  At present, because of observational selection effects, we know of no
exoplanetary systems with any planetary masses close to that of the Earth. We
have therefore used computer models to see whether such planets could be
dynamically stable in the presence of the more massive planets known to be
present, and in particular whether planets with roughly an Earth mass could
remain confined to the classical habitable zone (HZ) for long enough for life
to have emerged.
  Measured stellar properties have been used to determine for each system the
present location of the HZ. We have also determined the critical distances from
the orbit of each giant planet within which an Earth-mass planet would suffer
large orbital changes. We then evaluated the present habitability of each and
every exoplanetary system by examining the penetration of these critical
distances into the HZ. The critical distances can be obtained by extensive
computer modelling of an exoplanetary system. This is far too time consuming to
apply to all of the 150 or so systems already known, and to keep up with the
latest discoveries. Therefore, in earlier work we studied a few systems in
great detail, and developed a speedier means of obtaining the critical
distances. We summarize this comparatively quick method here. We can then
evaluate comparatively quickly the present habitability of each exoplanetary
system by examining the penetration of the critical distance(s) into the HZ.
The results are encouraging for astrobiology.
