Free Astronomy Magazine September-October 2019

13 SEPTEMBER-OCTOBER 2019 a computer model to simulate the chem- istry and temperature of planetary atmos- pheres and to figure out how they respond to different host stars. Then, she entered the features of those atmospheres in a model that simulates the planetary spec- trum and shows how Earth-like planets’ spectra could appear in future telescopes that will be dedicated to those kinds of ob- servations. Among these instruments, the most eagerly awaited are PLATO (PLAne- tary Transits and Oscillations of stars satel- lite), HabEx (Habitable Exoplanet Observa- tory), and LUVOIR (Large UV Optical In- frared surveyor). Giada Arney’s simulations suggest that nearby mid-late K dwarfs could be excellent targets for biosignature research. In addi- tion to the advantages already seen, these stars offer access to a wide range of wave- lengths for planets in their habitable zones, also with the constraint of the so-called IWA (Inner Working Angle), which defines the smallest angular separation between a planet and a star to which the planet can be resolved and observed directly. This is the most relevant limit for future telescopes, which, in some ways, will be forced to tar- get stars near the Sun. According to Giada Arney, the most interesting targets are 61 Cyg A and B (11.4 ly), Epsilon Indi (11.8 ly), Groombridge 1618 (15.9 ly), and HD 156026 (36 Ophiuchi C, 19.5 ly). The first three offer a signal-to-noise ratio 1.6 times better than Tau Ceti (11.9 ly), the nearest G dwarf after the Sun and Alpha Centauri A. The fourth offers a signal-to-noise ratio 1.4 times better than the G dwarf 82 Eridani (19.8 ly). Unfortunately, so far, no planets poten- tially similar to ours have been discovered around these stars. In short, the list of potential targets is sadly short and the only other two candi- date Earths known in the habitable zone of K dwarfs (Kepler-442 b and Kepler-62 f) are 1100-1200 light-years away, well be- yond the limits of an acceptable IWA. Hoping to find biosignatures in such a small number of atmospheres (we are not even certain they resemble the terrestrial one) is a true act of faith, but somewhere we must start! activity, with a significantly smaller num- ber of intense flares. Recent measurements performed in the UV and X-ray spectrum of a group of young K dwarfs (10 to about 600 million years old) by a team of researchers led by Tyler Richey-Yowell (Arizona State Univer- sity, Tempe) has demonstrated that any planets in their habitable zones would re- ceive a flux of radiation 5 to 50 times less intense than they would receive orbiting around early M dwarfs and a flow from 50 to 1000 times less intense than they would receive orbiting around late M dwarfs (the smallest but also the most harmful stars). In her theoretical study, Giada Arney used !