Distant Planets Could Have Plants of "Alien" Colors

John Roach
for National Geographic News
April 12, 2007
Scientists may be able to determine the color of extraterrestrial plant life while studying distant planets, according to a pair of new studies.

Researchers have developed a way to analyze the light emitted by a given planet's parent star and determine how that light interacts with various chemicals in the planet's atmosphere.

This gives scientists an idea of the wavelengths, or colors, of light that reach the planet's surface.

On Earth, red light is most abundant, while blue light is most energetic, or useful to plants. So plants tend to absorb these colors and reflect the less useful green light, said Nancy Kiang, a biologist at NASA's Goddard Institute for Space Studies in New York.

But other stars may emit different wavelengths of light, and their planets may have different chemicals in their atmospheres, she explained. So the dominant colors reaching the surfaces of other planets may be unlike those on Earth.

This means that distant worlds could theoretically feature plants that are red, yellow, or blue.

"We're actually predicting what pigments absorb … but you can conjecture what range of colors they might wind up reflecting since they're not absorbing them," Kiang said.

Search for Alien Plant Life

Kiang and colleagues used computer models to develop their method of studying light on distant worlds. Two related papers on the process appear in the March issue of the journal Astrobiology.

The models simulate different kinds of light emitted by stars that are hotter and cooler than the sun. The models then weigh how that light would interact with a planet's chemistry, what light would reach the ground, how plants would use that light, and other variables.

Co-author Victoria Meadows is an astrobiologist at the California Institute of Technology in Pasadena. She said the research will also help scientists detect life on extrasolar planets.

(Read related story: "Earthlike Planet Spied in Distant Solar System" [January 26, 2006].)

Photosynthesis—the process plants use to turn light into sugar—produces detectable signs of life at a global scale, she explained.

Photosynthesis on Earth is responsible for atmospheric oxygen and ozone. The same may be true on other planets, she said.

"That oxygen and ozone on Earth can be seen from space. It can be seen from a very great distance, so it's a good thing to go after [when searching for extraterrestrial life]," she said.

"It's not only the fact that we think photosynthesis is highly likely if there's surface life, but also the signs of photosynthesis are relatively easy to detect," Meadows said.

Planet Finders

The new research, the authors added, will guide the development of future telescopes designed to search for life on other planets.

NASA's Kepler spacecraft, scheduled to launch in October 2008, will be able to detect Earth-size planets in habitable zones around distant stars. This will help scientists understand whether these types of planets are common.

(See National Geographic magazine's "Search for Other Earths.")

Scientists hope future space telescopes, such as NASA's proposed Terrestrial Planet Finder and the European Space Agency's Darwin, will be able to find and study nearby extrasolar planets.

The telescopes will allow scientists to determine what a given planet's surface is made of and what chemicals are in its atmosphere.

That information, combined with data on the wavelengths of light from the parent star, will allow scientists to determine what colors of light the planet's plants most likely use for photosynthesis—and what colors they reflect.

"That's what this research is relevant to," Meadows said.

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