Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse
Object: Gas giant planet
Name: CFBDSIR2149
Location: The AB Doradus moving group of stars
The giant planet doesn鈥檛 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.
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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 鈥� 鈥渇ailed鈥� stars that never grow big enough to ignite 鈥� 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 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 鈥� too small to be a brown dwarf. But to be certain, Delorme and colleagues needed some idea of its age. 鈥淚f you see a bright object, you cannot be sure if it鈥檚 small and very young, so it still has energy from its formation, or if it鈥檚 [old and] massive,鈥� says Delorme.
That鈥檚 where being in a stellar caravan comes in handy. Follow-up observations of the object鈥檚 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. 鈥淟ike when you kick a clod of sand, the grains don鈥檛 stick together anymore but they have the same common motion,鈥� 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 鈥� squarely in planet range. The team also estimate its temperature to be about 400 掳C.
All told, CFBDSIR2149 is now the best-studied 鈥渓onely鈥� planet, says co-author of the University of Montreal in Canada. 鈥淚t鈥檚 much better constrained than any other object of its kind,鈥� 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鈥檚 still possible that CFBDSIR2149 is not a member of the AB Doradus group, which would make it a very odd brown dwarf. 鈥淓stablishing membership definitively isn鈥檛 easy though, because the moving group members are spread across a wide swath of the sky, unlike members of a compact star cluster,鈥� says of the University of Toronto in Canada, who was not involved in the new work.
It鈥檚 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鈥檛 the only planet kicked to the curb. Planet surveys suggest there are more small worlds than large ones in the galaxy, and it鈥檚 easier to detach smaller ones from their stars.
鈥淚f it鈥檚 ejected, that would mean there are likely many frozen Earths and frozen Neptunes out in interstellar space,鈥� Delorme says.
Journal reference: Astronomy & Astrophysics,
