6With all the research programs focused on
identifying potentially habitable or inhab-
ited planets, the overall goal was to search
indiscriminately around stars of very differ-
ent sizes, leaving out, at most, those of the
first spectral types O, B and A, due to the fact
that their brief stay on the main sequence
does not give enough time for any planets
around them to become definitely habitable.
It is reasonable to suppose that the longer a
star remains on the main sequence, the
higher the probability that an orbiting Earth-
like planet can host life forms. The smaller
the initial mass of a star, the longer its per-
manence on the main sequence, so it goes
without saying that M dwarfs present them-
selves as a rather interesting target, not only
because their average life greatly exceeds
the current age of the universe, but also be-
cause they are the most common stars in the
galaxy, representing at least 75 % of the total
stellar population.
Furthermore, if we consider that M dwarfs’
weak surface brightness and small mass favor
the discovery of planets both with the transit
method and the radial velocity variation
method, it is no surprise that the largestF
or a long time
considered
ideal stars around
which to look for
biosignatures,
red dwarfs are
instead proving
increasingly
unsuitable be-
cause of the vio-
lent flares that
characterize them
and that interact
with their plane-
tary atmos-
pheres. Above,
we see a repre-
sentation of it.
On the left, plan-
etary systems
comparison. On
TRAPPIST- 1 plan-
ets, many hopes
were placed
which are now
fading. [NASA/
JPL-Caltech]K type star EN_l'Astrofilo 29/08/2019 15:46 Page 6