The following discussion is a modified version of a paper posted on astro-ph in December 2009. It provides an overview of how our collaborative work exploring planetary habitability developed within the context of the classic Habitable Zone concept, and then evolved into a concept of Ecodynamic Domains, which we have found more useful for our own purposes. It was published as part of our contribution to the current debate about whether the HZ concept should be developed to accommodate the broad range of possibilities for habitability which has emerged from recent work, or whether it should be superceded.
M. J. Heath & L. R. Doyle December 24, 2009

Circumstellar Habitable Zones to Ecodynamic Domains: A Preliminary Review and Suggested Future Directions

M. J. Heath
Ecospheres Project
L. R. Doyle
SETI Institute

Abstract.
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 focusses 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 emphasises ecodynamic perspectives, which explore the dynamic interactions between the biotic and abiotic factors which together comprise ecosystems.

this is the whole paper as a PDF

Habitable Zones & Ecodynamic Domains