Free Astronomy Magazine

31 MAY-JUNE 2014 PLANETOLOGY cheering each time that, starting from the ‘90s, new trans-Neptunian objects such as Quaoar, Sedna, Eris and many others were discovered; all invariably found to be of insignificant size, comparable at most to that of Pluto and thus included together with this in the dwarf planets’ category. Since the possible nature and location of Planet X have adapted over time to our astronom- ical knowledge and to the need to explain phenom- ena (also not strictly as- tronomical), from the ‘80s onward also oth- er alternative hy- potheses to the classic planetary body were put forward. One of the better known is that admitting the existence of a dark companion of the Sun, per- haps a very faint red dwarf, per- haps a brown dwarf, which was called Nemesis. The choice of the name of a ven- geance goddess of Greek mythology lies in the fact that the hypothetical stellar, or substellar, object was considered respon- sible for supposed cyclical mass extinctions on Earth, occurring approximately every 26 million years and triggered by its pas- sage in the Oort Cloud, which would have sent a large number of comets into the inner solar system and some toward our planet. After more recent and accurate pa- leontological analyses have shown that there is no evidence of periodicity in mass extinctions, also the Nemesis variant lost credibility, not without, however, some oc- casional rekindled interest for this theory. The last one is of 2010 and doubles the fre- quency of the extinctions, without though finding any confirmation in the sky where an object such as Nemesis would have not escaped the many orbiting infrared tele- scopes that in recent decades have scoured the entire sky. Another Planet X variant was suggested in the late ‘90s to interpret an anomaly in the trajectories of the long-period comets coming from the Oort Cloud. It was predicted that those comets were randomly coming from any direction, but after verifying that it was not so, some astronomers suggested that a perturbing object several times more massive than I n the back- ground, a ren- dering of the WISE satellite or- biting around Earth. Left: this graph shows the observational limits of WISE, based on the mass (Jupiter=1) and distance from the Sun of plane- tary bodies. [Penn State University]