Free Astronomy Magazine January-February 2015

PLANETOLOGY els of density and brightness, not directly correlated with the distance from the star (not visible in the ALMA image). The two orbital regions considered more interesting by researchers are the more darker ones, located between 20 and 30 AU, and around 70 AU from HL Tauri. The more reliable simulations show that these less dense and less bright rings are generated concurrently with the formation of planets, which accrete their mass by sweeping up the gas and dust that initially filled those now-empty gaps. To suggest the presence of planets are not just the dimmer regions of the disk, but also the more brilliant ones, whose density undula- tions are consistent with an action of grav- itational confinement generated by plan- etary masses. Put simply, the material not swept up by planets is pushed to the inner and outer edges of the orbit, where it con- denses while remaining seemingly unper- turbed (actually, it will be swept up at a later evolutionary stage of the planetary system). The correspondence between what predicted by the numerical simulations and what found on the ALMA image shows that of planetary systems, the so-called “core- accretion model”. In summary, it predicts that the remnants of a star formation end up arranged in a disk structure around the star itself and that in that disk the grav-ita- tional force is such to concentrate the dust and gas in conglomerates of ever increasing propor- tions, up to forming com- ets, asteroids and even- tually planets. Computer simulations indicate that to reach this last stage it takes several million years (besides a suitable mass) and that it should not be possible to find traces of the presence of planets in the disk of a star as young as HL Tauri. But the image produced by ALMA is in di- rect contrast with expectations. In fact, as can be easily seen the protoplanetary disk is divided into a series of concentric circles (that appear as ovals due to the view point perspective), characterized by different lev- I n the above vid- eo, the shape of HL Tauri’s disk seen from a dif- ferent perspec- tive than that from Earth. In the side video, a short presenta- tion of ALMA with a summary of what discus- sed in this article. Below, a spectac- ular overview of part of the 66 an- tennas that make up the ALMA observatory. The facility, located on the Chajnan- tor plateau (Chile) about 5000 me- ters above sea level, is part of ESO. [ESO/M. Kornmesser, Luis Calçada et al., S. Guisard]