Is there life in the Alpha Centauri system?

University of Hawaiʻi at Mānoa
Louise Good, (808) 381-2939
Editor/Media Contact, Institute for Astronomy
Svetlana Berdyugina, (956) 220-6724
Visiting Scientist, UH NASA Astrobiology Institute
Posted: Aug 6, 2015

The Alpha Centauri A and B stars with their habitable zones.
The Alpha Centauri A and B stars with their habitable zones.

Scientists from the University of Hawaiʻi, including Jeff Kuhn, David Harrington and John Messersmith, are part of a team headed by Prof. Dr. Svetlana Berdyugina (Kiepenheuer Institut fuer Sonnenphysik and the University of Freiburg, Germany), a visiting scientist at the University of Hawaiʻi NASA Astrobiology Institute, that has proposed a sensitive technique for detecting life on other planets. This technique could be instrumental in searching for life in the planetary system nearest to the sun, Alpha Centauri, with existing telescopes.

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. 

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 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.

There are three stars in the Alpha Centauri 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 extremely hot, 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. 

Full caption for graphic: 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)

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