Plants growing on other planets are likely to be coloured the whole spectrum of the rainbow except for blue, researchers have concluded.

Two studies carried out on the likely appearance of extra-terrestrial undergrowth suggest that it is as likely to be red or yellow as green.

Astrobiologists said that the colour of the plants, if they exist, will be dependent on the quality of light available from their local sun. The studies were carried out to help to guide the search for life on other planets by identifying the colours that astronomers should search for.

On Earth the commonest colour of foliage is green because it is the shade that is of least use to the plants from the spectrum of light emitted by the sun when they photo-synthesise. The spectrum of light from other stars will differ, and plants growing on other planets are expected to evolve to take advantage of the best available light.

If red light is the colour that is of least use in ET’s forests and meadows the foliage on his planet is most likely to be red.

By analysing the spectrum of light available from stars, the study shows, astronomers will know what sort of colours to look for that would indicate plant life on any orbiting planet.

Blue, however, is an unlikely colour for alien plantlife as it is an energetic colour that makes it particularly valuable as a source of energy for plants through photosynthesis, making it far more likely that they would absorb rather than reflect it. It also has the disadvantage of being the wave-length closest to ultraviolet, which on Earth has a destructive effect on DNA.

Nancy Kiang, of Nasa’s God-dard Institute of Space Studies, in the United States, said: “All the colours of the rainbow are just as likely as green, given the range of stars. There’s a full range of colours but we think plants are unlikely to be blue.”

The studies were carried out as part of the work required before two projects — one by Nasa and the other by the European Space Agency (ESA) — to put telescopes into space to search for life on other planets.

Nasa’s Terrestrial Planet Finder telescope and ESA’s Darwin instrument are intended to be put into orbit around Earth in about ten years.

The two studies, published in the journal Astrobiology, looked at the light absorbed and reflected by plants on Earth. Dr Kiang said: “This work broadens our understanding of how life may be detected on Earth-like planets around other stars, while simultaneously improving our understanding of life on Earth.”

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