As the Voyager 1 spacecraft was about to leave the Solar System in 1990, the American astronomer Carl Sagan asked that spacecraft’s cameras be turned towards its home planet some 3 billion kilometres away. 

The resulting photograph is called the Pale Blue Dot and shows Earth as a tiny bluish-white speck against the vast emptiness of space. Sagan later used this phrase for the title of a book about his vision of humanity’s future in space.

Given Earth’s distinctive colour, an interesting question is what colour an alien Earth orbiting another star might be. Today, we get an answer of sorts from Siddharth Hegde at the Max Planck Institute for Astronomy in Germany and Lisa Kaltenegger at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

They point out that Earth’s colour is intimately linked to its habitability and, in particular, to the colour of water which covers 70 per cent of the Earth’s surface. However, the colour is also influenced by other environments such as desert, snow, lichen-covered rock and the 60 per cent of land covered by vegetation.

The vegetation, in particular, gives rise to the famous “red edge” that an alien viewer should see as the Earth rotates.  It is the result of the increased absorption of red light by photosynthesis as an ocean disappears from view and is replaced by tree-covered land.

If an exoplanet is anything like Earth, particularly in the amount of liquid water at the surface, then its colour should be an important clue, say Hegde and Kaltenegger. Assuming a clear atmosphere that gives a view of the surface, these guys estimate the colour of alien Earths based on the percentage of surface covered in water, tree-like vegetation, bacterial mats, endoliths, which live inside rocks, and so on. 

They conclude that it ought to be possible to assess the habitability of exoplanets that can be directly viewed in this way, a process that should help to focus interest on important exoplanets. 

Bluish dots, for example, would gain priority over Mars-like red dots, since the red planet is entirely devoid of life as far as we can tell.

That could well turn out to be a handy technique. Today, the number of alien Earths stands at three–Gliese 581d, HD 85512b and Gliese 667Cc. But that number is set to grow dramatically in the coming months and years as observaotries such as NASA’s Kepler spacecraft provide more data. So a way of filtering the most interesting exoplanets will surely be of much use.

Ref: arxiv.org/abs/1209.4098: Colors Of Extreme exoEarth Environments