Free Astronomy Magazine September-October 2019

SEPTEMBER-OCTOBER 2019 with the results recently appearing in The Astrophysical Journal Letters . The first rea- son for preferring K dwarfs is because they represent 12% of the main sequence stel- lar population, compared to about 8% of G dwarfs and 3% of F dwarfs. Moreover, K dwarfs are the longest-lived among the solar-type stars: 17-70 billion years, against 8-12 billion years for G dwarfs and 2-4 bil- lion for F dwarfs (the stay time of the lat- ter ones in the main sequence can certainly be considered insufficient for the develop- ment of particularly-evolved life). Even the planet-star contrast ratio is more favorable for K dwarfs. A K2V, for exam- ple, has a brightness that is just 1/3 rd that of the Sun (G2V), while a K6V emits just 1/10 th the sunlight. It will, therefore, be less difficult to directly study a candidate Earth in the habitable zone of a K dwarf than a similar planet in the habitable zone of a G dwarf. From a distant planet, the Earth would ap- pear 10 billion times less bright than the Sun, but, if it was in the habitable zone of a K dwarf, it would be “only” 1 billion times less bright and proportionally less difficult to observe directly. Compared to M dwarfs, K dwarfs offer two notable advantages: a decidedly shorter main pre-sequence phase of less than 100 million years and a much quieter surface D uring the planned four- year primary mis- sion, ESA’s obser- vatory PLATO will observe hundreds of thousands of stars, leading to the discovery and characterization of thousands of new exoplanets, some of which might lie in the habitable zone around Sun-like stars. [ESA/DLR]