88
S. Masiero and M. S. Erculiani
9.2.3 Ecological Limit to Life
As described in McKay [1], temperature is key both because of its influence on liquid
water and because it can be directly estimated from orbital and climate models of
exoplanetary systems. Life can grow and reproduce at temperatures as low as −15
◦ C,
and as high as 122
◦ C. Studies of life in extreme deserts show that on a dry world,
even a small amount of rain, fog, snow, and even atmospheric humidity can be
adequate for photosynthetic production producing a small but detectable microbial
community. Life is able to use light at levels less than 10
−5 of the solar flux at Earth.
UV or ionizing radiation can be tolerated by many microorganisms at very high levels
and is unlikely to be life limiting on an exoplanet. Biologically available nitrogen
may limit habitability. Levels of O 2 over a few percent on an exoplanet would be
consistent with the presence of multicellular organisms and high levels of O 2 on
Earth-like worlds indicate oxygenic photosynthesis. Other factors such as pH and
salinity are likely to vary and not limit life over an entire planet or moon.
9.2.4 Mass Versus Gravity
In order to understand if exoplanets could support life, we can understand what
gravity acceleration could experience an organism on it. From the physics we know
that the superficial gravity depends on the mass m and radius r according to Newton’s
formula a(r) = −Gm ˆ
r/ | r |
2 , with G the universal gravitational constant. It follows
that planets with different masses and sizes would have different gravity values on
their surfaces. In [2] a considerable number of the extrasolar planets have a surface
gravity very similar to that of Earth. It is true that in the Solar System the mass
of rocky planets smaller than Venus grows with the square root of the mass while
for gaseous giant exoplanets, the surface gravity linearly grows with the mass. For
planets between 1 and 100 Earth masses the surface gravity is roughly similar to that
of Earth.
In the ExoBioLaboratory a Super-Earth scenario will be considered for four different stars, the surface gravity being about 1 g (the gravity on the Earth) (Fig. 9.1).
9.2.5 Gravitropism
Gravitropism is the movement or growth of plants in response to gravity. Depending
on the gravity of an exoplanet, we can expect different height of plants, if present
there. On Earth the height limit is represented by the remarkable 130 m of sequoias.
Beyond this limit, gravity can create crucial mechanical damage to the plants. In
mosses, hypergravity up to 10 g can increase the size of chloroplasts, photosynthesis
and growth, particularly in Specimens of Physcomitrella patens [3].
S. Masiero and M. S. Erculiani
9.2.3 Ecological Limit to Life
As described in McKay [1], temperature is key both because of its influence on liquid
water and because it can be directly estimated from orbital and climate models of
exoplanetary systems. Life can grow and reproduce at temperatures as low as −15
◦ C,
and as high as 122
◦ C. Studies of life in extreme deserts show that on a dry world,
even a small amount of rain, fog, snow, and even atmospheric humidity can be
adequate for photosynthetic production producing a small but detectable microbial
community. Life is able to use light at levels less than 10
−5 of the solar flux at Earth.
UV or ionizing radiation can be tolerated by many microorganisms at very high levels
and is unlikely to be life limiting on an exoplanet. Biologically available nitrogen
may limit habitability. Levels of O 2 over a few percent on an exoplanet would be
consistent with the presence of multicellular organisms and high levels of O 2 on
Earth-like worlds indicate oxygenic photosynthesis. Other factors such as pH and
salinity are likely to vary and not limit life over an entire planet or moon.
9.2.4 Mass Versus Gravity
In order to understand if exoplanets could support life, we can understand what
gravity acceleration could experience an organism on it. From the physics we know
that the superficial gravity depends on the mass m and radius r according to Newton’s
formula a(r) = −Gm ˆ
r/ | r |
2 , with G the universal gravitational constant. It follows
that planets with different masses and sizes would have different gravity values on
their surfaces. In [2] a considerable number of the extrasolar planets have a surface
gravity very similar to that of Earth. It is true that in the Solar System the mass
of rocky planets smaller than Venus grows with the square root of the mass while
for gaseous giant exoplanets, the surface gravity linearly grows with the mass. For
planets between 1 and 100 Earth masses the surface gravity is roughly similar to that
of Earth.
In the ExoBioLaboratory a Super-Earth scenario will be considered for four different stars, the surface gravity being about 1 g (the gravity on the Earth) (Fig. 9.1).
9.2.5 Gravitropism
Gravitropism is the movement or growth of plants in response to gravity. Depending
on the gravity of an exoplanet, we can expect different height of plants, if present
there. On Earth the height limit is represented by the remarkable 130 m of sequoias.
Beyond this limit, gravity can create crucial mechanical damage to the plants. In
mosses, hypergravity up to 10 g can increase the size of chloroplasts, photosynthesis
and growth, particularly in Specimens of Physcomitrella patens [3].