  It is currently unknown how common life is on exoplanets, or how long planets
can remain viable for life. To date, we have a superficial notion of
habitability, a necessary first step, but so far lacking an understanding of
the detailed interaction between stars and planets over geological timescales,
dynamical evolution of planetary systems, and atmospheric evolution on planets
in other systems. A planet mass, net insolation, and atmospheric composition
alone are insufficient to determine the probability that life on a planet could
arise or be detected. The latter set of planetary considerations, among others,
underpin the concept of the habitable zone (HZ), defined as the circumstellar
region where standing bodies of liquid water could be supported on the surface
of a rocky planet. However, stars within the same spectral class are often
treated in the same way in HZ studies, without any regard for variations in
activity among individual stars. Such formulations ignore differences in how
nonthermal emission and magnetic energy of transient events in different stars
affect the ability of an exoplanet to retain its atmosphere.In the last few
years there has been a growing appreciation that the atmospheric chemistry, and
even retention of an atmosphere in many cases, depends critically on the
high-energy radiation and particle environments around these stars. Indeed,
recent studies have shown stellar activity and the extreme space weather, such
as that created by the frequent flares and coronal mass ejections (CMEs) from
the active stars and young Sun, may have profoundly affected the chemistry and
climate and thus habitability of the early Earth and terrestrial type
exoplanets. The goal of this white paper is to identify and describe promising
key research goals to aid the field of the exoplanetary habitability for the
next 20 years.
