Most people are familiar with the pale blue dot image of Earth taken by Voyager in 1990. Its blueness is significant, of course, because it is Earth's abundant liquid water that makes it look that way.

But if you looked at the light that is reflected from Earth carefully, you would see several interesting features. One, caused by vegetation, is called "red edge". Green plants absorbs a lot of red light creating a big, sudden jump in reflectivity in the red bit of the visible light spectrum. An alien, if it could get a good look, would be able to tell than Earth had plenty of vegetation because of this red edge.

A paper recently uploaded to arXiv (accepted for publication in the journal Astrobiology) looks at what features, like vegetation, we might be able to discern on a planet outside of our solar system.

Siddharth Hegde and Lisa Kaltenegger at the Max Planck Institute for Astronomy in Heidelberg, Germany, then looked at how extremophiles -- creatures that live and even thrive in extreme conditions here on Earth -- might fare in the different kinds of habitat. "Extremophiles provide us with the minimum known envelope of environmental limits for life on our planet," the researchers say.

I wrote about a similar topic at New Scientist in August when Hajime Kawahara at Tokyo Metropolitan University and Yuka Fujii at the University of Tokyo in Japan, published a paper that describes how they created 2D maps of what the light from an Earth-like planet would look like with various features on its surface (see 'Finding a Blue Marble' here). By watching a planet over time their technique is able to build up a more detailed image - a blue marble, rather than a pale blue dot. Maps like these may one day provide us with an indication of what the environment is like on a faraway exoplanet.

Hegde and Kaltenegger's paper covers a more rough and ready technique that could be a first step in deciding which exoplanets are the ones we should be studying in more detail – the planets where organisms, albeit extreme ones, can survive here on Earth.

They note, however, that the characteristics of vegetation (or any organism with chlorophyll) could vary depending on their planet's host star. The signature of chlorophyll near a hot star could have a blue, rather than red, edge to protect a plant's leaves from overheating. Or on a planet that orbits a cool, dim star chlorophyll may appear black as it tries to absorb as much light as possible across the whole range of the visible spectrum.

A day where we have to use these techniques to decide which of an abundance of potential Earth 2.0s to travel to seems a long, long way away. But that doesn't mean we can't start daydreaming about which we will aim for first.

(Hat tip to Technology Review for flagging up the paper.)