Astronomers have identified a few planets that wander through space without a parent star, some relatively close to our own solar system. The latest orphan object, named PSO J318.5-22, is 80 light years away from Earth. It is one of the lowest-mass free-floating objects found yet, with a mass six times that of Jupiter. Studying relatively low-mass objects that have no host star could give insight into the ‘early years’ of gas giants like Jupiter.
Exoplanets are usually discovered through indirect methods such as wobbling or dimming of their host stars induced by the planet. A few have been identified using direct imaging, but these exoplanets are difficult to study as they are right next to their much brighter host stars. Exoplanets like PSO J318.5-22 and another nearby orphan called CFBDSIR J214947.2-040308.9 (CFBDSIR2149 for short) have no host stars and so will be much easier to study. CFBDSIR2149 is 100 light-years from Earth and so astronomers have been able to study its atmosphere in great detail.
Free-floating planets are essentially rogue planetary-mass objects that are not committed to any particular star. Many candidates have been found before and recent studies indicate there may be large numbers of such rogues in our galaxy, potentially numbering more than the main sequence stars present. Previously astronomers had been unable to date such objects and thus were unable to determine if they were planets or brown stars (‘failed’ stars without the bulk to trigger enough reaction to shine as other stars do).
PSO J318.5-22 was identified using the Pan-STARRS 1 (PS1) wide-field survey telescope on Haleakala, Maui. It had a faint and unique heat signature and later observations using other telescopes showed it has properties similar to gas-giant planets orbiting around young stars. Astronomers were able to determine its distance from Earth by monitoring its position over two year. Based on the distance and its motion through space, the team concluded it is part of a collection of young stars called the Beta Pictoris moving group that formed about 12 million years ago.
CFBDSIR2149 seems to be part of a nearby group of young stars known as the AB Doradus Moving Group. The object was discovered through observations from the Canada-France-Hawaii Telescope, while the X-shooter spectrograph on ESO’s Very Large Telescope at the Paranal Observatory was used to examine the object’s atmosphere in detail. The AB Doradus Moving Group is the closest group to our Solar System. The stars within it are thought to have formed all at the same time; they drift through space together. Using the atmospheric characterization combined with the age and metallicity constraints from the possibility of the planet belonging to the AB Doradus Moving Group, the mass of CFBDSIR2149 is inferred to be approximately 4–7 times the mass of Jupiter, with a temperature of about 430ºC. The object’s age would be 50 to 120 million years, which is the same age as the moving group. The age of the object is only possible to know if the object is indeed part of the moving group; there is a small probability that the planet is only associated with the group by chance.
CFBDSIR2149, PSO J318.5-22 and other free-floating objects are thought to form as regular planets that have been kicked out of their home systems, or started as lone objects like ‘failed’ stars or the smallest stars.
The image is an artist’s impression an infrared view of CFBDSIR J214947.2-040308.9 with an image of the central parts of the Milky Way from the VISTA infrared survey telescope in the background. It appears blueish in this near-infrared view as much of the light at longer infrared wavelengths is absorbed by methane and other molecules in the planet's atmosphere. The object is so cool that in visible light it would only shine dimly with a deep red colour when seen close-up.
