August 6, 2015

Searching for Life in the Alpha Centauri System

The polarized light reflected from the leaf contains a footprint of the leaf's biopigments.
These biosignatures can be detected with a polarization filter, shown here
as a pair of sunglasses. Illustration: Svetlana Berdyugina

A new technique enables scientists to search for traces of life on exoplanets in reflected light

(August 6, 2015)  A new approach to searching for life on other planets: An international team has discovered that biopigments of plants, so-called biological photosynthetic pigments, leave behind unique traces in the light they reflect. Prof. Dr. Svetlana Berdyugina from the Institute of Physics of the University of Freiburg and the Freiburg Kiepenheuer Institute for Solar Physics studied these biosignatures together with researchers from the University of Hawaiʻi at Mānoa, USA, and the University of Aarhus, Denmark, with the help of polarization filters: If biopigments were present as a sign of life on a planet, they would leave behind a detectable polarized signature in the reflected light. The scientists have now published their findings in the International Journal of Astrobiology.

Figure 1: A green leaf absorbs almost all red, green and blue light (RGB), but it reflects
and transmits infrared light (shown in grey). The reflected infrared light is only weakly polarized
due to the reflection of a healthy leaf, but the reflected RGB light is strongly polarized due to
biopigments. Measuring the amount of polarized light at different colors reveals the signature
of the leaf biopigments. Green sand reflects and polarizes sunlight almost equally in all
wavelengths, which distinguishes it from a leaf that is a similar color.
Similarly, yellow plants are different from yellow sand, etc. (Credit: S. Berdyugina)

Photosynthetic pigments are plant substances that absorb and reflect particular wavelengths of visible light, making them appear in color in the reflected wave ranges. Biopigments are what gives plants, algae, bacteria, and human skin and eyes their colorful appearance. Chlorophyll pigments in plant leaves, for instance, absorb blue to red light but reflect a small part of green in the visible spectrum and thus appear green. An exception is infrared light: half of it is reflected and the other half passes through the leaf. Carotenoids absorb blue and red light but reflect yellow light and are thus typically red, orange, or yellow in color.

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