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G. Vladilo et al.
greenhouse instability [4]. This regime, characterized by high water vapour content,
is extremely hard to model even with state-of-the-art, 3D climate models [19].
14.3.2 Applications to Studies of Habitability for Complex
Life
By using the methodology described above we can estimate the index h 050 for a
broad range of factors that affect the planetary climate. Studies performed with the
ESTM show that the parameters that most heavily impact T s (ϕ, t) and h 050 are the
insolation, the atmospheric properties (pressure and composition) and the albedo
(surface and clouds) [18]. The index h 050 can be applied to individual exoplanets or
to perform statistical studies of exoplanetary habitability.
Kepler-452b is an example of an Earth-size planet (R = 1.63 R ⊕ ) in the HZ of a
sun-like star [20] that has been studied with the ESTM [21]. Under the assumption
of a rocky-dominated nature, T s (ϕ, t) and h 050 were computed for a broad range of
climate factors. Even if the insolation of Kepler-452b is only 10% larger than that
received by Earth, the constraints of habitability for complex life are very stringent.
For most choices of parameters, habitable solutions with h 050 > 0.2 are only found
if the CO 2 partial pressure is pCO 2 < 0.04 bar. At this limiting value of pCO 2 ,
the planet is habitable only if the total pressure is p < 2 bar. In all cases, the habitability index h 050 drops to zero if the orbital eccentricity is e > 0.3. Changes of
rotation period and axis tilt affect h 050 due to their impact on the equator–pole temperature difference, which affects the possible existence of polar caps. Variations
of h 050 resulting from the luminosity evolution of the host star were estimated with
the aid of stellar evolutionary tracks [22]. Only a small combination of parameters
yields habitability-weighted lifetimes > 2 Gyr, sufficiently long to develop atmospheric biosignatures still detectable at the present time [21]. This study illustrates
the importance of exploring the parameter space of climate factors in order to assess
the potential of individual planets to host complex life.
Thanks to the flexibility of the ESTM, it is possible to run a large number of
climate simulations and perform statistical studies of habitability. An example of
this statistical approach is the study of the bistability of the planetary climate as a
function of the initial conditions of the simulations. An intriguing result of this type of
study is that the planetary conditions that support climate bistability are remarkably
similar to those required for the sustenance of multicellular life on the planetary
surface [23]. The statistical approach can also be used to build up multi-parameter
HZs by calculating h 050 as a function of the planet insolation, S, and other climateimpacting parameters [7, 24, 25]. At variance with the classic HZ, thanks to the
low value of the upper thermal limit (T 2 = 50
◦ C), the inner edge of the complexlife HZ can be calculated without simulating the conditions that drive the runaway
greenhouse instability.
G. Vladilo et al.
greenhouse instability [4]. This regime, characterized by high water vapour content,
is extremely hard to model even with state-of-the-art, 3D climate models [19].
14.3.2 Applications to Studies of Habitability for Complex
Life
By using the methodology described above we can estimate the index h 050 for a
broad range of factors that affect the planetary climate. Studies performed with the
ESTM show that the parameters that most heavily impact T s (ϕ, t) and h 050 are the
insolation, the atmospheric properties (pressure and composition) and the albedo
(surface and clouds) [18]. The index h 050 can be applied to individual exoplanets or
to perform statistical studies of exoplanetary habitability.
Kepler-452b is an example of an Earth-size planet (R = 1.63 R ⊕ ) in the HZ of a
sun-like star [20] that has been studied with the ESTM [21]. Under the assumption
of a rocky-dominated nature, T s (ϕ, t) and h 050 were computed for a broad range of
climate factors. Even if the insolation of Kepler-452b is only 10% larger than that
received by Earth, the constraints of habitability for complex life are very stringent.
For most choices of parameters, habitable solutions with h 050 > 0.2 are only found
if the CO 2 partial pressure is pCO 2 < 0.04 bar. At this limiting value of pCO 2 ,
the planet is habitable only if the total pressure is p < 2 bar. In all cases, the habitability index h 050 drops to zero if the orbital eccentricity is e > 0.3. Changes of
rotation period and axis tilt affect h 050 due to their impact on the equator–pole temperature difference, which affects the possible existence of polar caps. Variations
of h 050 resulting from the luminosity evolution of the host star were estimated with
the aid of stellar evolutionary tracks [22]. Only a small combination of parameters
yields habitability-weighted lifetimes > 2 Gyr, sufficiently long to develop atmospheric biosignatures still detectable at the present time [21]. This study illustrates
the importance of exploring the parameter space of climate factors in order to assess
the potential of individual planets to host complex life.
Thanks to the flexibility of the ESTM, it is possible to run a large number of
climate simulations and perform statistical studies of habitability. An example of
this statistical approach is the study of the bistability of the planetary climate as a
function of the initial conditions of the simulations. An intriguing result of this type of
study is that the planetary conditions that support climate bistability are remarkably
similar to those required for the sustenance of multicellular life on the planetary
surface [23]. The statistical approach can also be used to build up multi-parameter
HZs by calculating h 050 as a function of the planet insolation, S, and other climateimpacting parameters [7, 24, 25]. At variance with the classic HZ, thanks to the
low value of the upper thermal limit (T 2 = 50
◦ C), the inner edge of the complexlife HZ can be calculated without simulating the conditions that drive the runaway
greenhouse instability.