Free Astronomy Magazine March-April 2015

SOLAR SYSTEM interested in Scholz’s Star and started to spectroscopically analyze it in white light with the Southern African Large Telescope, and in near-infrared light with the Magellan telescope at the Las Campanas Observatory. It was immediately evident that the red dwarf is moving away at a speed of about 80 km/s, and its apparent motion across the sky, estimated at just 1 arcsecond per year (corresponding to a tangential velocity of 3 km/s), revealed that it is moving away from a direction almost coincident with that of the Sun. It was indeed the case to investi- gate the matter further, and to that end Eric Mamajek and colleagues simulated 10,000 possible orbits of Scholz's Star and of the Sun around the centre of the Galaxy. From the calculations resulted that even by taking into account all observational uncertainties, about 70,000 years ago that red dwarf pas- sed just 0.82 light-years from our star, a dis- tance equivalent to 52,000 astronomical units (AU) or 54 times closer than Proxima Centauri, and actually inside the outer part of the Oort Cloud, which extends from 20,000 AU from the Sun out to about 1.5 light-years. According to the researchers in- volved in the study (including Scott Baren- feld, California Institute of Technology, and Valentin D. Ivanov, European Southern Ob- servatory), the probability that Scholz’s Star really penetrated the outer Oort Cloud is equal to 98% (with 79% of the simulations placing it at less than 1.1 light-years from the Sun), while the probability that it may have reached the inner Oort Cloud, name- ly between 2,000 and 20,000 AU, is just 1 in 10,000. (Only in the latter case the proba- bilities of triggering a comets shower in the inner solar system would be rather high.) The uniqueness of this discovery, made offi- cial in February in The Astrophysical Journal Letters , is shown by the fact that the best candidate to date for a close encounter with the Sun was the star HIP 85605, which in about 332,000 years (subject to a wide mar- gin of uncertainty) should pass at a dis- tance of only 0.33 light-years, hence on the edge between the outer and inner part of the Oort Cloud. But the same Mama- jek’s team has recent- ly shown that the distance of HIP 85605 had been underestimated by about ten times (20 light-years instead of 200) and that conse- quently the predictions were incorrect; with- out considering that what interest us most are the past intrusions rather than the fu- ture ones, as are in fact the former that can have the most immediate consequences on our planet. Mamajek and colleagues esti- mate that, based on the high speed and the low mass of Scholz's Star (which is actually a binary system consisting of a small red dwarf of 0.08 solar masses and a substar of 0.06 solar masses), its transit through the Oort Cloud could not have been particularly devastating from the dynamic point of view, but it could still have pushed an indeter- minable number of cometary nuclei towards the planetary system, along highly elliptical C omparison between the sizes of the solar system with those of the Oort Cloud. Also shown is the trajectory of Scholz’s Star, with the position oc- cupied 70,000 years ago.