Free Astronomy Magazine September-October 2014

EXOPLANETS the axis (perhaps 50°, consistent with that of the disk of debris surrounding the star beyond the planetary orbits) generates, in the presence of strong surface magnetic activ- ity, a series of pseudo-plane- tary signals. It is purely a case that these si- gnals are of the same magnitude of the orbital pe- riods within the habitable zone. When Robert- son's team remo- ved from the overall radial ve- locity of GJ 581 (dominated by planet “b” with its period of 5.36 days) the “noise” plainly attributable to the magnetic surface ac- tivity of the star, the signal until then attri- buted to planet “d” decreased significantly L eft, an infrared image of the GJ 581 system. The star (blurred in the centre of the picture) ap- pears stretched due to the debris disk surrounding it. The graphic projection of the structure shows the planetary sys- tem and the disk itself drawn to scale. [ESA/Her- schel/PACS/Jean- François Lestrade, Observatoire de Paris] Below, a comparison be- tween the size of the Earth and the size of GJ 581c, the only planet near the habit- able zone of GJ 581. [Virginia Keler] the radial velocity of the star. H α is a spe- cific wavelength (656.281 nm) of visible red light, very sensitive to changes in magnetic activity and thus indicative of the photo- spheric and chromospheric ac- tivity level in stars. The spectra available to researchers covered two different periods, from De- cember 2005 to September 2007 and from January 2010 to July 2011. The H α line behaviour in those two periods allowed to correct the rotation period at- tributed to GJ 581, bringing it from approximately 93 days to a more truthful 130±2 days, which is exactly twice the orbital period of GJ 581d and four times that of GJ 581g. Was this pure coincidence? No. Evidently we are in the presence of har- monics linked to the star’s rota- tion period, which combined with a suitable inclination of