14 SETI in Rocky Exoplanets …
133
A parameter of special interest for building up the HZ for complex life is the
atmospheric columnar mass, N atm = p/g, where p and g are the surface pressure
and gravitational acceleration, respectively. The atmospheric columnar mass has a
strong impact on the climate because it affects the energy transport both along the
surface (horizontal transport) and between the surface and the top of the atmosphere
(vertical transport). The atmospheric columnar mass also acts as a protective shield
for life potentially present on the planetary surface, by absorbing and degrading
cosmic rays of stellar or Galactic origin. For a planet with Earth-like characteristics
(including the magnetic field), the surface dose of secondary particles of cosmic rays
exceeds 100 mSv/year when N atm < 300 g/cm
2 [26]. By displaying h 050 as a function
of S and N atm , it is possible to build up an atmospheric mass habitable zone (AMHZ)
for complex life [7]. The calculations of the AMHZ can be repeated for different
atmospheric compositions, showing the impact of greenhouse gases, such as CO 2 ,
on the location of the HZ. The AMHZ calculated with the index h 050 is generally
narrower than the classic HZ, providing tight constraints for the search of exoplanets
capable of hosting multicellular life. By decreasing N atm , the inner edge of the HZ
gets closer to the star due to the decrease of the greenhouse heating of the surface.
However, this effect becomes negligible at low values of N atm and below 300 g/cm
2
the only net result is a significant rise of the surface dose of radiation [7, 26].
14.4 Conclusions
By investigating the thermal limits of multicellular life it is possible to introduce
an operational definition of complex-life habitability that can be applied to SETIoriented studies of exoplanets. Based on experimental data of terrestrial life, the temperature interval 0 ≤ T (
◦ C) ≤ 50 is suitable for the emergence of complex life and
for the biological generation of atmospheric O 2 , a biosignature potentially detectable
with spectroscopic observations of exoplanetary atmospheres. With the aid of dedicated climate models, it is possible to predict the surface temperature distribution
of rocky exoplanets by combining observational data with a parameterization of the
climate factors that are currently unconstrained by observations. By modelling the
surface temperature we can assess which range of parameter space is suitable for the
sustenance of complex life and the detection of atmospheric biomarkers.
For future applications of this methodology it is desirable to upgrade climate models to be able to simulate a broad range of stellar, orbital and planetary conditions,
including the climate impact of biological feedbacks. By adopting a multi-parameter
approach to the study of planetary habitability it will be possible to broaden the
concept of habitable zone [24, 25, 27]. To assess the universal validity of the temperature limits deduced from the properties of terrestrial life, we need to improve
our understanding of the physical mechanisms that govern the thermal response of
life processes. Statistical studies of exoplanets with properties suitable to sustain
complex life can be applied to study the potential distribution of intelligent life in
the Galactic Habitable Zone [28].
133
A parameter of special interest for building up the HZ for complex life is the
atmospheric columnar mass, N atm = p/g, where p and g are the surface pressure
and gravitational acceleration, respectively. The atmospheric columnar mass has a
strong impact on the climate because it affects the energy transport both along the
surface (horizontal transport) and between the surface and the top of the atmosphere
(vertical transport). The atmospheric columnar mass also acts as a protective shield
for life potentially present on the planetary surface, by absorbing and degrading
cosmic rays of stellar or Galactic origin. For a planet with Earth-like characteristics
(including the magnetic field), the surface dose of secondary particles of cosmic rays
exceeds 100 mSv/year when N atm < 300 g/cm
2 [26]. By displaying h 050 as a function
of S and N atm , it is possible to build up an atmospheric mass habitable zone (AMHZ)
for complex life [7]. The calculations of the AMHZ can be repeated for different
atmospheric compositions, showing the impact of greenhouse gases, such as CO 2 ,
on the location of the HZ. The AMHZ calculated with the index h 050 is generally
narrower than the classic HZ, providing tight constraints for the search of exoplanets
capable of hosting multicellular life. By decreasing N atm , the inner edge of the HZ
gets closer to the star due to the decrease of the greenhouse heating of the surface.
However, this effect becomes negligible at low values of N atm and below 300 g/cm
2
the only net result is a significant rise of the surface dose of radiation [7, 26].
14.4 Conclusions
By investigating the thermal limits of multicellular life it is possible to introduce
an operational definition of complex-life habitability that can be applied to SETIoriented studies of exoplanets. Based on experimental data of terrestrial life, the temperature interval 0 ≤ T (
◦ C) ≤ 50 is suitable for the emergence of complex life and
for the biological generation of atmospheric O 2 , a biosignature potentially detectable
with spectroscopic observations of exoplanetary atmospheres. With the aid of dedicated climate models, it is possible to predict the surface temperature distribution
of rocky exoplanets by combining observational data with a parameterization of the
climate factors that are currently unconstrained by observations. By modelling the
surface temperature we can assess which range of parameter space is suitable for the
sustenance of complex life and the detection of atmospheric biomarkers.
For future applications of this methodology it is desirable to upgrade climate models to be able to simulate a broad range of stellar, orbital and planetary conditions,
including the climate impact of biological feedbacks. By adopting a multi-parameter
approach to the study of planetary habitability it will be possible to broaden the
concept of habitable zone [24, 25, 27]. To assess the universal validity of the temperature limits deduced from the properties of terrestrial life, we need to improve
our understanding of the physical mechanisms that govern the thermal response of
life processes. Statistical studies of exoplanets with properties suitable to sustain
complex life can be applied to study the potential distribution of intelligent life in
the Galactic Habitable Zone [28].