This bright blue marble is CFBDSIR2149. Located only 100 light-years away from Earth, this lonely cyan gem may be the very first errant planet detected by humans. And apparently it's quite beautiful, looking similar to Neptune* while being four to seven times as big as Jupiter.
CFBDSIR2149—I'm just going to call it Melancholia, even if it's not on a collision course with Earth—is not the first errant planet candidate. But according to the European Southern Observatory, it's the first time astronomers are pretty sure it's a real planet ejected from a solar system similar to ours, as opposed to a failed star.
Scientists have been detecting small isolated celestial bodies since the 1990s but they can't tell if they are real planets or failed stars that never ignited; the latter are known as cool brown dwarf stars. They couldn't study their age or atmosphere because they were too close to bright stars. Unlike those, Melancholia is the first time that astronomers have been able to study an errant body in such detail. And their conclusions seem to be pretty clear:
The team's statistical analysis of the object's proper motion—its angular change in position across the sky each year—shows an 87% probability that the object is associated with the AB Doradus Moving Group, and more than 95% probability that it is young enough to be of planetary mass, making it much more likely to be a rogue planet rather than a small "failed" star. More distant free-floating planet candidates have been found before in very young star clusters, but could not be studied in detail.
The team—lead by Philippe Delorme from Institut de planétologie et d'astrophysique de Grenoble, CNRS/Université Joseph Fourier, France—was actually hunting for brown dwarves when they were able to detect this object using ESO's Very Large Telescope and the Canada-France-Hawaii Telescope.
Unlike failed brown dwarfs, this object was most probably a planet that was ejected from a solar system like ours thanks to the interaction of gravitational forces. You know, as if someone told Jupiter to GTFO. According to Delorme:
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. 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.
The planet seems to be associated with the AB Doradus Moving Group, a group of about 30 stars that move at the same speed and have the same age and composition. The astronomers believe that this planet may have been ejected from one of the solar systems in the group. [European Southern Observatory]
* The planet is blue because of its atmospheric composition. This artist rendering—made by the ESO team—shows how it would look like if it were illuminated by a star like any planet in our solar system.