Free Astronomy Magazine March-April 2015

SOLAR SYSTEM T his diagram shows in log- arithmic scale the distances within our solar system and the exten- sion of the Oort Cloud, as far as the Alpha Cen- tauri system. Also shown is the current posi- tion of the Voy- ager 1 probe. [NASA] The graph below is the best estimate of the trajectory travelled by Scholz’s Star in the region of gravitational in- fluence of the Sun (red curve). The crosses re- present 100 years intervals. [E. Ma- majek et al.] however easy, in that they are not typically very bright and given that in the vicinity of the Sun, where the search should be made, there are mainly red dwarfs, whose number grows as their masses decrease. But there is though a way to know if a par- ticular star has passed or will pass close to us, which is to measure both the tangential and radial velocities. The first corresponds to the apparent motion of the star project- ed on the sky, while the second is the veloc- ity along the line of view. By being able to know both speed extents it is possible to de- duce the actual motion of the star in the three spatial dimensions, and by knowing its distance it is possible to calculate its trajec- tory back in time and in the future. Clearly, the further away in time and space we go, the more the margin of error increases. It is evident at this point that the best candi- dates for flybys with the solar system are stars which are still relatively close, with an extremely low tangential velocity and a mark- ed radial velocity. These features are well suit- ed to a very faint red dwarf star (magnitude 18.3), discovered in 2013 in the constellation Monoceros by the astronomer Ralf-Dieter Scholz, of the Leibniz-Institut für Astrophysik Potsdam, Germany. Although located at “just” 20 light-years from Earth, that dwarf star, known as WISE J072003.20-084651.2, or more simply “Scholz’s Star”, had never been noticed before due to the fact of being proj- ected on a region of the galactic plane teem- ing with stars. Towards the end of that same year, a group of researchers led by Eric Mamajek, of the University of Rochester (NY, USA), became