Free Astronomy Magazine September-October 2015

42 PLANETOLOGY nal, are a problem not easily solvable. Among the teams of astronomers who have more recently dived into this kind of research there is that of Megan Bedell (University of Chicago) and Jorge Melén- dez (Universidade de São Paulo), which along with some colleagues carried out during the last four years with the use of HARPS a series of observations of a specific sample of 63 stars, selected for their re- markable resemblance to the Sun. On the basis of the results, the team identified three stars among those sam- pled, whose variations in the radial velocities are similar to those expected in the presence of a Jupiter twin in orbit at about 5 astronomical units. By analyzing different indexes of these stars’ magnetic activ- ity, especially the one referring to the H and K lines of ionized calcium, where the contribu- tion of active regions on the signal can be more reliably de- tected, Bedell, Meléndez and colleagues realized that for two of the three stars the radial velocity measured is apparent and that the periodicity observ- ed is entirely due to photospheric activity. The signal from the third sun-like star turned out instead to be more interesting. The star, called HIP 11915, located 186 light-years from Earth in the constellation of Cetus, has a visual magnitude of 8.6, and besides diameter, temperature and metallicity has also in common with the Sun another important parameter, its age, T he ESO’s structure, at the La Silla site in Chile, home to the 3.6-metre telescope and its HARPS spectro- graph. [Serge Brunier, ESO] Below, an anima- tion of the HIP 11915 system. [ESO/ M. Korn- messer]