Alien plants might not necessarily be green

Green, yellow or even red-dominant plants may live on extra-solar planets, speculate NASA scientists who believe they have found a way to predict the colour of plants on planets in other solar systems.

The scientists, whose reports appear in the March issue of U.S. journal, Astrobiology, studied light absorbed and reflected by organisms on Earth, and determined that if astronomers were to look at the light given off by planets circling distant stars, they might predict that some planets have mostly non-green plants.

Besides improving our understanding of life on Earth, the findings can potentially improve current methods of searching for life elsewhere in the universe, scientists say.

"We can identify the strongest candidate wavelengths of light for the dominant colour of photosynthesis on another planet," said Nancy Kiang, lead author of the study and a biometeorologist at NASA's Goddard Institute for Space Studies, New York.

Kiang and her colleagues calculated what the stellar light would look like at the surface of Earth-like planets whose atmospheric chemistry is consistent with the different types of stars they orbit. By looking at the changes in that light through different atmospheres, researchers identified colours that would be most favourable on other planets for photosynthesis, which is a process by which plants use energy from sunlight to produce sugar.

In the process of photosynthesis, plants convert energy from the sun into chemical energy in the form of glucose, or sugar - NASA AmesOrganisms that live in different light environments absorb the light colours that are most available. For example, there is a type of bacteria that inhabit murky waters where there is little visible light, and so they use infrared radiation during photosynthesis.

It has long been known that the chlorophyll in most plants on Earth absorbs blue and red light and less green light, so chlorophyll appears green. According to scientists, the Sun has a specific distribution of colours of light, emitting more of some colours than others. Gases in Earth's air also filter sunlight, absorbing different colours. As a result, more red light particles reach Earth's surface than blue or green light particles, so plants use red light for photosynthesis.

But not all stars have the same distribution of light colours as our Sun. Study scientists say they now realize that photosynthesis on extrasolar planets will not necessarily look the same as on Earth. Scientists expect each planet to have different dominant colours for photosynthesis, based on the planet’s atmosphere where the most light reaches the planet’s surface. The dominant photosynthesis might even be in the infrared.

"It makes one appreciate how life on Earth is so intimately adapted to the special qualities of our home planet and Sun," Kiang said.

More information is available from NASA's press release.