Free Astronomy Magazine July-August 2016

G raphical re- presentation of the Large Syn- optic Survey Tele- scope inside its dome. This is an- other potential candidate for the discovery of Plan- et Nine. [Todd Mason, Mason Productions Inc./ LSST Corporation] Scott Kenyon (left) and Benjamin Bromley (below) have produced a study that com- plements that of Li and Adams. tem to gravitationally perturb Planet Nine’s orbit. As expected in these cases, not only the planet's orbit would have become considerably wider, but it would have also become decidedly more elliptical, far more than that of any planet familiar to us. In spite of the many variables involved, the positive thing about this scenario is that it can be verified – assuming, that is, to be one day able to directly observe Planet Nine: if its spectral prop- erties were to match those of a small gas giant, this would show that it comes from within the solar system; if, instead, it was more like a giant Pluto, then it could have formed in a much outer region, which is not very likely given the decreas- ing density of the protoplanetary disk at increasing distances from the Sun. Once the shape of the orbit of the hypothetical planet is exactly known, also its eccentric- ity will provide some information about the history of this object. And what about the scenarios according to which Planet Nine would actually be an exoplanet (whether rogue or not) captured by the Sun in very remote times? According to Li and Adams there is just a 2 percent chance that things happened exactly like that. But not all simulations provide equally pessimistic results. A re- cent study conducted by Alexander Mustill and Melvyn Davies (Lund University, Sweden), to- gether with Sean Ray- mond (Laboratoire d'As- trophysique de Bordeaux, France), has produced very different results which led the authors to conclude that it is highly likely that Planet Nine is a captured exoplanet. Also in this case such event would have taken place during the first million years of existence of the solar system, when the Sun was still in the open stellar cluster in which it was born, along with another thousand stars of va-