Free Astronomy Magazine January-February 2019

8 JANUARY-FEBRUARY 2019 PLANETOLOGY Let’s open a brief parenthesis on this survey technique, to remind by summary how it works. Each star moves in the galaxy and, regardless of its real spatial trajectory, we can see it moving away or approaching (it rarely appears motionless). If the star moves away, the lines of the chemical elements vis- ible in its spectrumwill be slightly redshifted when compared to the reference lines pro- duced in the laboratory. If the star ap- proaches, the shift of the lines will be towards the blue. In any case, for each star, we can know the exact value of the dis- placement, which remains unchanged for millennia if in the meantime no mass intervenes to modify it. A planet orbiting a star can make the radial velocity oscillate significantly. The magnitude of the oscillation essen- tially depends on the mass of the star, the mass of the planet, the distance separat- ing the two bod- ies, and their position with re- spect to the ob- server. The farther the planet moves from the straight line passing through the star and the observer, the lower the oscillation in the radial velocity of the star will be. An Earth-sized planet can displace a dwarf star no more than a few meters per second, an amount easily achieved by phenomena related to the mag- netic activity of the star and the mass differ- ence between the two objects. All of these radial velocity details underline that the search for non-transiting extrasolar planets requires high power instruments and very high-resolution spectroscopes, as well as long periods of observation. The large team of researchers mentioned above who dis- covered Barnard’s Star b (led by Ignasi Ribas, Institut d’Estudis Espacials de Catalunya, IEEC, and Institut de Ciències de l’Espai, CSIC, Barcelona) worked based on this high-reso- lution radial velocity measurement method. The study started by reviewing archive data collected from the end of the last cen- tury through 2015. The database had sug- gested the presence of a potentially sig- nificant signal, with a period of about 230 days, but without a sufficient sampling to accurately characterize it. To confirm and understand the nature of that signal, the I n the back- ground, an artistic view of Barnard’s Star b seen from space. [ESO/M. Korn- messer] T his video shows Barnard’s Star and its super-Earth. [ESO/M. Kornmesser]