An alien colour chart has been devised by astronomers and biologists to help them recognise extraterrestrial life.
The colours represent 137 chemical fingerprints of different types of micro-organism, some of which hail from the most extreme environments on Earth.
If similar life forms are abundant on an "exoplanet" outside the Solar System, they could be identified from the wavelengths of light they reflect.
In the same way, a distant observer looking at the Earth would note a greenish tint due to its vegetation.
Photosynthesising plants appear green because they absorb light wave lengths corresponding to other colours. Only green visible light is reflected back.
The scientists, led by Siddharth Hegde from the Max Planck Institute for Astronomy in Heidelberg, Germany, wrote in the journal Proceedings of the National Academy of Sciences: "This catalogue provides a broad scope of surface signatures for life on exoplanets, which could provide different conditions from those on Earth, allowing, for example, extremophiles on Earth to become the predominant life form.
"Much of the history of life on Earth has been dominated by microbial life, and it is likely that life on exoplanets evolves through single-celled stages prior to multicellular creatures.
"Here, we present the first database, to our knowledge, for such surface features in preparation for the next generation of space and ground-based telescopes that will search for a wide variety of life on exoplanets."
Around 2,000 planets have been discovered orbiting distant stars, some of which occupy the "habitable zone" where temperatures are mild enough to allow liquid surface water and possibly Earth-like life.
The next phase of research on exoplanets will involve probing their atmospheres and reflected light for life signatures.
Indirect biosignatures - by-products of life - could be found such as ozone, derived from oxygen, and methane. But searching for the equivalent of leafy green on the planets' surfaces would provide more direct evidence of life, scientists believe.
The presence of an alien organism covering large swathes of a planet's surface could be confirmed from its pigmentation.
In the new study, researchers assembled cultures of 137 difference species of micro-organisms spanning a variety of colours and from environments ranging from the Atacama desert to Hawaiian seawater.
The colours included a deep red, yellow, pink, green and brown.
There are plans to collect more samples and add more living colours to the catalogue.
But the scientists point out that detecting the colour-prints of organisms living on the surfaces of planets will be highly challenging.
Currently it is not possible to measure light directly from an Earth-sized planet because it is drowned out by much brighter starlight.
The scientists add: "Our freely available database can be used as inputs to models of rocky exoplanets as well as help plan observational strategies to detect a variety of life on extrasolar planets."