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Detecting Life beyond Our Solar System

Photosynthetic biopigments

Scientists found that the light reflected by photosynthetic pigments in living organisms such as plants is polarized differently from the light reflected by nonliving things such as rocks, sand, or water. By searching for this polarization fingerprint, it may be possible to detect plant life on other planets. (Credit: S. Berdyugina)

Is there life in the Alpha Centauri system? In an article published in the International Journal of Astrobiology, IfA scientists have proposed a sensitive technique for detecting photosynthetic organisms in extrasolar planetary systems to try to find out. Headed by Svetlana Berdyugina (Kiepenheuer Institut fuer Sonnenphysik and the University of Freiburg, Germany, and a visiting scientist at the University of Hawai‘i NASA Astrobiology Institute), the group includes IfA’s Jeff Kuhn, David Harrington, and John Messersmith, and biologist Tina Šantl-Temkiv of Aarhus University, Denmark.

Photosynthetic biopigments

The team has measured various biological photosynthetic pigments in the laboratory. They absorb almost all solar light of specific colors in the visible and convert it into chemical bonds to store energy. For example, chlorophyll pigments absorb blue to red light and reflect a small part of green in the visible, as seen in green plants.

All infrared light is reflected, and this is employed in agriculture to monitor water content in crops. Such biopigments are contained in plants, algae, bacteria, and even in human skin (carotenoids) and eyes (rhodopsin), creating the colored beauty of our world. They can also help find life on the surfaces of other planets.

The scientists have found that the part of visible light reflected by various plants with vibrant colors oscillates in certain directions, while incident light oscillates in all directions. Thanks to this peculiarity, this reflected light can be detected remotely by using polarizing filters (similar to Polaroid sunglasses or 3-D movie goggles) when viewed at specific angles even if the star is millions of times brighter than the planet. The team found that each biopigment has its own colored footprint in such polarized light.

Modeled spectra reflected off distant exo-Earth surfaces have demonstrated the advantage of using polarized light to distinguish photosynthetic biosignatures from minerals, ocean water, and the atmosphere. The high contrast of the biosignatures in the polarized light is the key to finding them in the overwhelmingly bright stellar light that usually hides the exoplanetary signals.

Earlier, Berdyugina and her team employed polarized light to see for the first time the blue color of an exoplanet. Now this method can help to see colors of life on other planets, even at large distances from the Sun.

Our neighbor, the triple stellar system Alpha Centauri

This technique could be instrumental in searching for life in the planetary system nearest to the Sun, Alpha Centauri, with existing telescopes. There are three stars in this system. While scientists are interested in finding life around all these stars, Alpha Centauri B, only 4.37 light years from Earth, seems optimal for life searches with current telescopes.

In 2014, a small planet was discovered around Alpha Centauri B. Unfortunately, this exoplanet is ten times closer to the star than Mercury is to the Sun, so its surface is melting under the stellar heat, and it probably has no atmosphere. At a distance where planets like Earth with liquid water on their surface could exist (the “habitable zone”), no planets have been found as yet, but scientists are continuing to search for one. If such a planet is found, or even before that, it is possible to search for photosynthetic biosignatures in the Alpha Centauri B spectrum. Using the proposed polarization technique, this task becomes even more feasible.