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thermal tolerance increases with increasing complexity of the organism. The study
of metabolic processes casts light on the supramolecular mechanisms of thermal
response, suggesting that aerobic metabolism sets tighter thermal limits than other
high-level functions typical of multicellular organisms [13].
The above considerations suggest that any form of multicellular, aerobic life will
be characterized by stringent thermal limits due to the combined constraints that
originate at the molecular level and at higher levels of structural and functional
complexity. A better understanding of these mechanisms may eventually lead to the
definition of universal thermal limits for specific forms of life. For the moment, in
lack of better indications, we adopt the thermal limits 0 ≤ T (
◦ C) ≤ 50, representative of terrestrial multicellular poikilotherms, as a criterion of long-term habitability
of complex life outside Earth. The fact that such limits are shared by multicellular
poikilotherms emerged from independent evolutionary pathways on Earth is consistent with this assumption.
14.3 Modelling the Surface Temperature of Rocky
Exoplanets
Based on the above discussion we use the ambient temperature as a tool for assessing
the capability of a planet to host complex life. The surface temperature of exoplanets
can be calculated by inserting observational data in dedicated climate models. Unfortunately, only a small amount of data can be measured for individual exoplanets [3].
The data obtained from the transit and radial velocity surveys may include planetary
structural parameters (radius, mass), orbital parameters (semi-major axis, eccentricity), and properties of the host star (luminosity, spectral type, chemical composition,
and age). Other planetary quantities that impact the climate, but are currently not
measurable (e.g. rotation period, axis tilt, geography, surface pressure, atmospheric
composition) must be treated as free model parameters.
Climate models for exoplanets must deal with the intrinsic complexity of the
climate system, characterized by different components, processes, feedbacks and
time scales [14]. In addition, exoplanet climate models must be adapted to simulate
conditions which are not treated in Earth climate models. Given these difficulties,
a hierarchy of climate models should be employed in exoplanet studies. Models of
higher complexity, tested with well-known climates of rocky planets (present Earth,
Mars, Venus, paleo-Earth), should be used to validate models of lower complexity.
Once properly validated, models of low complexity can be used to explore the huge
parameter space that characterizes the stellar, orbital and planetary properties of
rocky exoplanets.
Classic studies of the HZ have used single atmospheric column calculations, with
a simplified, radiative-convective treatment of the vertical transport and an albedo
representative of the mean planetary albedo [4, 5]. With an idealized treatment of
the latitudinal energy transport, Energy Balance Models (EBMs) can simulate latitu-
G. Vladilo et al.
thermal tolerance increases with increasing complexity of the organism. The study
of metabolic processes casts light on the supramolecular mechanisms of thermal
response, suggesting that aerobic metabolism sets tighter thermal limits than other
high-level functions typical of multicellular organisms [13].
The above considerations suggest that any form of multicellular, aerobic life will
be characterized by stringent thermal limits due to the combined constraints that
originate at the molecular level and at higher levels of structural and functional
complexity. A better understanding of these mechanisms may eventually lead to the
definition of universal thermal limits for specific forms of life. For the moment, in
lack of better indications, we adopt the thermal limits 0 ≤ T (
◦ C) ≤ 50, representative of terrestrial multicellular poikilotherms, as a criterion of long-term habitability
of complex life outside Earth. The fact that such limits are shared by multicellular
poikilotherms emerged from independent evolutionary pathways on Earth is consistent with this assumption.
14.3 Modelling the Surface Temperature of Rocky
Exoplanets
Based on the above discussion we use the ambient temperature as a tool for assessing
the capability of a planet to host complex life. The surface temperature of exoplanets
can be calculated by inserting observational data in dedicated climate models. Unfortunately, only a small amount of data can be measured for individual exoplanets [3].
The data obtained from the transit and radial velocity surveys may include planetary
structural parameters (radius, mass), orbital parameters (semi-major axis, eccentricity), and properties of the host star (luminosity, spectral type, chemical composition,
and age). Other planetary quantities that impact the climate, but are currently not
measurable (e.g. rotation period, axis tilt, geography, surface pressure, atmospheric
composition) must be treated as free model parameters.
Climate models for exoplanets must deal with the intrinsic complexity of the
climate system, characterized by different components, processes, feedbacks and
time scales [14]. In addition, exoplanet climate models must be adapted to simulate
conditions which are not treated in Earth climate models. Given these difficulties,
a hierarchy of climate models should be employed in exoplanet studies. Models of
higher complexity, tested with well-known climates of rocky planets (present Earth,
Mars, Venus, paleo-Earth), should be used to validate models of lower complexity.
Once properly validated, models of low complexity can be used to explore the huge
parameter space that characterizes the stellar, orbital and planetary properties of
rocky exoplanets.
Classic studies of the HZ have used single atmospheric column calculations, with
a simplified, radiative-convective treatment of the vertical transport and an albedo
representative of the mean planetary albedo [4, 5]. With an idealized treatment of
the latitudinal energy transport, Energy Balance Models (EBMs) can simulate latitu-