  With a fleet of exploratory space missions on the horizon, the study of
target specific biospheres is crucial for accurately determining the
probability of the existence of microbial life on various planetary bodies and
prioritising targets accordingly. Although previous studies have compared the
potential habitability of objects in our solar system by bulk characteristics,
it is less common that precise qualitative methods are developed for ranking
candidates hospitable to microbial life on a local environment basis. In this
review we create a planetary environmental database and use it to motivate a
list of primary habitability candidates and essential criteria for microbial
survival. We then propose a new method, the Microbial Habitability Index (MHI)
which uses a metric of microbial survival factor values in target environments
compared with appropriate Earth analogues to assess their potential for life.
We arrive at a selection of eight primary candidates and from this set conclude
that Europa, Mars, and Enceladus have the highest potential for facilitating
microbial survival.
