Free Astronomy Magazine September-October 2015

PLANETOLOGY estimated at 4.0 ± 0.6 billion years (4.6 billion in the Sun’s case). In order to identify the “noise” introduced in the ra- dial velocity of HIP 11915 by its photo- spheric activity, the researchers careful- ly examined nearly sixty high-resolu- tion spectra record- ed with HARPS (al- so during previous works) between 2003 and 2015. The average signal detected, corresponding to a radial velocity of 10-11 m/sec, is theoret- ically compatible with a solar type activity, but HIP 11915 did not prove as active as the Sun, to the extent that once taken into account all possible stellar activity indexes and excluded any 5 minutes oscillation pe- riod, as the one affecting the Sun, what re- mains to be attributed is the larger part of the radial velocity, a part compatible with an extrasolar planet having a mass identi- cal to that of Jupiter, located on a low-ec- centricity orbit (about 0.1) with a revolu- tion period of about 10 years. Given that the chemical composition of HIP 11915 is 98% similar to the Sun’s, and that in the case of our star the abundances of the elements cannot be completely unrelated to the fact that there are rocky planets inner to Jupiter, it is likely that also between HIP 11915 and its giant planet, codenamed HIP 11915 b, there are rocky planets not yet discovered due to instru- mental limitations. And until they are dis- covered (assuming that they exist), HIP 11915 may be cautiously defined only a twin of the Sun-Jupiter system. In any case, additional observations will be necessary since Bedell/Meléndez’s team has noticed a suspicious coincidence be- tween the trend of the stellar activity phases and the gravitational pull due to the supposed “Jupiter”. In recent years, the two signals have in fact grown at an equal pace, a correlation not found in older observations and which lends itself to more than one interpretation. One could be that seeing the stellar activ- ity reaching a maximum phase while HIP 11915 b goes to line up with star and ob- server. But the picture could also be dif- ferent, as the stellar activity may have un- known peculiarities. Moreover, HIP 11915 b may be more massive than Jupiter, since in not knowing the inclination of its orbit, its current mass estimation is in fact a lower value. We will just have to wait for the results of new researches, which surely will not take long to arrive. Nevertheless, HIP 11915 is to date one of the most prom- ising candidates for hosting a planetary system similar to our own. T he key protag- onists of the discovery of HIP 11915 b, Megan Bedell and Jorge Meléndez, photo- graphed from the dome of the 3.6- metre telescope; in the background, view of the other facilities of La Silla Observatory. [ESO] n