Free Astronomy Magazine July-August 2016

PLANETOLOGY rious sizes. The high stellar density within the cluster fostered for about 100 million years the close encounters between stars, which could pass as close as a few hundred astronomical units from each other. Dur- ing those ‘flybys’, if a planet of a star were to find itself at the border edge of its sys- tem (for the reasons foreseen in Li and Adams’ scenario), it would have stood a good chance of being stolen. Mustill and colleagues calculated that if a planet had been stolen from another star by the Sun, its current orbit should be sim- ilar to the one it had around its original star (in the case of the two stars’ masses being comparable, otherwise its orbit would have been smaller), and that the relative velocity at which the two stars met had to be very low and similar to the orbital speed of the stolen planet, i.e. about 1 km/s. If the actual orbit of Planet Nine were to have a semi-major axis of about 700 AU, we could already have an idea of how the Sun got closer to the ‘despoiled’ star (or vice versa if you prefer). In fact, in order for a transfer of that type to take place, the minimum separation between the stars must be less than three times the distance between the parent star and the exoplanet, i.e. less than 2,100 AU in our case. Given that the Kuiper Belt was not destroyed by the encounter, we can say that the minimum distance between the two stars was surely greater than 150 AU. At this point, for assessing the verisimili- tude of the scenario according to which the explanet was captured by the Sun, we need to know how often and the distance within which these close approaches between stars occur in a typical stellar cluster consist- ing of a thousand stars. Studies carried out in this regard over the past decade show that most of the stars have at least one low speed ‘near miss’ with others within 1,000 AU, with minimum distances of 250 AU. It follows that Planet Nine could really be an exoplanet, and, in that case, we would have a unique opportunity to study up close an object otherwise unreachable with current technology. But first we must fig- ure out where it is hiding. n A lexander Mus- till, first author of a study accord- ing to which Plan- et Nine would ac- tually be an exo- planet. Right, the Wide-field Infra- red Survey Explor- er telescope, the instrument that more than others may have come closer to photo- graph the elusive Planet Nine.[NASA]