How to spot vegetation on Earth-like planets


In December 1990, scientists analysing data from the Galileo spacecraft found compelling evidence for the existence of life in space.

The data famously came from the craft’s first fly by of Earth, a planet on which life seemed a definite possibility.  The exercise led to the establishment of a number of criteria that if found elsewhere, would point to the existence of life

Among the most persuasive of these criteria is the vegetation red edge–a sharp increase in the reflectance of light at a wavelength of around 700 nm.  This is the result of chlorophyll absorbing visible light but reflecting near infrared strongly which Galileo found strong evidence for.

Assuming alien plant life is similar to ours,  could we spot a vegetative red edge on an Earth-like planet orbiting a star several light years away, asks Luc Arnold at the Saint-Michel-l’Observatoire in France and amis.

At that distance the planet would be a point-like dot so the first question to consider is whether the red edge would be visible at all in such an averaged view of the planet, especially because the planet is likely to rotate during the observation times necessary at that distance. What difference does the viewing angle make, after all the Earth’s continents, where the vegetation signal is strongest, are mainly in the northern hemisphere? And what if the planet was at a different epoch than ours, such as in the middle of an ice age.

Arnold and co have worked all this out and say that none of these factors can significantly reduce the vegetation red signal. They’ve even worked out what the Earth’s  signal would have looked like during an ice age and found that the vital signs are hard to suppress.

So if another Earth is out there, the vegetaion red edge should show up like a sore thumb.

Except for one thing–it’ll be visible only if you have a large enough telescope. The next big one that NASA hopes to send up is the Terrestrial Planet Finder. ESA has  a similar instrument in the works called Darwin.

The teams behind these instruments say they could spot Earth-like planets orbiting stars at distance s of up to  30 light years with an exposure measured in a couple of hours.

Arnold and friends say that spotting  the signs of life on such a planet would be much harder. The vegetation red edge might only be seen with an exposure of 18 weeks with a telescope like the Terrestrial Planet finder’s. An 18 week exposure of a planet orbiting another star isan almost impossible  task .

So when might we eventually see vegetation on another planet? The Terrestrial Planet Finder looks unlikely to be launched before 2025 and even then might not have the oomph to do the trick.

But should it spot Earth-like planets, the motivation for building a telescope that could spot vegetation would suddenly become a lot stronger.

Ref: The Earth as an Extrasolar Planet: The Vegetation Spectral Signature Today and During the last Quaternary Climatic Extrema

Comments are closed.