By measuring the photon signatures left when light bounces off photosynthesizing cells, astronomers may soon have a new tool for detecting extraterrestrial organisms.

"When you look at objects in the solar system, what's a high-probability way of determining whether or not that planet has life?" said Neill Reid, a Space Telescope Science Institute astrobiologist. "Circular polarization has the potential to be a signature of life."

Other proposed signs of life include gases produced by biological processes, or infrared light reflected by radiation-tolerant microbes. However, those methods are limited. Not all microbes are radiation-tolerant, and nonbiological processes can sometimes produce gases typically associated with life.

The new technique exploits the tendency of some photons to adopt a corkscrewed rather than up-and-down wavelength after hitting a molecule. Those photons are said to be circularly polarized, and their precise path is dictated by the molecule that sets them spinning.

Put a spectrometer calibrated to detect life-specific circular polarized light on a telescope, and you'd have "a powerful remote sensing technique for generic life searches," the researchers wrote Monday in the Proceedings of the National Academy of Sciences.



Critical to their system is the tendency of all living creatures to be made from groups of so-called homochiral molecules. Chirality refers to the "handedness" of molecules, which often contain the same constituent atoms arranged in mirror-image forms. The amino acids in all Earthly life are left-handed, while DNA and RNA are made from right-handed molecules.

That configuration produced the circular polarization wavelengths identified in two species of photosynthesizing bacteria by Reed's team, which was led by STSI colleague Bill Sparks and University of

Hertfordshire astrophysicist James Hough.

Whether those signatures are also present in more complex organisms has yet to be determined. However, the researchers consider the bacteria, believed to be direct-line descendants of species that evolved three billion years ago, to be a plausible stand-in for early life elsewhere in the universe.

It's also possible that the homochirality seen on Earth is not universal. "You can conceive of life where homochirality is not so widespread," said Sandra Pizzarello, an Arizona State University biochemist who was not involved in the study. "It happens that on

Earth, amino acids are left-handed, but if you think of the possibilities out there, that's not strictly necessary.

But Pizzarello still called the work "ingenious." Any search for extraterrestrial life relies on Earth-based assumptions, and those the researchers made are at least sound, she said.

"They say that photosynthesis is plausibly commonplace, and that's true," Pizzarello said. Adding weight to the assumption of widespread homochirality is the predominance of left-handedness among amino acids found in meteorites that struck Earth.

The researchers' life-detecting apparatus is presently restricted to a laboratory bench, but could eventually be installed in large telescopes and space-faring satellites, said Reid. The researchers next plan to measure circular polarization in other organisms, and to analyze circular polarized light reflected from Mars.

See Also:

Citation: "Detection of circular polarization in light scattered from photosynthetic microbes." By William B. Sparks, James Hough,

Thomas A. Germer, Feng Chen, Shiladitya DasSarma, Priya DasSarma, Frank

T. Robb, Nadine Manset, Ludmilla Kolokolova, Neill Reid, F. Duccio

Macchetto, and William Martin. Proceedings of the National Academy of

Sciences*, Vol. 106, No. 16, April 20, 2009.*

Images: 1. The relatively Earth-like Gliese 581c, envisioned by Karen Wehrstein for NASA 2. PNAS

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