  In this paper we discuss and illustrate the hypothesis that life
substantially alters the state of a planetary environment and therefore,
modifies the limits of the HZ as estimated for an uninhabited planet. This
hypothesis lead to the introduction of the Habitable Zone for Inhabited planets
(hereafter InHZ), defined here as the region where the complex interaction
between life and its abiotic environment is able to produce plausible
equilibrium states with the necessary physical conditions for the existence and
persistence of life itself. We support our hypothesis of an InHZ with three
theoretical arguments, multiple evidences coming from observations of the Earth
system, several conceptual experiments and illustrative numerical simulations.
Conceptually the diference between the InHZ and the Abiotic HZ (AHZ) depends on
unique and robust properties of life as an emergent physical phenomenon and not
necesarily on the particular life forms bearing in the planet. Our aim here is
to provide conceptual basis for the development of InHZ models incorporating
consistently life-environment interactions. Although previous authors have
explored the effects of life on habitability there is a gap in research
developing the reasons why life should be systematically included at
determining the HZ limits. We do not provide here definitive limits to the InHZ
but we show through simple numerical models (as a parable of an inhabited
planet) how the limits of the AHZ could be modified by including plausible
interactions between biota and its environment. These examples aim also at
posing the question that if limits of the HZ could be modified by the presence
of life in those simple dynamical systems how will those limits change if life
is included in established models of the AHZ.
