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.
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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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The Alpha Centauri A and B stars with their habitable zones (green ovals)
as seen projected on the sky. The habitable zones appear as an ovals because the planets' orbits
are inclined to our line of sight. For the same reason, the distance between the A and B stars
appears shortened. If there are planets in the habitable zones (blue dots), photosynthetic
biopigments could be detected with the proposed polarimetric technique. Sizes of the
stars and planets are not to scale. (1 AU = the distance between Earth and the sun)
[Credit: S. Berdyugina] |
The scientists discovered that the part of the visible light spectrum reflected in colors by various plants oscillates in particular directions, meaning that it becomes polarized. Each biopigment leaves behind a colorful footprint in the polarized light. The researchers can detect this biosignature with the help of polarization filters, which function similarly to polarized sunglasses or 3D movie glasses. The signature in the polarized light of plants on distant planets would also be detectable in this way. The high contrast of the biosignatures in the polarized light could be the key for finding them in overwhelmingly bright stellar light that hides exoplanetary signals.
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Artist’s impressions of Earth-like planets covered by photosynthetic organisms
with terrestrial-like biopigments studied by the team
[Credit: S. Berdyugina & C. Giebink] |
"This technique could be instrumental in searching for life in Alpha Centauri, the planetary system closest to the Sun," says Berdyugina. The astrophysicist points out that the star Alpha Centauri B is optimal for searches with current telescopes because it is one of the closest to Earth. No planet has yet been found in Alpha Centauri B's habitable zone -- the distance from a central star at which a planet can have liquid water on its surface, the precondition for life as we know it on Earth. "Even before such a planet is found, we can use the polarization technique to search for biosignatures that point to life," says Berdyugina. Larger telescopes will be needed to examine more distant planetary systems. Until astronomers build such telescopes, the team aims to search for photosynthetic footprints in the light of the Alpha Centauri system.
Source: Albert-Ludwigs-Universität Freiburg [August 07, 2015]
