Free Astronomy Magazine September-October 2014

EXOPLANETS the other spectrometric, because depend- ing on whether the region is in the hemi- sphere that rotates toward the observer or in that moving in the opposite direction, there will be a corresponding farness or closeness of the star in relation to the same observer. This entirely apparent movement is more marked the more the stellar rota- tion axis is perpendicular with respect to the line of sight, and in the short term is impossible to differentiate it from the ef- fect that a planet would have on the radial velocity of the star. In the short term that is, because when observations are extend- ed over periods of months or, better yet, years, it is unlikely that a star’s active re- gions can continue to mimic the cyclic orbit of a planet. If then that star does not show O n the side, the position of GJ 581 in rela- tion to the Sun and other refer- ence stars. In the star chart below, the position on the celestial sphere of GJ 581, shown by the red circle. photometric variations and is characterized by low surface activity, it is natural to attri- bute any periodic variation of the radial ve- locity to one or more planets. And this is ex- actly the case of GJ 581. But are we sure that we can extend to a red dwarf the typical solar knowledge gathered about active regions? On that type of red dwarf the surface mag- netic fields could be much more persistent than is the case on the Sun, or there could be some structures that are not necessarily as dark as the spots but equally capable of inhibiting the convective cells’ mo- tion, as seems to be sug- gested by the studies conducted on a very fa- mous red dwarf, Bar- nard's Star. To verify whether, even in the absence of signif- icant spots, GJ 581 can display surface magnet- ic phenomena capable of simulating the exis- tence of its controver- sial planets, Robertson’s team started to look for a correlation between the modulation of the H-alpha line (H α ) and