars probably formed as part of a larger cluster between 50 million and 120 million years ago but got ripped away when it ran into a more massive obstacle, like a huge gas cloud.
The wanderers are no longer gravitationally linked, but they are headed in the same direction. "Like when you kick a clod of sand, the grains don't stick together anymore but they have the same common motion," Delorme says.
Frozen rogues?
The team calculates that CFBDSIR2149 has an 87 per cent probability of being an AB Doradus member. If so, it is also likely to be somewhere between 50 and 120 million years old, and would be between 4 and 7 times the mass of Jupiter – squarely in planet range. The team also estimate its temperature to be about 400 °C.
All told, CFBDSIR2149 is now the best-studied "lonely" planet, says co-author Étienne Artigau of the University of Montreal in Canada. "It's much better constrained than any other object of its kind," he says. Other candidates are either so distant and dim that they
show themselves only through gravitational effects, or their age or mass is too uncertain to be definitively classified as planets.
It's still possible that CFBDSIR2149 is not a member of the AB Doradus group, which would make it a very odd brown dwarf. "Establishing membership definitively isn't easy though, because the moving group members are spread across a wide swath of the sky, unlike members of a compact star cluster," says Ray Jayawardhana of the University of Toronto in Canada, who was not involved in the new work.
It's also not clear whether the world was ejected from its own solar system or formed alone from a small cloud of collapsing gas. But if it was ejected, it probably wasn't the only planet kicked to the curb. Planet surveys suggest there are more small worlds than large ones in the galaxy, and it's easier to detach smaller ones from their stars.
"If it's ejected, that would mean there are likely many frozen Earths and frozen Neptunes out in interstellar space," Delorme says.
