Free Astronomy Magazine September-October 2014

22 EXOPLANETS moment in which the gravitational effects produced by its mass on the motion of its host star are detected. In other words, the planet pulls the star toward it, even if just slightly, and if that movement occurs along the line of sight it will induce an oscillation of the natural radial velocity of the star (it is worth reminding that the radial velocity is maximum if the star is moving precisely toward the observer or in the opposite di- rection, while it is equal to zero if it moves along a trajectory inclined at 90° with re- spect to the line of sight). To determine whether a star is moving to- ward or away from the observer, it is neces- sary to break down its light into the various colours and obtain the so-called “spec- trum”. The stars spectrum is typically char- acterized by the presence of dark and light thin lines, which according to their posi- tions (wavelengths) are indicative of the presence in the stellar atmosphere (from the photosphere to the corona) of certain chemical elements. These positions vary with respect to standard laboratory posi- tions if the stars are moving toward or away from the observer. The best spectro- graphs available today are capable of mea- suring such small displacements of the spectral lines that the corresponding veloc- ities are in the order of 1 m/s. It is logical to assume that any phenomena occurring at the “surface” of a star is able to disturb even much less accurate measurements; as for example the case of a star characterized by very modest global pulsations, which can easily produce planetary-type signals. But even in the absence of pulsations there are equivocal situations, such as the pres- ence of large groups of spots. In fact, the intense magnetic fields characterizing them subtract energy to the convective cells over which they occur, and this loss of energy results in a lower brightness of those spe- cific regions and in a lower rising velocity of the plasma (hydrogen and other ionized atoms) from the deeper layers. This has two major effects, one photometric, since the presence of a group of spots dims –almost imperceptibly– the star’s brightness, and I f the signals that mimicked the presence of the three inexis- tent GJ 581 plan- ets had been generated by a big spot like the one shown here, the researchers would have re- corded a photo- metric variation of the red dwarf. As this was not the case, it is very likely that on that type of stars the surface magnetic activity produces phenomena not yet known. [D. Aguilar (CfA)]