  The concept of the Circumstellar Habitable Zone has served the scientific
community well for some decades. It slips easily off the tongue, and it would
be hard to replace. Recently, however, several workers have postulated types of
habitable bodies which might exist outside the classic circumstellar habitable
zone (HZ). These include not only bodies which orbit at substantial distances
from their parent stars, but also snowball worlds with geothermally-maintained
internal oceans and even densely-atmosphered worlds with
geothermally-maintained surface oceans, which have been ejected from unstable
planetary systems into interstellar space. If habitability is not a unique and
diagnostic property of the HZ, then the value of the term has been compromised
in a fundamental way. At the same time, it has become evident that multiple
environmental states, differing in important ways in their habitability, are
possible even for geophysically similar planets subject to similar levels of
insolation, within the classic HZ. We discuss an approach to investigations of
planetary habitability which focuses on planetary-scale ecosystems, which are
here termed - ecospheres. This is following a usage popular amongst ecologists,
such as Huggett (1999), rather than that of authors such as Strughold (1953)
and Dole (1964), who used it as a term for the HZ. This approach emphasizes
ecodynamic perspectives, which explore the dynamic interactions between the
biotic and abiotic factors which together comprise ecosystems.
