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)

The search for life on other planets is fascinating, challenging and enlightening. Scientists from the University of Hawaiʻi at Mānoa’s Institute for Astronomy, including Astronomer Jeff Kuhn, Instrument and Telescope Project Scientist David Harrington and Software Engineer John Messersmith, are part of a team headed by Professor 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 developed a new approach to searching for life on other planets. Biologist Tina Santl-Temkiv of Aarhus University, Denmark, is also a team member.

New technique finds photosynthetic biopigment signatures

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 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.

Searching for biosignatures to find life on other planets

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 10 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.

An xxx news release

—By Louise Good