Seeking life on alien worlds? Look to plants

If earthlings ever venture out among the stars, one of the first places we will visit will surely be the star Gliese 581, located only 20.5 light-years from Earth. This star is known to have at least three terrestrial planets in orbit around it – with 5, 8 and 17 times Earth’s mass. For now, the distances between us and other stars are prohibitively great, making any actual journey impossible.

The first signs of life beyond our solar system might come from the gentle breathing out of plants on an alien world.

Plants take in carbon dioxide and release oxygen in the process of photosynthesis. Nancy Kiang of the NASA Goddard Institute for Space Studies in New York told us that only photosynthesis from land plants and ocean plankton is known to be able to produce atmospheric oxygen at a global scale.

Nancy Kiang: And we figure that photosynthesis is so widespread over the Earth, it creates a global signature of life that you can detect by telescopes. But on another planet, maybe it will look different. And we have to know what we’re looking for.

Kiang and her colleagues are taking the first steps toward looking for the global signature of living plants on distant worlds, by simulating the possible light spectra hitting the surfaces of these worlds.

Nancy Kiang: It’s a thrill to recognize that the possibility of life elsewhere is very strong, so that we might not be alone in the universe. That life might not be like life on Earth, but it could be out there. It could follow the same rules as life on Earth.

Kiang explained that photosynthesis on Earth produces two kinds of significant spectral signatures by plants and algae in the oceans that create global scale signs of life.

Nancy Kiang: One is the impact on the composition of the atmosphere, specifically the amount of oxygen in our atmosphere, photosynthesis by plants and green algae and other kinds of algae produces the oxygen that we breathe. The other kind is the distinct reflectance signature of plants on land. What that means is that plants reflect light in different wavelengths. We know that they reflect in the green, that’s why we see them in green. They also reflect very strongly in the infrared, so satellites can actually pick up these reflectance signatures and then distinguish plants from the land surface and find them. If plants exist on a planet in another solar system, maybe they will be reflecting in different wavelengths.

So we came up with rules for how pigments absorb for light. And given that, we wanted to apply those rules then for how pigments would absorb wavelengths in other solar systems on other planets. We saw that we have to simulate the spectrum of light hitting the surface of those planets. So what we did was that we looked at a variety of different stars, both hotter and cooler than our sun. We simulated an Earth-like, Earth-size planet in the habitable zone around those stars. Habitable zone meaning that the planet can have a temperature such that it will have liquid water that can support life.

We simulated the light spectrum hitting the surface of these planets, and we identified particular wavelengths that looked like they will be very likely candidates for photosynthetic pigments to absorb.

And we found that you could have quite the full spectrum of colors. For hotter stars than our sun, you have the peak of our photon flux shifted a little bit more to the blue. So the pigments could favor absorption of blue radiation much more. For stars much cooler than our sun, there’s a type of star with much less visible radiation than our sun, but much more infrared radiation. So perhaps these photosynthetic organisms would favor infrared radiation for photosynthesis.

It’s a thrill to recognize that the possibility that life elsewhere is very strong, so that we might not be alone in the universe. That life might not be like life on Earth, but it could be out there. It could follow the same rules as life on Earth. So if we study the Earth, we have to actually understand life on Earth much better in order to look for life ‘out there.’ This work has made me personally really appreciate how intimately adapted life here is to our particular home planet and sun. So we should really treasure life as we have it on Earth because maybe it might be hard for us to adapt to life on another planet even if we manage to go there.

Our thanks to Research Corporation, a foundation for the advancement of science.

Our thanks to:
Nancy Kiang
NASA Goddard Institute for Space Studies
Columbia University
New York, NY

June 29, 2007

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