Free Astronomy Magazine

EXOPLANETS in flux during transits is more evi- dent; finally, the reduced stellar mass can facilitate the detection of possible changes in the radial velocity, allowing to trace back the planet's mass. For how differ- ent from the Sun, it is not thus a coincidence that a large sample of red dwarfs has been included among the stars to be monitored with Kepler. One of those red dwarfs was precisely Kepler-186, which in the first two years of constant photometric monitoring (one measurement every 29.4 minutes) showed no less than 4 separate series of transits, which astronomers attributed to as many planets, named Kepler- 186b, Kepler-186c, Kepler-186d, Kepler-186e – the letter “a” is not used for the planets in that it identifies the star – and with or- bital periods ranging from 3.9 to 22.4 days. Such rapid revolution periods are indicative of short distances from the red dwarf – in this case be- tween 5 and 19 million kilome- tres – and, therefore, of much higher surface temperatures than those on Earth. None of those four planets can consequently be considered an analogue to Earth, even though their di- mensions are very interesting in all having a diameter less than 1.5 Earth diameters, with one, the “186b”, which diameter is only 8% larger than that of our planet. But then, at the end of the third year of monitoring that distant planetary system, as- tronomers made official the ex- istence of a fifth planet, named Kepler-186f, which was observed transiting over the disk of the little star every 129.9 days at an average distance from the same of 58.7 million kilometres. Such distance places it within the out- T he graph shows the signals of the 5 planets discov- ered around Ke- pler-186. The gray dots are the individual photo- metric measure- ments, the blue dots the medians and the solid red lines represent the more proba- ble theoretical light curves. The star’s dimming is given in parts per million (ppm). [E. Quintana et al.] Below: a compari- son between the transit of the Earth on the Sun and of Kepler- 186f on its star: the second is about 4 times deeper and there- fore easier to de- tect. [Wendy Stenzel]