Free Astronomy Magazine May-June 2016

PLANETOLOGY to their mutual attraction. The fact is, how- ever, that with this technique the planets re- main invisible, in the sense that they are not directly observable, and thus do not lend themselves to an easy spectroscopic investi- gation. It is true that if we subtract the stel- lar spectrum, it is possible with difficulty to extract the more obvious lines left by the chemical elements of the atmosphere of a planet in transit, but if there is no atmo- sphere, then no information can be obtain- ed. This problem does not exist for a par- ticular class of planets, so-called ‘free-float- ing’, that roam through space since not bound by the gravitational relationship with the star around which they formed. The latest of these objects was discovered inside a young stellar association known as the Beta Pictoris moving group, whose members are characterized by the common motion of translation in space. The new planet, called PSO J318.5338-22.8603 (PSO J318-22 for short), has been detected by a team of researchers led by Katelyn Allers, Bucknell University, through spectroscopic observations made in the near infrared with the Gemini North telescope. The survey made it possible to calculate the rotational and radial velocities of the object. The latter, combined with the already known tangen- tial velocity (and spatial position), has con- firmed that PSO J318-22 is a member of the Beta Pictoris moving group, allowing to at- tribute to the object the same age of the other members, about 23 million years old. Once established its age, the extremely red colour and apparent brightness, Allers and colleagues were able, through appropri- ate evolutionary models, to determine that the object is not, as we would have expected, a failed star (a brown dwarf) but a rogue planet with roughly 8.3 times the mass of Jupiter and a ‘surface’ tempera- ture of 1,130 Kelvin. By combining the spectroscop- ic data with equally up-to- date data on the photo- metric variability of PSO J318-22, the researchers also found that its axis of rotation appears inclined of at least 29 degrees, and that its rotation period is between 5 and 10.2 hours, with an equatorial velocity that is consistent with the relationship between mass and velocity found in the giant planets of our solar system. PSO J318-22 is the third known planet in the Beta Pictoris mov- ing group (the only rogue one), while the others are 51 Eridani b, slightly larger than Jupiter, and Beta Pictoris b, about 11 Jupi- ter masses. The intermediate mass of PSO J318-22 and the absence of a star in its im- mediate vicinity make it an excellent labo- ratory for testing our theories of planetary evolution. INVESTIGATED THE ATMOSPHERE OF A SUPER-EARTH After several unsuccessful attempts, astron- omers were able to examine the composi- tion of a super-Earth’s atmosphere for the first time. The planet in question, Cancri e A real image of the rogue planet PSO J318- 22, which, by being very young and hot, it is par- ticularly bright in the red and near- infrared. [N. Met- calfe/Pan-STARRS 1 Science Consor- tium]