Free Astronomy Magazine September-October 2015

PLANETOLOGY type, show magnetic activity cycles that generally result in a “surface" randomly affected by active regions, with spots, fa- culae and other pheno- mena. These, in turn- ing towards the ob- server or moving away from him, alter the bal- ance between blueshift and redshift, and mimic the presence of plan- ets. In other words, a star can show a certain radial velocity even though its distance from the Earth remains unaffected and it is thus virtually devoid of planets. Observations over months and years and appropriate meth- ods for interpreting the stellar activity, permit researchers to eliminate false planetary sig- nals and establish whether any residual radial velocity is con- struable with the existence of one or more planets, as well as calculate their period and mass. In the case of searches for Jupi- ter’s equivalents around Sun’s equivalents, the picture is more difficult to fill in, as the planet’s revolution period could equal that of the star’s activity cycle, exactly as it happens in our sys- tem, where the two periods are both close to 11 years. Since it is true that the solar activity cycle may deviate from the average duration by a few years, that there are long solar minimums, and that in the long run the dis- tribution of the active regions of one side and the other of the central meridian compensate it- self, if we were to apply the same to a star like the Sun, it would ap- pear relatively simple to separate a stable signal from one not equally as stable. But this is not so, because two (or more) signals manifesting themselves in comparable time scale, with overlapping phases or not, and providing a unique and complex sig- T he HARPS spectrograph, photographed both closed and open during some tests. The vessel containing the “heart” of the in- strument is kept under vacuum during the opera- tional phases. [ESO]