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 K 2 V, for exam-
ple, has a brightness that is just 1 / 3 rdthatof the Sun (G 2 V), while a K 6 V emits just
1 / 10 ththe 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 surfaceD
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]K type star EN_l'Astrofilo 29/08/2019 15:46 Page 12