14 SETI in Rocky Exoplanets …
131
dinal and seasonal variations of the surface temperature [15] and have been applied
to studies of planetary habitability [16]. By incorporating single column, radiativeconvective calculations and a schematic treatment of the clouds in classic EBMs, one
obtains 2D (vertical and latitudinal) models with seasonal dependence of the surface
temperature [6, 17]. A refinement of this last type of model is the Earth-like planet
surface temperature model (ESTM), which incorporates a physically-based description of the meridional transport validated with models of higher complexity [18].
The ESTM provides fast estimates of the zonal surface temperature, T s = T s (ϕ, t),
as a function of latitude, ϕ, and time, t. The temperature distribution predicted in this
way can be used to characterize the habitability of exoplanets.
14.3.1 A Temperature-Dependent Index of Complex-Life
Habitability
To introduce a quantitative index of habitability we define a temperature interval,
(T 1 , T 2 ), which we consider optimal for the maintainance of multicellular life and the
production of atmospheric biosignatures. From a modelization of the zonal surface
temperature, T s = T s (ϕ, t), such as that provided by the ESTM, we then calculate a
habitability function
H (ϕ, t) =
1 if T 1 ≤ T s (ϕ, t) ≤ T 2
0 otherwise
.
(14.1)
By averaging H (ϕ, t) in ϕ (weighting latitude zones according to their area) and t
(over one orbital period), we obtain the index of mean planetary habitability
h =
+
π
2
−
π
2
dϕ
P
0 dt [H (ϕ, t) cos ϕ]
2P
.
(14.2)
The choice of the temperature limits (T 1 , T 2 ) is a critical point of this operational
definition of habitability. Based on the thermal limits for complex life discussed
above, we adopt T 1 = 0
◦ C and T 2 = 50
◦ C in Eq. (14.1) and we call h 050 the index
calculated from Eq. (14.2) with these limits. The index h 050 , normalized between
0 and 1 by construction, provides a quantitative estimate of the mean planetary
habitability for multicellular life. Moreover, the temperature range that maximizes
h 050 is the same range in which terrestrial cyanobacteria and plants produce oxygen
[7]. This is important because oxygen is a potential atmospheric biosignature and is
probably necessary for the emergence of complex life [10].
In addition to the above considerations, driven by biological arguments, the index
h 050 presents an important advantage in terms of climate calculations: the low value
of the upper thermal limit, T 2 = 50
◦ C, avoids the necessity to perform climate calculations in a regime of high temperatures that may lead to the onset of runaway
131
dinal and seasonal variations of the surface temperature [15] and have been applied
to studies of planetary habitability [16]. By incorporating single column, radiativeconvective calculations and a schematic treatment of the clouds in classic EBMs, one
obtains 2D (vertical and latitudinal) models with seasonal dependence of the surface
temperature [6, 17]. A refinement of this last type of model is the Earth-like planet
surface temperature model (ESTM), which incorporates a physically-based description of the meridional transport validated with models of higher complexity [18].
The ESTM provides fast estimates of the zonal surface temperature, T s = T s (ϕ, t),
as a function of latitude, ϕ, and time, t. The temperature distribution predicted in this
way can be used to characterize the habitability of exoplanets.
14.3.1 A Temperature-Dependent Index of Complex-Life
Habitability
To introduce a quantitative index of habitability we define a temperature interval,
(T 1 , T 2 ), which we consider optimal for the maintainance of multicellular life and the
production of atmospheric biosignatures. From a modelization of the zonal surface
temperature, T s = T s (ϕ, t), such as that provided by the ESTM, we then calculate a
habitability function
H (ϕ, t) =
1 if T 1 ≤ T s (ϕ, t) ≤ T 2
0 otherwise
.
(14.1)
By averaging H (ϕ, t) in ϕ (weighting latitude zones according to their area) and t
(over one orbital period), we obtain the index of mean planetary habitability
h =
+
π
2
−
π
2
dϕ
P
0 dt [H (ϕ, t) cos ϕ]
2P
.
(14.2)
The choice of the temperature limits (T 1 , T 2 ) is a critical point of this operational
definition of habitability. Based on the thermal limits for complex life discussed
above, we adopt T 1 = 0
◦ C and T 2 = 50
◦ C in Eq. (14.1) and we call h 050 the index
calculated from Eq. (14.2) with these limits. The index h 050 , normalized between
0 and 1 by construction, provides a quantitative estimate of the mean planetary
habitability for multicellular life. Moreover, the temperature range that maximizes
h 050 is the same range in which terrestrial cyanobacteria and plants produce oxygen
[7]. This is important because oxygen is a potential atmospheric biosignature and is
probably necessary for the emergence of complex life [10].
In addition to the above considerations, driven by biological arguments, the index
h 050 presents an important advantage in terms of climate calculations: the low value
of the upper thermal limit, T 2 = 50
◦ C, avoids the necessity to perform climate calculations in a regime of high temperatures that may lead to the onset of runaway