Lichens and algae could be the first life forms we find on Earth-like exoplanets, by looking for their light signatures in a planet's distinctive colouring.
Astronomers have found several rocky worlds in the habitable zone, the region around a star where liquid water can exist on a planet's surface, and many more are thought to exist. As telescopes get more sensitive, we should be able to collect light reflected off such planets and look for clues to their surface conditions.
Seen from space, Earth gives off a large amount of near-infrared light, which is reflecting off the chlorophyll in plants. We might see a similar "red edge" on distant exoplanets if they also host green vegetation.
But Siddharth Hegde and Lisa Kaltenegger of the Max Planck Institute for Astronomy in Heidelberg, Germany, think it is possible that many rocky worlds will have extreme heat, dryness or acidity, and that hardier life forms will dominate their surfaces. So what would these organisms look like from a distance?
To find out the pair looked at the light reflected by some of Earth's more extreme life forms: lichens in arid regions, bacterial mats in very hot water and red algae in acid mine drainage. They calculated that seen from afar each type of organism would create a unique colour pattern. Lichens, for instance, appear more yellow than the algae or bacteria.
Finding these patterns wouldn't necessarily mean life is present, but it could be a step towards narrowing down exoplanets for more detailed searches, the authors say.
It's an attractive idea, says Nicolas Cowan of Northwestern University in Evanston, Illinois, and it is far more likely that a given planet would contain microbial life than trees. But the work has its limits, he cautions. For instance, atmospheres on other planets may be very different from our own and could scatter light in ways we wouldn't expect.
"Nature may be more creative than our little corner of the cosmos would have us believe," Cowan says.