9 Exobiolab: Different Life on Different Planets
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they do not need to use extra green light. But not all stars have the same distribution
of light colors as our Sun. Photosynthesis on extrasolar planets will not necessarily
look the same as on Earth.
The graph of Fig. 9.2 shows the intensity of light by color (wavelength) that
reaches the surface of Earth-like planets orbiting different types of stars. From hotter
to cooler, the star types are F, G, K, and M. Our Sun is represented by the yellow
line (G2 star). A planet orbiting an F2 star (red line) has more blue light at the
surface, whereas Earth and the K2 star planet receive more red light. Planets around
M stars receive much less visible light but much more infrared light. Atmospheric
gases such as ozone (O 3 ), oxygen (O 2 ), water vapor (H 2 O), and carbon dioxide (CO 2 )
absorb light at specific wavelengths, producing the pronounced dips that astronomers
might someday detect in exoplanets’ atmospheres. On diagram’s horizontal axis, the
colored bar marks the wavelengths from 0 to 0.4 microns as UV, 0.4 to 0.7 as visible,
and longer than 0.7 as infrared.
9.3.1 F Star—White Bulb
Main sequence stars brighter than the Sun (spectral types F and A and the very shortlived B and O) emit more blue and ultraviolet light than the Sun. Given sufficient time
for Earth-type photosynthetic life to evolve (about hundreds of millions to billions
of years), planets around such stars could develop an oxygen atmosphere with a
layer of ozone that blocks more energetic but potentially harmful ultraviolet light,
but which anyway would transmit more blue light to the ground than on the Earth. In
response, life could evolve a type of photosynthesis that strongly absorbs blue light,
and probably green as well. In contrast, yellow, orange, and red wavelengths of light
would likely be reflected by such plants, so the foliage would have the bright colors
found during autumn in Earth’s deciduous forests all year round. On the other hand,
some plants may reflect some blue light due to its overabundance and potential to
“burn” photosynthetic organisms (like sunburn from ultraviolet exposure on Earth).
9.3.2 G Star—White-Yellow Bulb
Type G stars are stars similar to our Sun. The basic model is our Solar System with
the Sun as a star. In the process of photosynthesis, plants convert energy from the
Sun into chemical energy in the form of glucose, or sugar. The chlorophyll in plants
absorbs more blue and red light from sunlight, and less green light. Chlorophyll is
green, because it reflects green light more than blue and red one.
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they do not need to use extra green light. But not all stars have the same distribution
of light colors as our Sun. Photosynthesis on extrasolar planets will not necessarily
look the same as on Earth.
The graph of Fig. 9.2 shows the intensity of light by color (wavelength) that
reaches the surface of Earth-like planets orbiting different types of stars. From hotter
to cooler, the star types are F, G, K, and M. Our Sun is represented by the yellow
line (G2 star). A planet orbiting an F2 star (red line) has more blue light at the
surface, whereas Earth and the K2 star planet receive more red light. Planets around
M stars receive much less visible light but much more infrared light. Atmospheric
gases such as ozone (O 3 ), oxygen (O 2 ), water vapor (H 2 O), and carbon dioxide (CO 2 )
absorb light at specific wavelengths, producing the pronounced dips that astronomers
might someday detect in exoplanets’ atmospheres. On diagram’s horizontal axis, the
colored bar marks the wavelengths from 0 to 0.4 microns as UV, 0.4 to 0.7 as visible,
and longer than 0.7 as infrared.
9.3.1 F Star—White Bulb
Main sequence stars brighter than the Sun (spectral types F and A and the very shortlived B and O) emit more blue and ultraviolet light than the Sun. Given sufficient time
for Earth-type photosynthetic life to evolve (about hundreds of millions to billions
of years), planets around such stars could develop an oxygen atmosphere with a
layer of ozone that blocks more energetic but potentially harmful ultraviolet light,
but which anyway would transmit more blue light to the ground than on the Earth. In
response, life could evolve a type of photosynthesis that strongly absorbs blue light,
and probably green as well. In contrast, yellow, orange, and red wavelengths of light
would likely be reflected by such plants, so the foliage would have the bright colors
found during autumn in Earth’s deciduous forests all year round. On the other hand,
some plants may reflect some blue light due to its overabundance and potential to
“burn” photosynthetic organisms (like sunburn from ultraviolet exposure on Earth).
9.3.2 G Star—White-Yellow Bulb
Type G stars are stars similar to our Sun. The basic model is our Solar System with
the Sun as a star. In the process of photosynthesis, plants convert energy from the
Sun into chemical energy in the form of glucose, or sugar. The chlorophyll in plants
absorbs more blue and red light from sunlight, and less green light. Chlorophyll is
green, because it reflects green light more than blue and red one.