Free Astronomy Magazine

EXOPLANETS That planet orbits around Kepler-186 (one of more than 156,000 stars which have been monitored for years by Kepler) also known as KIC 8120608, KOI 571 and – after its sky coordinates – 2MASS J19543665 +4357180. It is a red dwarf with spectral type M1, a diameter of about 660,000 kilo- metres (47% of that of the Sun), a surface temperature of nearly 3800 kelvin and an abundance of iron which is half that of the Sun. Kepler-186 is located 492 light-years from Earth, in the constellation Cyg- nus, and has an apparent visual magnitude of 14.9 (in the extreme red end of the visible spectrum, while in the infrared it reaches a magnitude of 11-12). Red dwarf stars are very com- mon as, in fact, they ac- count for 3/4 of the stellar population of our Galaxy (and perhaps of the entire universe) and are an ideal target in the search for planets similar to our own. This is due to several reasons: their habitable zones are more internal and smaller than those of stars like the Sun and more massive, and hence the most interesting planets transit more often on the stellar disk, shortening the time for their discovery; in addition, the ratio between the star’s diameter and the planets’ diameter is more favourable than in other cases, and consequently the drop A comparison between the size of Earth and Kepler-186f, and between the size of the Kepler-186 planetary system with that of our inner solar sys- tem. All elements are drawn to scale. Kepler- 186f’s orbit is as large as that of Mercury. Left: an animation show- ing the effects of a planetary tran- sit. [NASA Ames/SETI Insti- tute/JPL-Caltech]