Free Astronomy Magazine November-December 2019

11 NOVEMBER-DECEMBER 2019 EXOPLANETS whose atmosphere water vapor has now been detected. In 2017, Kepler itself discov- ered a second super-Earth in the system, K2- 18 c. K2-18 c turned out to be less interesting than the other, orbiting just 9 million km (0.06 AU) from the star and with a revolu- tion period of 9 days, placing it far beyond the internal limit of the K2-18 habitable zone, which extends between 18 and 37.5 million km (0.12-0.25 AU). Even before the second planet was discovered, its outermost sibling had already attracted the attention of astronomers due to the fact that it is well-placed in the K2-18 habitable zone at about 21 million km (0.143 AU) and with a revolution period slightly smaller than 33 days. At that distance, the planet receives about 1440 W/m 2 from the star, a value very