Free Astronomy Magazine November-December 2019

NOVEMBER-DECEMBER 2019 similar to what the Earth receives from the Sun, 1370 W/m 2 , albeit with a different spec- tral energy distribution. Models indicate that that level of irradiation can produce a surface temperature (a term sometimes am- biguous) between 200 K and 320 K, or be- tween about -70 °C and +50 °C, practically the same measurable range on the Earth's surface (assuming also that the average albedo of K2- 18 b is the same, i.e., about 0.3). These assumptions prompted researchers to collect spectra of the K2-18 b atmosphere whenever the Hubble Space Telescope became available, the only in- strument theoretically capable of collecting an infrared signal (1.1-1.7 µ m) intense enough to allow the recognition of the most abundant molecules. The observa- tions aimed to exploit the transits in front of the star disk to obtain the overall spec- trum of the star together with the planet, from which one can then subtract the single out-of-transit star spectrum. The two teams of re- searchers who worked on the complex data process- ing were led by Angelos Tsiaras (University College London) and Björn Ben- neke (Université de Mon- tréal). Both teams analyzed eight K2-18 b transits recorded between 2015 and 2017. Benneke's team included in the process three observations from the Spitzer Space Tele- scope. The results of the long work of the two teams were published re- spectively in Nature Astron- omy and The Astronomical Journal . The conclusions T his is another possible vi- sion of the K2-18 b surface, espe- cially if it is tidally locked, al- ways displaying the same hemi- sphere to the star. On the left, Björn Benneke, leader of one of the two teams of researchers who discovered water vapor on K2-18 b. [Amélie Philibert]