Plants on Other Planets May Not be Green

Tigran Khanzadyan30th of April 2007

Based on 2 Astrobiology papers by Kiang et al. (2007) and a press release from SPITZER (NASA)

Outline or 5Ws

• Why are plants green (on Earth)?

• Photosynthesis: What is it?

• Possibility of life out there ... (Where?)

• Which methods where used?

• Who would benefit from this research?

Why are plants green?• Seams to be the easiest

question to answer, yes?

• From the school biology classes we (me) remember that it’s because of chlorophyll in them. Chlorophyll absorbs the blue and red light from sun and reflects the green - hence we see the plants in green.

• But we see some plants have different colors and furthermore there are plants in sea which are not green ...

In fact ...• Plants under the water are “tuned” differently to take

advantage of the energy which is reaching them.

• Plants do not have to be green at all. The reason that the plants are green is their ancestral connection with green algae which had selective advantage for life on land after the buildup of atmospheric O2, which shifted the surface spectral photon flux from a peak at 600nm to 685nm.

• But evan now in the murky depths of Earth's waters lurk photosynthetic bacteria that appear purple to the human eye, employing light in the infrared spectrum to store energy; more archaic plants—such as lichens and moss—utilize more of the blue spectrum in visible light. There are even red, shade-dwelling vascular plants.

Photosynthesis• Photosynthesis efficiently converts light energy to

electrochemical energy for oxidation-reduction (“redox”) reactions.

• H2A is a reducing substrate such as H2O or H2S. In case of H2O we have oxygenic photosynthesis, but in case of H2S we have an-oxygenic photosynthesis.

Photosynthesis

• So we know that photosynthesis can occur as oxygenic and un-oxygenic. Both occur on Earth, but the dominant is oxygenic (good for us). But in the earlier stages of Earth evolution (like 3Gy ago) the situation was quite opposite.

• Photosynthesis is a very widespread, very successful process. It is the dominant form of life. It can be detectable at the global scale and you can see it from really big distances from the planet - provided that some criteria are met, like no or reduced cloud cover and the presence of widespread vegetation.

Possibility of life out there ...

• At the present there are more than 250 extra-solar planet discovered around nearby stars. There must be much more out there ...

• But we are looking for ones situated in a “habitable zone” of the stars. There must be ones ...

• The widespread and diverse nature of conditions for existing life on other planets provided by photosynthesis process is the key factor.

• Oxygenic photosynthesis in particular is quite possible to occur in wide range of stellar types.

Which methods where used?

• The team by Kiang et al used previously simulated planetary atmospheric compositions for Earth-like planets around observed F2V and K2V, modeled M1V and M5V stars, and around the active M4.5V star AD Leo.

• They used Earth’s atmospheric composition as well as very low O2 content in case an-oxygenic photosynthesis dominates.

• By using line-by-line radiative transfer model, they calculate the incident spectral photon flux densities at the surface of the planet and under water.

Surface incident photon flux densities at solar noon for the Earth and for the planets in the habitable

zone of F, K and M stars

And they found that ...• They identified bands of available photosynthetically relevant

radiation and found that photosynthetic pigments on planets around :

- F2V stars may peak in absorbance in the blue

- K2V in the red-orange

- M stars in the NIR, in bands 0.93-1.1um, 1.1-1.4um, 1.5-1.8um, and 1.8-2.5um. Underwater organisms will be restricted to wavelengths shorter than 1.1um.

- M star with an-oxygenic photosynthesis will have photon fluxes above 1.6um curtailed by methane.

Who would benefit from this research?

• There are many missions planned to for the near future to detect Earth size planets and analyze the light from them : NASA’s Kepler, ESA’s Corot, NASA’s Space Interferometry Mission, Terrestrial Planet Finder and ESA’s Darwin.

• We should be able to determine those spectra for signs of life.

Recently ...

Gliese 581

• Host star is M dwarf and the planet is within the “Habitable zone”

• Orbits around the star in every 13 days

• 1.5 times Earth diameter

• 5 times Earth mass, g=22m/s/s