Light could reveal plant life on other planets

Plant life growing on planets orbiting nearby stars could soon be detected using a new technique, scientists claim.

Light reflected from vegetation has a particular signature that makes it stand out, even against the dazzling brilliance of a star, researchers have discovered.

That could make it possible to spot life in a star system before the planet harbouring it has been found.

The scientists suggest using the technique to target Alpha Centauri B, the star closest to the sun at a distance of 4.37 light years.

No planet has yet been identified orbiting in the star’s “habitable zone” — the region where temperatures are mild enough to allow liquid surface water and possibly Earth-like life — but the possibility of an as-yet undiscovered small, habitable planet in the system remains.

The new method relies on measuring polarised light, which has waves all oscillating in the same direction.

Lead scientist Professor Svetlana Berdyugina, from the University of Freiburg in Germany, said: “This technique could be instrumental in searching for life in Alpha Centauri, the planetary system closest to the sun.

“Even before such a planet is found, we can use the polarisation technique to search for biosignatures that point to life.”

Larger telescopes than exist today would be needed to study more distant star systems, she said.

Plants appear green as their chlorophyll pigments reflect light wavelengths giving rise to that colour. Other biopigments are responsible for red, orange and yellow plant colours.

Prof Berdyugina’s team discovered that visible light reflected in colours by various plants has an identifiable polarisation. Like a skipping rope flicked up and down or from side to side, the light waves oscillate in a certain way.

Each biopigment leaves a polarisation “footprint” in its reflected light, which can be detected with the help of special filters that function like polarised sunglasses.

Because they stand out so strongly, the biosignatures could be visible even against the background of brilliant starlight that makes it virtually impossible to view extrasolar planets directly.

The research is published in the International Journal of Astrobiology.

At least one unconfirmed planet is thought to orbit Alpha Centauri B, but not in its habitable zone.