Is there life on the Alpha Centauri system? Press release, August 5, 2015.

Colossus scientists have proposed a sensitive technique for detecting photosynthetic organisms in extrasolar planetary systems

The search for life on other planets is fascinating, challenging, and enlightening. If successful, it will teach us about ourselves, where we come from, and what our destiny is.

Colossus scientists headed by Prof. Dr. Svetlana Berdyugina (Kiepenheuer Institut fuer Sonnenphysik and the University of Freiburg, Germany, and a visiting scientist at the University of Hawaii NASA Astrobiology Institute) and Dr. Jeff Kuhn (University of Hawaii, UH) and including David Harrington (UH), John Messersmith (UH) and biologist Tina Santl-Temkiv of Aarhus University, Denmark, have developed a new approach to searching for life on other planets.

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. (Figure 1).

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.

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)

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 (Figure 1). Thanks to this peculiarity, this reflected light can be detected remotely by using polarizing filters (similar to Polaroid sunglasses or 3D 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.

These results will be published by the International Journal of Astrobiology, Cambridge University Press [1]. Earlier Prof. Berdyugina and her team employed polarized light to see for the first time the blue color of an exoplanet [2]. 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 (Figure 2).

Figure 2: 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)

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.

For more distant stars and planetary systems telescopes significantly larger than 30m are needed. The Colossus will search for life and advanced Earth-like civilizations within 60 light years from the Sun [3]. We may encounter alien "Earths" with photosynthetic organisms similar to ours (Figure 3).

Figure 3: Artist's impressions of Earth-like planets covered by photosynthetic organisms with terrestrial-like biopigments studied by the team. (Credit: S. Berdyugina and C. Giebink).

This research was supported at KIS/Freiburg by the European Research Council (ERC) Advanced Grant HotMol (www.hotmol.eu) the Leibniz Association (WGL) grant InnoPol, and the UH NASA Astrobiology Institute team.

References:

[1] Berdyugina, S.V., Kuhn, J.R., Harrington, D.M., Santl-Temkiv, T., Messersmith, E.J.: Remote Sensing of Life: Polarimetric Signatures of Photosynthetic Pigments as Sensitive Biomarkers, International Journal of Astrobiology, in press (2015) PDF

[2] Berdyugina, S.V., Berdyugin, A.V., Fluri D.M., Piirola V.: Polarized reflected light from the exoplanet HD189733b: First multicolor observations and confirmation of detection, Astrophysical Journal Letters, 728, L6-L10 (2011) PDF

[3] Kuhn, J.R., Berdyugina, S.V.: Global warming as a detectable thermodynamic marker of Earth-like extrasolar civilizations: the case for a telescope like Colossus, International Journal of Astrobiology, vol. 14, pp. 401-410 (2015) PDF

Contacts:


Svetlana Berdyugina

Jeff Kuhn