M. Güdel (University of Vienna, Department of Astrophysics, Austria),
R. Dvorak (University of Vienna, Department of Astrophysics, Austria),
N. Erkaev (Siberian Federal University, Russian Federation),
J. Kasting (Penn State, Department of Geosciences, United States),
M. Khodachenko (Austrian Academy of Sciences / SINP, Moscow State University, Russia),
H. Lammer (Austrian Academy of Sciences, Space Research Institute, Austria),
E. Pilat-Lohinger (University of Vienna, Department of Astrophysics, Austria),
H. Rauer (DLR, Institut für Planetenforschung, Germany),
I. Ribas (Institut d'Estudis Espacials de Catalunya - CSIC, Spain),
B. Wood (Naval Research Laboratory, Space Science Division, United States)

With the discovery of hundreds of exoplanets and a potentially huge number of Earth-like planets waiting to be discovered, the conditions for their habitability have become a focal point in exoplanetary research. The classical picture of habitable zones primarily relies on the stellar flux allowing liquid water to exist on the surface of an Earth-like planet with a suitable atmosphere. However, numerous further stellar and planetary properties constrain habitability. Apart from "geophysical" processes depending on the internal structure of a planet, a complex array of astrophysical factors additionally determine habitability. Among these, variable stellar UV, EUV, and X-ray radiation, stellar and interplanetary magnetic fields, ionized winds, and energetic particles control the constitution of upper planetary atmospheres and their physical and chemical evolution. Short- and long-term stellar variability necessitates full timedependent studies to understand planetary habitability at any point in time. Furthermore, dynamical effects in planetary systems and transport of water to Earth-like planets set fundamentally important constraints. We will review these astrophysical conditions for habitability under the crucial aspects of the long-term evolution of stellar properties, the consequent extreme conditions in the early evolutionary phase of planetary systems, and the important interplay between properties of the host star and its planets.

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