  The field of exoplanetary science is making rapid progress both in
statistical studies of exoplanet properties as well as in individual
characterization. As space missions provide an emerging picture of formation
and evolution of exoplanetary systems, the search for habitable worlds becomes
one of the fundamental issues to address. To tackle such a complex challenge,
we need to specify the conditions favorable for the origin, development and
sustainment of life as we know it. This requires the understanding of global
(astrospheric) and local (atmospheric, surface and internal) environments of
exoplanets in the framework of the physical processes of the interaction
between evolving planet-hosting stars along with exoplanetary evolution over
geological timescales, and the resulting impact on climate and habitability of
exoplanets. Feedbacks between astrophysical, physico-chemical atmospheric and
geological processes can only be understood through interdisciplinary studies
with the incorporation of progress in heliophysics, astrophysics, planetary,
Earth sciences, astrobiology, and the origin of life communities. The
assessment of the impacts of host stars on the climate and habitability of
terrestrial (exo)planets and potential exomoons around them may significantly
modify the extent and the location of the habitable zone and provide new
directions for searching for signatures of life. Thus, characterization of
stellar ionizing outputs becomes an important task for further understanding
the extent of habitability in the universe. The goal of this white paper is to
identify and describe promising key research goals to aid the theoretical
characterization and observational detection of ionizing radiation from
quiescent and flaring upper atmospheres of planet hosts as well as properties
of stellar coronal mass ejections and stellar energetic particle events.
