Galactic Environments of the Sun and Cool Stars
  The importance of understanding the current and historical galactic environments of cool stars is discussed. The penetration of interstellar gas into a stellar astrosphere is a function of the interaction of the star with the interstellar cloud surrounding the star, and this factor needs to be understood if an efficient search for life-bearing planets is to be made. For the Sun, both current and historical galactic conditions are such that if a solar wind were present, it would have excluded most inflowing interstellar matter from the inner regions of the heliosphere for the past few million years. Variations in heliosphere size over the recent historical path of the Sun are estimated, along with estimates of astrosphere sizes for selected nearby stars. Considering only possible effects due to encounters with interstellar clouds, stable planetary climates are more likely for inner than outer planets. 
Priscilla C. Frisch