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

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.