The quest for life in the Universe is often affected by the free use of
extrapolations of our phenomenological geocentric knowledge. We point out that
the existence of a living organism, and a population of organisms, requires the
existence of available energy or, more precisely, available power per unit
volume (Sect. 1). This is not a geocentric concept, but a principle that
belongs to the foundations of thermodynamics. A quest about availability in the
Universe is justified. We discuss the case in which power comes from mining
(Sect. 2), and from thermal disequilibrium (Sect. 3). Thermal disequilibrium
may show up in two ways: on planets without a star (Sect. 4), and on planets
where the surface thermal disequilibrium is dominated by the incoming photon
flux from the nearest star (Sect. 6). In the first case we study the
availability by simulating the structure of the planet with a simple model that
contains the general features of the problem. For the first case we show that
the availability is in general very small (Sect. 5). In the second case we show
that the availability is in general large; the order of magnitude depends first
of all on the star's temperature and the planet's orbit, but is also controlled
by the greenhouse gases present on the planet.Comment: 30 pages, 10 figures, to be published in Nuovo Cimento
